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USGS2015

Simulation of groundwater flow and analysis of the effects of water-management options in the North Platte Natural Resources District, Nebraska

The North Platte Natural Resources District (NPNRD) has been actively collecting data and studying groundwater resources because of concerns about the future availability of the highly inter-connected surface-water and groundwater resources. This report, prepared by the U.S. Geological Survey in cooperation with the North Platte Natural Resources District, describes a groundwater-flow model of the North Platte River valley from Bridgeport, Nebraska, extending west to 6 miles into Wyoming. The model was built to improve the understanding of the interaction of surface-water and groundwater resources, and as an optimization tool, the model is able to analyze the effects of water-management options on the simulated stream base flow of the North Platte River. The groundwater system and related sources and sinks of water were simulated using a newton formulation of the U.S. Geological Survey modular three-dimensional groundwater model, referred to as MODFLOW–NWT, which provided an improved ability to solve nonlinear unconfined aquifer simulations with wetting and drying of cells. Using previously published aquifer-base-altitude contours in conjunction with newer test-hole and geophysical data, a new base-of-aquifer altitude map was generated because of the strong effect of the aquifer-base topography on groundwater-flow direction and magnitude. The largest inflow to groundwater is recharge originating from water leaking from canals, which is much larger than recharge originating from infiltration of precipitation. The largest component of groundwater discharge from the study area is to the North Platte River and its tributaries, with smaller amounts of discharge to evapotranspiration and groundwater withdrawals for irrigation. Recharge from infiltration of precipitation was estimated with a daily soil-water-balance model. Annual recharge from canal seepage was estimated using available records from the Bureau of Reclamation and then modified with canal-seepage potentials estimated using geophysical data. Groundwater withdrawals were estimated using land-cover data, precipitation data, and published crop water-use data. For fields irrigated with surface water and groundwater, surface-water deliveries were subtracted from the estimated net irrigation requirement, and groundwater withdrawal was assumed to be equal to any demand unmet by surface water. The groundwater-flow model was calibrated to measured groundwater levels and stream base flows estimated using the base-flow index method. The model was calibrated through automated adjustments using statistical techniques through parameter estimation using the parameter estimation suite of software (PEST). PEST was used to adjust 273 parameters, grouped as hydraulic conductivity of the aquifer, spatial multipliers to recharge, temporal multipliers to recharge, and two specific recharge parameters. Base flow of the North Platte River at Bridgeport, Nebraska, streamgage near the eastern, downstream end of the model was one of the primary calibration targets. Simulated base flow reasonably matched estimated base flow for this streamgage during 1950–2008, with an average difference of 15 percent. Overall, 1950–2008 simulated base flow followed the trend of the estimated base flow reasonably well, in cases with generally increasing or decreasing base flow from the start of the simulation to the end. Simulated base flow also matched estimated base flow reasonably well for most of the North Platte River tributaries with estimated base flow. Average simulated groundwater budgets during 1989–2008 were nearly three times larger for irrigation seasons than for non-irrigation seasons. The calibrated groundwater-flow model was used with the Groundwater-Management Process for the 2005 version of the U.S. Geological Survey modular three-dimensional groundwater model, MODFLOW–2005, to provide a tool for the NPNRD to better understand how water-management decisions could affect stream base flows of the North Platte River at Bridgeport, Nebr., streamgage in a future period from 2008 to 2019 under varying climatic conditions. The simulation-optimization model was constructed to analyze the maximum increase in simulated stream base flow that could be obtained with the minimum amount of reductions in groundwater withdrawals for irrigation. A second analysis extended the first to analyze the simulated base-flow benefit of groundwater withdrawals along with application of intentional recharge, that is, water from canals being released into rangeland areas with sandy soils. With optimized groundwater withdrawals and intentional recharge, the maximum simulated stream base flow was 15–23 cubic feet per second (ft 3 /s) greater than with no management at all, or 10–15 ft 3 /s larger than with managed groundwater withdrawals only. These results indicate not only the amount that simulated stream base flow can be increased by these management options, but also the locations where the management options provide the most or least benefit to the simulated stream base flow. For the analyses in this report, simulated base flow was best optimized by reductions in groundwater withdrawals north of the North Platte River and in the western half of the area. Intentional recharge sites selected by the optimization had a complex distribution but were more likely to be closer to the North Platte River or its tributaries. Future users of the simulation-optimization model will be able to modify the input files as to type, location, and timing of constraints, decision variables of groundwater withdrawals by zone, and other variables to explore other feasible management scenarios that may yield different increases in simulated future base flow of the North Platte River.

Nebraska↗

USGS2016

Effects of water-supply reservoirs on streamflow in Massachusetts

State and local water-resource managers need modeling tools to help them manage and protect water-supply resources for both human consumption and ecological needs. The U.S. Geological Survey, in cooperation with the Massachusetts Department of Environmental Protection, has developed a decision-support tool to estimate the effects of reservoirs on natural streamflow. The Massachusetts Reservoir Simulation Tool is a model that simulates the daily water balance of a reservoir. The reservoir simulation tool provides estimates of daily outflows from reservoirs and compares the frequency, duration, and magnitude of the volume of outflows from reservoirs with estimates of the unaltered streamflow that would occur if no dam were present. This tool will help environmental managers understand the complex interactions and tradeoffs between water withdrawals, reservoir operational practices, and reservoir outflows needed for aquatic habitats. A sensitivity analysis of the daily water balance equation was performed to identify physical and operational features of reservoirs that could have the greatest effect on reservoir outflows. For the purpose of this report, uncontrolled releases of water (spills or spillage) over the reservoir spillway were considered to be a proxy for reservoir outflows directly below the dam. The ratio of average withdrawals to the average inflows had the largest effect on spillage patterns, with the highest withdrawals leading to the lowest spillage. The size of the surface area relative to the drainage area of the reservoir also had an effect on spillage; reservoirs with large surface areas have high evaporation rates during the summer, which can contribute to frequent and long periods without spillage, even in the absence of water withdrawals. Other reservoir characteristics, such as variability of inflows, groundwater interactions, and seasonal demand patterns, had low to moderate effects on the frequency, duration, and magnitude of spillage. The reservoir simulation tool was used to simulate 35 single- and multiple-reservoir systems in Massachusetts over a 44-year period (water years 1961 to 2004) under two water-use scenarios. The no-pumping scenario assumes no water withdrawal pumping, and the pumping scenario incorporates average annual pumping rates from 2000 to 2004. By comparing the results of the two scenarios, the total streamflow alteration can be parsed into the portion of streamflow alteration caused by the presence of a reservoir and the additional streamflow alteration caused by the level of water use of the system. For each reservoir system, the following metrics were computed to characterize the frequency, duration, and magnitude of reservoir outflow volumes compared with unaltered streamflow conditions: (1) the median number of days per year in which the reservoir did not spill, (2) the median duration of the longest consecutive period of no-spill days per year, and (3) the lowest annual flow duration exceedance probability at which the outflows are significantly different from estimated unaltered streamflow at the 95-percent confidence level. Most reservoirs in the study do not spill during the summer months even under no-pumping conditions. The median number of days during which there was no spillage was less than 365 for all reservoirs in the study, indicating that, even under reported pumping conditions, the reservoirs refill to full volume and spill at least once during nondrought years, typically in the spring. Thirteen multiple-reservoir systems consisting of two or three hydrologically connected reservoirs were included in the study. Because operating rules used to manage multiple-reservoir systems are not available, these systems were simulated under two pumping scenarios, one in which water transfers between reservoirs are minimal and one in which reservoirs continually transferred water to intermediate or terminal reservoirs. These two scenarios provided upper and lower estimates of spillage under average pumping conditions from 2000 to 2004. For sites with insufficient data to simulate daily water balances, a proxy method to estimate the three spillage metrics was developed. A series of 4,000 Monte Carlo simulations of the reservoir water balance were run. In each simulation, streamflow, physical reservoir characteristics, and daily climate inputs were randomly varied. Tobit regression equations that quantify the relation between streamflow alteration and physical and operational characteristics of reservoirs were developed from the results of the Monte Carlo simulations and can be used to estimate each of the three spillage metrics using only the withdrawal ratio and the ratio of the surface area to the drainage area, which are available statewide for all reservoirs. A graphical user-interface for the Massachusetts Reservoir Simulation Tool was developed in a Microsoft Access environment. The simulation tool contains information for 70 reservoirs in Massachusetts and allows for simulation of additional scenarios than the ones considered in this report, including controlled releases, dam seepage and leakage, demand management plans, and alternative water withdrawal and transfer rules.

Massachusetts↗

USGS2017

Sequence stratigraphy, seismic stratigraphy, and seismic structures of the lower intermediate confining unit and most of the Floridan aquifer system, Broward County, Florida

Deep well injection and disposal of treated wastewater into the highly transmissive saline Boulder Zone in the lower part of the Floridan aquifer system began in 1971. The zone of injection is a highly transmissive hydrogeologic unit, the Boulder Zone, in the lower part of the Floridan aquifer system. Since the 1990s, however, treated wastewater injection into the Boulder Zone in southeastern Florida has been detected at three treated wastewater injection utilities in the brackish upper part of the Floridan aquifer system designated for potential use as drinking water. At a time when usage of the Boulder Zone for treated wastewater disposal is increasing and the utilization of the upper part of the Floridan aquifer system for drinking water is intensifying, there is an urgency to understand the nature of cross-formational fluid flow and identify possible fluid pathways from the lower to upper zones of the Floridan aquifer system. To better understand the hydrogeologic controls on groundwater movement through the Floridan aquifer system in southeastern Florida, the U.S. Geological Survey and the Broward County Environmental Planning and Community Resilience Division conducted a 3.5-year cooperative study from July 2012 to December 2015. The study characterizes the sequence stratigraphy, seismic stratigraphy, and seismic structures of the lower part of the intermediate confining unit aquifer and most of the Floridan aquifer system. Data obtained to meet the study objective include 80 miles of high-resolution, two-dimensional (2D), seismic-reflection profiles acquired from canals in eastern Broward County. These profiles have been used to characterize the sequence stratigraphy, seismic stratigraphy, and seismic structures in a 425-square-mile study area. Horizon mapping of the seismic-reflection profiles and additional data collection from well logs and cores or cuttings from 44 wells were focused on construction of three-dimensional (3D) visualizations of eight sequence stratigraphic cycles that compose the Eocene to Miocene Oldsmar, Avon Park, and Arcadia Formations. The mapping of these seismic-reflection and well data has produced a refined Cenozoic sequence stratigraphic, seismic stratigraphic, and hydrogeologic framework of southeastern Florida. The upward transition from the Oldsmar Formation to the Avon Park Formation and the Arcadia Formation embodies the evolution from (1) a tropical to subtropical, shallow-marine, carbonate platform, represented by the Oldsmar and Avon Park Formations, to (2) a broad, temperate, mixed carbonate-siliciclastic shallow marine shelf, represented by the lower part of the Arcadia Formation, and to (3) a temperate, distally steepened carbonate ramp represented by the upper part of the Arcadia Formation. In the study area, the depositional sequences and seismic sequences have a direct correlation with hydrogeologic units. The approximate upper boundary of four principal permeable units of the Floridan aquifer system (Upper Floridan aquifer, Avon Park permeable zone, uppermost major permeable zone of the Lower Floridan aquifer, and Boulder Zone) have sequence stratigraphic and seismic-reflection signatures that were identified on cross sections, mapped, or both, and therefore the sequence stratigraphy and seismic stratigraphy were used to guide the development of a refined spatial representation of these hydrogeologic units. In all cases, the permeability of the four permeable units is related to stratiform megaporosity generated by ancient dissolution of carbonate rock associated with subaerial exposure and unconformities at the upper surfaces of carbonate depositional cycles of several hierarchical scales ranging from high-frequency cycles to depositional sequences. Additionally, interparticle porosity also contributes substantially to the stratiform permeability in much of the Upper Floridan aquifer. Information from seismic stratigraphy allowed 3D geomodeling of hydrogeologic units—an approach never before applied to this area. Notably, the 3D geomodeling provided 3D visualizations and geocellular models of the depositional sequences, hydrostratigraphy, and structural features. The geocellular data could be used to update the hydrogeologic structure inherent to groundwater flow simulations that are designed to address the sustainability of the water resources of the Floridan aquifer system. Two kinds of pathways that could enable upward cross-formational flow of injected treated wastewater from the Boulder Zone have been identified in the 80 miles of high-resolution seismic data collected for this study: a near-vertical reverse fault and karst collapse structures. The single reverse fault, inferred to be of tectonic origin, is in extreme northeastern Broward County and has an offset of about 19 feet at the level of the Arcadia Formation. Most of the 17 karst collapse structures identified manifest as columniform, vertically stacked sagging seismic reflections that span early Eocene to Miocene age rocks equivalent to much of the Floridan aquifer system and the lower part of the overlying intermediate confining unit. In some cases, the seismic-sag structures extend upward into strata of Pliocene age. The seismic-sag structures are interpreted to have a semicircular shape in plan view on the basis of comparison to (1) other seismic-sag structures in southeastern Florida mapped with two 2D seismic cross lines or 3D data, (2) comparison to these structures located in other carbonate provinces, and (3) plausible extensional ring faults detected with multi-attribute analysis. The seismic-sag structures in the study area have heights as great as 2,500 vertical feet, though importantly, one spans about 7,800 feet. Both multi-attribute analysis and visual detection of offset of seismic reflections within the seismic-sag structures indicate faults and fractures are associated with many of the structures. Multi-attribute analysis highlighting chimney fluid pathways also indicates that the seismic-sag structures have a high probability for potential vertical cross-formational fluid flow along the faulted and fractured structures. A collapse of the seismic-sag structures within a deep burial setting evokes an origin related to hypogenic karst processes by ascending flow of subsurface fluids. In addition, paleo-epigenic karst related to major regional subaerial unconformities within the Florida Platform generated collapse structures (paleo-sinkholes) that are much smaller in scale than the cross-formational seismic-sag structures.

Florida↗

USGS2011

Status and understanding of groundwater quality in the San Diego Drainages Hydrogeologic Province, 2004: California GAMA Priority Basin Project

Groundwater quality in the approximately 3,900-square-mile (mi 2 ) San Diego Drainages Hydrogeologic Province (hereinafter San Diego) study unit was investigated from May through July 2004 as part of the Priority Basin Project of the Groundwater Ambient Monitoring and Assessment (GAMA) Program. The study unit is located in southwestern California in the counties of San Diego, Riverside, and Orange. The GAMA Priority Basin Project is being conducted by the California State Water Resources Control Board in collaboration with the U.S. Geological Survey (USGS) and the Lawrence Livermore National Laboratory. The GAMA San Diego study was designed to provide a statistically robust assessment of untreated-groundwater quality within the primary aquifer systems. The assessment is based on water-quality and ancillary data collected by the USGS from 58 wells in 2004 and water-quality data from the California Department of Public Health (CDPH) database. The primary aquifer systems (hereinafter referred to as the primary aquifers) were defined by the depth interval of the wells listed in the California Department of Public Health (CDPH) database for the San Diego study unit. The San Diego study unit consisted of four study areas: Temecula Valley (140 mi 2 ), Warner Valley (34 mi 2 ), Alluvial Basins (166 mi 2 ), and Hard Rock (850 mi 2 ). The quality of groundwater in shallow or deep water-bearing zones may differ from that in the primary aquifers. For example, shallow groundwater may be more vulnerable to surficial contamination than groundwater in deep water-bearing zones. This study had two components: the status assessment and the understanding assessment. The first component of this study-the status assessment of the current quality of the groundwater resource-was assessed by using data from samples analyzed for volatile organic compounds (VOC), pesticides, and naturally occurring inorganic constituents, such as major ions and trace elements. The status assessment is intended to characterize the quality of groundwater resources within the primary aquifers of the San Diego study unit, not the treated drinking water delivered to consumers by water purveyors. The second component of this study-the understanding assessment-identified the natural and human factors that affect groundwater quality by evaluating land use, well construction, and geochemical conditions of the aquifer. Results from these evaluations were used to help explain the occurrence and distribution of selected constituents in the study unit. Relative-concentrations (sample concentration divided by benchmark concentration) were used as the primary metric for relating concentrations of constituents in groundwater samples to water-quality benchmarks for those constituents that have Federal and (or) California benchmarks. For organic and special-interest constituents, relative-concentrations were classified as high (> 1.0), moderate (> 0.1 and ≤1.0), and low (≤0.1). For inorganic constituents, relative concentrations were classified as high (> 1.0), moderate (> 0.5 and ≤1.0), and low (≤0.5). Grid-based and spatially weighted approaches were then used to evaluate the proportion of the primary aquifers (aquifer-scale proportions) with high, moderate, and low relative-concentrations for individual compounds and classes of constituents. One or more of the inorganic constituents with health-based benchmarks were high (relative to those benchmarks) in 17.6 percent of the primary aquifers in the Temecula Valley, Warner Valley, and Alluvial Basins study areas (hereinafter also collectively referred to as the Alluvial Fill study areas because they are composed of alluvial fill aquifers), and in 25.0 percent of the Hard Rock study area. Inorganic constituents with health-based benchmarks that were frequently detected at high relative-concentrations included vanadium (V), arsenic (As), and boron (B). Vanadium and As concentrations were not significantly correlated to either urban or agricultural land use indicating natural sources as the primary contributors of these constituents to groundwater. The positive correlation of B concentration to urban land-use was significant which indicates that anthropogenic activities are a contributing source of B to groundwater. The correlation of V, As and B concentrations to pH was positive, indicating that in alkaline groundwater these constituents are being desorbed from, or being inhibited from adsorbing to, particle surfaces. Inorganic constituents with aesthetic benchmarks that were detected at high relative-concentrations include manganese (Mn), iron (Fe), and total dissolved solids (TDS). In the Alluvial Fill study areas, Mn and TDS were detected at high relative-concentrations in 13.7 percent of the primary aquifers, and Fe in 6.9 percent. In the Hard Rock study area, Mn was detected at high relative-concentrations in 33.3 percent of the primary aquifers, and TDS in 16.7 percent; Fe was not detected at high relative-concentrations. Total dissolved solids concentrations were significantly correlated to agricultural land use suggesting that agricultural practices are a contributing source of TDS to groundwater. Manganese and Fe concentrations were highest in groundwater with low dissolved oxygen and pH indicating that the reductive dissolution of oxyhydroxides may be an important mechanism for the mobilization of Mn and Fe in groundwater. TDS concentrations were highest in shallow wells and in modern (< 50 yrs) groundwater which indicates anthropogenic activities as a source of TDS concentrations in groundwater. The relative-concentrations of organic constituents with health-based benchmarks were high in 3.0 percent of the primary aquifers in the Alluvial Fill study areas. A single detection in the Alluvial Basins study area of the discontinued gasoline oxygenate methyl tert-butyl ether (MTBE) was the only organic constituent detected at a high relative-concentration; high relative-concentrations of these constituents were not detected in the Hard Rock study area. Twelve of 88 VOCs and 14 of 123 pesticides and pesticide degradates analyzed in grid wells were detected. Chloroform was the only VOC detected in more than 10 percent of the grid wells. The herbicides simazine, atrazine, and prometon were each detected in greater than 10 percent of the grid wells. Perchlorate was detected in 22 percent of the grid wells sampled. The understanding assessment showed a significant correlation of trihalomethanes (THMs) and solvents to urban land-use, indicating that detections of these constituents are more likely to occur in groundwater underlying urbanized areas of the study unit. MTBE concentrations were negatively correlated to the distance from the nearest leaking underground fuel tank, indicating that point sources are the most significant contributing factor for MTBE concentrations to groundwater in the study unit. The positive correlation of THM and herbicide concentrations to modern groundwater was significant, as was the negative correlation of herbicide concentrations to pH and anoxic groundwater. The negative correlation of herbicides to pH and anoxic groundwater was likely due to the fact that these constituents were detected more frequently in shallow wells where groundwater conditions tend to be oxic with relatively low pH.

California↗

USGS2014

Status and understanding of groundwater quality in the Klamath Mountains study unit, 2010: California GAMA Priority Basin Project

Groundwater quality in the Klamath Mountains (KLAM) study unit was investigated as part of the Priority Basin Project of the California Groundwater Ambient Monitoring and Assessment (GAMA) Program. The study unit is located in Del Norte, Humboldt, Shasta, Siskiyou, Tehama, and Trinity Counties. The GAMA Priority Basin Project is being conducted by the California State Water Resources Control Board in collaboration with the U.S. Geological Survey (USGS) and the Lawrence Livermore National Laboratory. The GAMA Priority Basin Project was designed to provide a spatially unbiased, statistically robust assessment of the quality of untreated (raw) groundwater in the primary aquifer system. The assessment is based on water-quality data and explanatory factors for groundwater samples collected in 2010 by the USGS from 39 sites and on water-quality data from the California Department of Public Health (CDPH) water-quality database. The primary aquifer system was defined by the depth intervals of the wells listed in the CDPH water-quality database for the KLAM study unit. The quality of groundwater in the primary aquifer system may be different from that in the shallower or deeper water-bearing zones; shallow groundwater may be more vulnerable to surficial contamination. This study included two types of assessments: (1) a status assessment , which characterized the status of the current quality of the groundwater resource by using data from samples analyzed for volatile organic compounds, pesticides, and naturally occurring inorganic constituents, such as major ions and trace elements, and (2) an understanding assessment , which evaluated the natural and human factors potentially affecting the groundwater quality. The assessments were intended to characterize the quality of groundwater resources in the primary aquifer system of the KLAM study unit, not the quality of treated drinking water delivered to consumers by water purveyors. Relative-concentrations (sample concentrations divided by the health- or aesthetic-based benchmark concentrations) were used for evaluating groundwater quality for those constituents that have Federal or California regulatory or non-regulatory benchmarks for drinking-water quality. A relative-concentration greater than (>) 1.0 indicates a concentration greater than a benchmark, and a relative-concentration less than or equal to (≤) 1.0 indicates a concentration less than or equal to a benchmark. Relative-concentrations of organic constituents were classified as “high” (relative-concentration > 1.0), “moderate” (0.1 < relative-concentration ≤ 1.0), or “low” (relative-concentration ≤ 0.1). For inorganic constituents, the boundary between low and moderate relative-concentration was set at 0.5. Aquifer-scale proportion was used in the status assessment as the primary metric for evaluating regional-scale groundwater quality. High aquifer-scale proportion is defined as the percentage of the area of the primary aquifer system with a relative-concentration greater than 1.0 for a particular constituent or class of constituents; percentage is based on an areal rather than a volumetric basis. Moderate and low aquifer-scale proportions were defined as the percentages of the primary aquifer system with moderate and low relative-concentrations, respectively. The KLAM study unit includes more than 8,800 square miles (mi 2 ), but only those areas near the sampling sites, about 920 mi 2 , are included in the areal assessment of the study unit. Two statistical approaches—grid-based and spatially weighted—were used to evaluate aquifer-scale proportions for individual constituents and classes of constituents. To confirm this methodology, 90 percent confidence intervals were calculated for the grid-based high aquifer-scale proportions and were compared to the spatially weighted results, which were found to be within these confidence intervals in all cases. Grid-based results were selected for use in the status assessment unless, as was observed in a few cases, a grid-based result was zero and the spatially weighted result was not zero, in which case, the spatially weighted result was used. The status assessment showed that inorganic constituents with human-health benchmarks were detected at high relative-concentrations in 2.6 percent of the primary aquifer system and at moderate relative-concentrations in 10 percent of the system. The high aquifer-scale proportion for inorganic constituents mainly reflected the high aquifer-scale proportions of boron. Inorganic constituents with secondary maximum contaminant levels were detected at high relative-concentrations in 13 percent of the primary aquifer system and at moderate relative-concentrations in 10 percent of the system. The constituents present at high relative-concentrations included iron and manganese. Organic constituents with human-health benchmarks were not detected at high relative-concentrations, but were detected at moderate relative-concentrations in 1.9 percent of the primary aquifer system. The 1.9 percent reflected a spatially weighted moderate aquifer-scale proportion for the gasoline additive methyl tert-butyl ether. Of the 148 organic constituents analyzed, 14 constituents were detected. Only one organic constituent had a detection frequency of greater than 10 percent—the trihalomethane, chloroform. The second component of this study, the understanding assessment , identified the natural and human factors that may have affected the groundwater quality in the KLAM study unit by evaluating statistical correlations between water-quality constituents and potential explanatory factors. The potential explanatory factors evaluated were aquifer lithology, land use, hydrologic conditions, depth, groundwater age, and geochemical conditions. Results of the statistical evaluations were used to explain the occurrence and distribution of constituents in the KLAM study unit. Groundwater age distribution (modern, mixed, or pre-modern), redox class (oxic, mixed, or anoxic), and dissolved oxygen concentration were the explanatory factors that best explained occurrence patterns of the inorganic constituents. High concentrations of boron were found to be associated with groundwater classified as mixed or pre-modern with respect to groundwater age. Boron was also negatively correlated to dissolved oxygen and positively correlated to specific conductance. Iron and manganese concentrations were strongly associated with low dissolved oxygen concentrations, anoxic and mixed redox classifications, and pre-modern groundwater. Specific conductance concentrations were found to be related to pre-modern groundwater, low dissolved oxygen concentrations, and high pH. Chloroform was selected for additional evaluation in the understanding assessment because it was detected in more than 10 percent of wells sampled in the KLAM study unit. Septic tank density was the only explanatory factor that was found to relate to chloroform concentrations.

California↗

USGS2013

California State Waters Map Series: Offshore of Santa Barbara, California

In 2007, the California Ocean Protection Council initiated the California Seafloor Mapping Program (CSMP), designed to create a comprehensive seafloor map of high-resolution bathymetry, marine benthic habitats, and geology within the 3-nautical-mile limit of California’s State Waters. The CSMP approach is to create highly detailed seafloor maps through collection, integration, interpretation, and visualization of swath sonar data, acoustic backscatter, seafloor video, seafloor photography, high-resolution seismic-reflection profiles, and bottom-sediment sampling data. The map products display seafloor morphology and character, identify potential marine benthic habitats, and illustrate both the surficial seafloor geology and shallow (to about 100 m) subsurface geology. The Offshore of Santa Barbara map area lies within the central Santa Barbara Channel region of the Southern California Bight. This geologically complex region forms a major biogeographic transition zone, separating the cold-temperate Oregonian province north of Point Conception from the warm-temperate California province to the south. The map area is in the southern part of the Western Transverse Ranges geologic province, which is north of the California Continental Borderland. Significant clockwise rotation—at least 90°—since the early Miocene has been proposed for the Western Transverse Ranges province, and geodetic studies indicate that the region is presently undergoing north-south shortening. Uplift rates (as much as 2.2 mm/yr) that are based on studies of onland marine terraces provide further evidence of significant shortening. The city of Santa Barbara, the main coastal population center in the map area, is part of a contiguous urban area that extends from Carpinteria to Goleta. This urban area was developed on the coalescing alluvial surfaces, uplifted marine terraces, and low hills that lie south of the east-west-trending Santa Ynez Mountains. Several beaches line the actively utilized Santa Barbara coastal zone, including Arroyo Burro Beach Park, Leadbetter Beach, East Beach, and “Butterfly Beach.” There are ongoing coastal erosion problems associated with both development and natural processes; between 1933–1934 and 1998, cliff erosion in the map area occurred at rates of about 0.1 to 1 m/yr, the largest amount (63 m) occurring at Arroyo Burro in the western part of the map area. In addition, development of the Santa Barbara Harbor, which began in 1928, lead to shoaling west of the harbor as the initial breakwater trapped sand, as well as to coastal erosion east of the harbor. Since 1959, annual harbor dredging has mitigated at least some of the downcoast erosion problems. The Offshore of Santa Barbara map area lies in the central part of the Santa Barbara littoral cell, which is characterized by littoral drift to the east-southeast. Drift rates have been estimated to be about 400,000 tons/yr at Santa Barbara Harbor. Sediment supply to the western and central parts of the littoral cell, including the map area, is largely from relatively small transverse coastal watersheds. Within the map area, these coastal watersheds include (from east to west) San Ysidro Creek, Oak Creek, Montecito Creek, Sycamore Creek, Mission Creek, Arroyo Burro, and Atascadero Creek. The Ventura and Santa Clara Rivers, the mouths of which are about 40 to 50 km southeast of Santa Barbara, are much larger sediment sources. Still farther east, eastward-moving sediment in the littoral cell is trapped by Hueneme and Mugu Canyons and then transported to the deep-water Santa Monica Basin. The offshore part of the map area consists of a relatively flat and shallow continental shelf, which dips gently seaward (about 0.4° to 0.8°) so that water depths at the 3-nautical-mile limit of California’s State Waters are about 45 m in the east and about 75 m in the west. This part of the Santa Barbara Channel is relatively well protected from large Pacific swells from the north and northwest by Point Conception and from the south and southwest by offshore islands and banks. The shelf is underlain by variable amounts of upper Quaternary shelf, estuarine, and fluvial sediments deposited as sea level fluctuated in the late Pleistocene. Seafloor habitats in the broad Santa Barbara Channel region consist of significant amounts of soft sediment and isolated areas of rocky habitat that support kelp-forest communities nearshore and rocky-reef communities in deep water. The potential marine benthic habitat types mapped in the Offshore of Santa Barbara map area are directly related to its Quaternary geologic history, geomorphology, and active sedimentary processes. These potential habitats, which lie within the Shelf (continental shelf) megahabitat, range from soft, unconsolidated sediment to hard sedimentary bedrock. This heterogeneous seafloor provides promising habitat for rockfish, groundfish, crabs, shrimp, and other marine benthic organisms.

California↗

USGS2019

Determination of study reporting limits for pesticide constituent data for the California Groundwater Ambient Monitoring and Assessment Program Priority Basin Project, 2004–2018—Part 1: National Water Quality Schedules 2003, 2032, or 2033, and 2060

The California Groundwater Ambient Monitoring and Assessment Program Priority Basin Project (GAMA-PBP) is a long-term cooperative project designed to assess the quality of groundwater resources used for public and domestic drinking water supplies in the State of California, to monitor and evaluate changes to that quality, to investigate the human and natural factors controlling water quality, and to improve the availability of comprehensive groundwater quality data and information. Between May 18, 2004, and May 3, 2018, the GAMA-PBP collected 3001 groundwater samples for analysis of pesticide constituents by the U.S. Geological Survey (USGS) National Water Quality Laboratory (NWQL)(note that ‘pesticide constituents’ includes parent compounds and degradates). Of these samples, 2994 were analyzed for pesticide constituents on schedules 2003, 2032, or 2033 (65 to 84 constituents), and 840 were analyzed for pesticide constituents on schedule 2060 (58 constituents). The original dataset reported by the NWQL to the USGS National Water Information System (NWIS) database contained a total of 2,688 detections of 78 pesticide constituents and 253,825 non-detections. In this original dataset, 33 percent of the 3,001 samples analyzed had reported detections of one or more pesticide constituents. This report describes the GAMA-PBP data-quality objectives for pesticide data, the procedures used to establish study reporting limits, and use of those reporting limits to censor the data from the NWQL so that the final data published by the GAMA-PBP meet these data-quality objectives. The final GAMA-PBP dataset for samples collected from May 2004 to May 2018, after censoring, had a total of 1,632 detections of 37 pesticide constituents. In the final GAMA-PBP dataset, 25 percent of the 3,001 samples analyzed had detections of one or more pesticide constituents. The presence of pesticides in groundwater is commonly evaluated by calculating detection frequencies. Detection frequencies for pesticides are sensitive to detection limits and method performance for concentrations near those limits; therefore, the two primary data quality issues addressed in the GAMA-PBP data-quality objectives for pesticides are (1) establishing criteria for classifying data from the laboratory as detections or non-detections for the purpose of data reporting by the project and (2) accounting for changes in analytical methods or method performance over time. The GAMA-PBP addresses these issues by developing study reporting limits that are used as the boundary between detections and non-detections for the reporting of GAMA-PBP results. These reporting limits are defined from method detection limits (MDLs) provided by the NWQL, unless examination of results from laboratory set blanks (LSBs) and GAMA-PBP field blanks indicates that a higher concentration censoring limit is warranted. The GAMA-PBP selected the MDL as the primary choice for defining study reporting limits for consistency with U.S. Environmental Protection Agency (EPA) guidelines for reporting detections of pesticides and other organic constituents. A five-step procedure is used to develop study reporting limits and censor the GAMA-PBP dataset accordingly. The effect of the censoring at each step is described to provide information about the relative effect of each step on the overall censoring of the dataset. Steps 1 and 2 can be implemented at the time the data are received, whereas steps 3−5 require information accumulated over an extended period. Step 1: Reject results that were most likely the result of specific contamination instances attributable to unusual field or laboratory conditions during sample collection or processing. Two such instances were identified, leading to rejection of 25 detections, which were assigned a data-quality indicator code of “Q” for “reviewed and rejected” in the NWIS database. Step 2: Use the NWQL MDLs in effect at the time each sample was analyzed as the reporting limit. A total of 506 detections were censored on this basis. Step 3: Use the maximum MDL established by the NWQL during July 2004–August 2018 (MDLmax) as the reporting limit. The rationale for using the MDLmax as the reporting limit is based primarily on the observation that the concentrations of MDLs generally increased over time. A total of 438 detections were censored on this basis. Step 4: Use the LSBs to identify periods of greater potential laboratory contamination bias and define raised reporting limits to be used during those periods. These periods were defined by using a moving average detection frequency approach. For consistency with the NWQL procedures for defining raised reporting limits on the basis of detections in LSBs, the raised reporting limits were defined as equal to three times the highest concentration measured in an LSB during the period. A total of 25 detections in groundwater samples analyzed during periods of increased laboratory contamination bias were censored. Step 5: Use the LSBs and field blanks to identify potential contamination bias from field or laboratory processes outside of the time periods identified in step 4. The NWQL protocols were used to define the MDLs from blanks analyzed outside of the periods identified in step 4. If an MDL defined from blanks was greater than the MDLmax, the MDL defined from blanks was used to censor the data. One constituent had a study reporting limit defined on this basis, and a total of 62 detections in groundwater samples were censored. As of 2019, the USGS NWIS database does not have the capability to store both the original value reported by the NWQL and the final value published by the GAMA-PBP that reflects application of the quality-control censoring described in this report. In the interim, while this capability is developed, the 1,031 results censored in steps 2−5 are blocked from public release in NWIS, and the GAMA-PBP has published the original and final values in a USGS data release accompanying this report. The entire GAMA-PBP final dataset for pesticide constituents on schedules 2003, 2032, or 2033, or on schedule 2060 is publicly available in that USGS data release, through the USGS GAMA-PBP public web portal, and through the California State Water Resources Control Board GAMA public groundwater information system.

California↗

USGS2016

Regional chloride distribution in the Northern Atlantic Coastal Plain aquifer system from Long Island, New York, to North Carolina

The aquifers of the Northern Atlantic Coastal Plain are the principal source of water supply for the region’s nearly 20 million residents. Water quality and water levels in the aquifers, and maintenance of streamflow, are of concern because of the use of this natural resource for water supply and because of the possible effects of climate change and changes in land use on groundwater. The long-term sustainability of this natural resource is a concern at the local community scale, as well as at a regional scale, across state boundaries. In 2010, the U.S. Geological Survey (USGS) began a regional assessment of the Northern Atlantic Coastal Plain aquifers. An important part of this assessment is a regional interpretation of the extent of saltwater and the proximity of saltwater to fresh-groundwater resources and includes samples and published interpretations of chloride concentrations newly available since the last regional chloride assessment in 1989. This updated assessment also includes consideration of chloride samples and refined interpretations that stem from the 1994 discovery of the buried 35 million year old Chesapeake Bay impact structure that has substantially altered the understanding of the hydrogeologic framework and saltwater distribution in eastern Virginia. In this study, the regional area of concern for the chloride samples and interpretations extends from the Fall Line in the west to the outer edge of the Continental Shelf in the east and from the eastern tip of Long Island in the north to about halfway down the North Carolina coast in the south. Discussions of chloride distribution are presented for each of the 10 regional aquifer layers of the Northern Atlantic Coastal Plain, including the offshore extents. Maps of interpreted lines of equal concentration or isochlors were manually prepared for nine of the regional aquifers; a map was not prepared for the surficial regional aquifer. The isochlor interpretations include the offshore extent of the nine regional aquifers and are presented on a 1:2,000,000 scale base map. Vertically, the chloride samples and interpretations range from deepest (oldest) to shallowest (youngest)—Potomac-Patuxent, Potomac-Patapsco, Magothy, Matawan, Monmouth-Mount Laurel, Aquia, Piney Point, Lower Chesapeake, and Upper Chesapeake regional aquifers. The approach of this study maximizes the overall density of chloride information and data by assessing relevant published interpretations, all USGS chloride samples, and all relevant offshore samples in one comprehensive interpretation. Published isochlors, where they were interpreted by regional aquifer, were used as much as possible for this regional isochlor assessment. Publication dates for the isochlors used range from 1982 to 2015, and the scales for the isochlors range from local (county or municipality) to state (sub-regional) to regional. The USGS National Water Information System database provided well sample data for the parts of aquifers that are mainly beneath the land areas and yielded 37,517 water-quality records for 1903 through 2011. Published data reports from four phases of research-related offshore coring (1976, 1993, 1997, 2009) were the main source of water-quality data for the parts of aquifers from the shoreline to the outer edge of the Continental Shelf and yielded samples from multiple depths of each of 13 cores. This study also used interpretations and offshore core data from the last regional chloride assessment (1989) which, in addition to 7 offshore cores, included water-quality data from about 500 wells, and borehole geophysics interpretations from a subset of 11 wells. All published information and data that were used in this study were considered time independent and did not assess the published interpretations or data for temporal trends. The approach used here examined only published interpretations and available chloride data, and did not directly use supplemental techniques that can provide insight into the distribution of saltwater, such as geochemical characterization, borehole geophysical information, and geochronology. Isochlor maps for this study are limited to manual interpretations of the 250-milligram per liter (mg/L) and 10,000-mg/L boundaries developed for 9 of the 10 regional aquifers that constitute the regional hydrogeologic framework of the Northern Atlantic Coastal Plain. For a given aquifer, the approach was to initially consider published isochlor interpretations, where available, then to modify the published interpretations, if necessary, to the extent indicated by the well and core samples. The final step was to interpolate isochlors to the full extent of each aquifer layer in areas with sufficient samples or cited interpretations, or to extrapolate isochlors in areas with no samples or where samples were sparse. The principal limitation of this study is that, because of its regional extent, data and information density can vary greatly, and thus confidence in interpretations can vary widely for onshore and offshore areas across the study area. In areas of sparse data, some samples of elevated chloride could be misinterpreted as being part of a regional elevated chloride trend, and in other cases, an elevated concentration could be misinterpreted as being of only local importance. The interpretive work of this study was applied to a 1:2,000,000 scale base map. Locations of isochlors, wells, cores, political boundaries, and shorelines are meant to be considered approximate. The isochlors presented in this study were manually interpreted for each aquifer unit as a conceptual representation of an equal concentration line approximately in the middle of an aquifer’s thickness. Differences in chloride concentration lines between the top and bottom of an aquifer could be substantial, especially for the thick parts of aquifers, but that information is not presented in this regional assessment. Although additional offshore chloride data are available compared to 27 years ago (1989), the offshore information remains sparse, resulting in less confidence in the offshore interpretations than in the onshore interpretations. Regionally, the 250- and 10,000-mg/L isochlors tend to map progressively eastward from the deepest to the shallowest aquifers across the Northern Atlantic Coastal Plain aquifer system but with some exceptions. The additional data, conceptual understanding, and interpretations in the vicinity of the buried Chesapeake Bay impact structure in eastern Virginia resulted in substantial refinement of isochlors in that area. Overall, the interpretations in this study are updates of the previous regional study from 1989 but do not comprise major differences in interpretation and do not indicate regional movement of the freshwater-saltwater interface since then.

Delaware, Maryland, New Jersey, New York, North Ca↗

USGS2010

Occurrence and distribution of organic chemicals and nutrients and comparison of water-quality data from public drinking-water supplies in the Columbia aquifer in Delaware, 2000-08

The U.S. Geological Survey, in cooperation with the Delaware Department of Natural Resources and Environmental Control and the Delaware Geological Survey, conducted a groundwater-quality investigation to (a) describe the occurrence and distribution of selected contaminants, and (b) document any changes in groundwater quality in the Columbia aquifer public water-supply wells in the Coastal Plain in Delaware between 2000 and 2008. Thirty public water-supply wells located throughout the Columbia aquifer of the Delaware Coastal Plain were sampled from August through November of 2008. Twenty-two of the wells in the sampling network for this project were previously sampled in 2000. Eight new wells were selected to replace wells no longer in use. Groundwater collected from the wells was analyzed for the occurrence and distribution of selected pesticides, pesticide degradates, volatile organic compounds, nutrients, and major inorganic ions. Nine of the wells were analyzed for radioactive elements (radium-226, radium-228, and radon). Groundwater-quality data were compared for sites sampled in both 2000 and 2008 to document any changes in water quality. One or more pesticides were detected in samples from 29 of the 30 wells. There were no significant differences in pesticide and pesticide degradate concentrations and similar compounds were detected when comparing sampling results from 2000 and 2008. Pesticide and pesticide degradate concentrations were generally less than 1 microgram per liter. Twenty-four compounds, 14 pesticides, and 10 pesticide degradates were detected in at least one sample; the pesticide degradates, metolachlor ethanesulfonic acid, deethylatrazine, and alachlor ethanesulfonic acid were the most frequently detected compounds, each found in more than 50 percent of samples. Almost 80 percent of the detected pesticides were agricultural herbicides, which reflects the prevalence and wide distribution of agriculture in sampled areas, as well the dominance of agricultural pesticides among the target analytes for this study. No concentration of a pesticide or pesticide degradate exceeded any regulatory standard. Dieldrin, an insecticide that has been banned for several decades, was detected at a concentration that exceeded a non-regulatory health-based screening level of 0.002 micrograms per liter at nine sites. Volatile organic compounds (VOCs) were generally detected at concentrations of less than 1 microgram per liter, although 7 of the 31 detected VOCs had concentrations greater than 1 microgram per liter. There were no significant differences in VOC concentrations from 2000 to 2008; however, among the resampled wells, the mean number of VOCs detected per well was significantly different over the 8-year period. The number of VOCs detected per well decreased in 73 percent of the resampled wells; the decrease ranged from one to eight fewer detections in 2008 than in 2000. Chloroform and methyl tert-butyl ether were the most frequently detected VOCs, at 90 percent and 63 percent, respectively, among the 30 wells. Solvents were the most frequently detected class of VOCs. All measured concentrations of VOCs in groundwater were below established standards for drinking water and below other health-based guidelines. There were no significant differences in nutrient or major-ion concentrations between 2000 and 2008, however, the medians of two field measurements, pH and dissolved oxygen, were significantly higher in 2008 than in 2000 in the resampled wells. Although pH and dissolved oxygen were higher, water was still acidic and predominantly oxic. Nitrate was the predominant nutrient species in the Columbia aquifer, with a 90-percent detection frequency. The median nitrate concentration in groundwater was 4.88 milligrams per liter, which was slightly lower than, but not significantly different from, the median of 5.23 milligrams per liter for the 2000 samples. Concentrations of nitrate exceeded the U.S. Environmental Protection Agency's Maximum Contaminant Level or Federal drinking-water standard of 10 milligrams per liter as nitrogen in samples from two wells. Eight of the 30 wells sampled had iron or manganese concentrations that exceeded the U.S. Environmental Protection Agency's Secondary Maximum Contaminant Level; nine samples exceeded the Health Advisory Limit set by the Delaware Division of Public Health of 20 milligrams per liter for sodium in drinking water. Two radiochemical isotopes, radium-226 and radon-222, were detected in all nine groundwater samples analyzed; five samples had detectable levels of radium-228 activity. None of the samples exceeded the U.S Environmental Protection Agency's Maximum Contaminant Level for radium or radon in drinking water. Although radioactive elements were more frequently detected in 2008 than in 2000, this increased detection frequency is more likely due to lower detection levels in 2008 than 2000. The average age of groundwater entering the screens of the production wells sampled in 2008 ranged from 6 to 35 years, with a median groundwater age of 22 years. Groundwater age was positively correlated with well depth and negatively correlated with dissolved oxygen. Data from the 22 resampled wells indicate a significant positive difference in the average modeled groundwater-sample-age results. The average groundwater age from samples collected in 2008 was generally 7 years older than the average groundwater age from samples collected in 2000.

Delaware↗

USGS2011

Hydrogeology, chemical characteristics, and water sources and pathways in the zone of contribution of a public-supply well in San Antonio, Texas

In 2001, the National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey initiated a series of studies on the transport of anthropogenic and natural contaminants (TANC) to public-supply wells (PSWs). The main goal of the TANC project was to better understand the source, transport, and receptor factors that control contaminant movement to PSWs in representative aquifers of the United States. Regional- and local-scale study areas were selected from within existing NAWQA study units, including the south-central Texas Edwards aquifer. The local-scale TANC study area, nested within the regional-scale NAWQA study area, is representative of the regional Edwards aquifer. The PSW selected for study is within a well field of six production wells. Although a single PSW was initially selected, because of constraints of well-field operation, samples were collected from different wells within the well field for different components of the study. Data collected from all of the well-field wells were considered comparable because of similar well construction, hydrogeology, and geochemistry. An additional 38 PSWs (mostly completed in the confined part of the aquifer) were sampled throughout the regional aquifer to characterize water quality. Two monitoring well clusters, with wells completed at different depths, were installed to the east and west of the well field (the Zarzamora and Timberhill monitoring well clusters, respectively). One of the monitoring wells was completed in the overburden to evaluate potential hydrologic connectivity with the Edwards aquifer. Geophysical and flowmeter logs were collected from one of the well-field PSWs to determine zones of contribution to the wellbore. These contributing zones, associated with different hydrogeologic units, were used to select monitoring well completion depths and groundwater sample collection depths for depth-dependent sampling. Depth-dependent samples were collected from the PSW from three different depths and under three different pumping conditions. Additionally, selected monitoring wells and one of the well-field PSWs were sampled several times in response to a rainfall and recharge event to assess short-term (event-scale) temporal variations in water quality. For comparison purposes, groundwater samples were categorized as being from regional aquifer PSWs, from the well field (wellhead samples), from the monitoring wells (excluding the overburden well), from the overburden well, from the PSW depth-dependent sampling, and from temporal sampling. Groundwater samples were analyzed for inorganic, organic, isotopic, and age-dating tracers to characterize geochemical conditions in the aquifer and provide understanding of the mechanisms of mobilization and movement of selected constituents from source areas to a PSW. Sources, tracers, and conditions used to assess water quality and processes affecting the PSW and the aquifer system included (1) carbonate host rock composition; (2) physicochemical constituents; (3) major and trace element concentrations; (4) saturation indices with respect to minerals in aquifer rocks; (5) elemental ratios, such as magnesium to calcium ratios, that are indicative of water-rock interaction processes; (6) oxidation-reduction conditions; (7) nutrient concentrations, in particular nitrate concentrations; (8) the isotopic composition of nitrate, which can point to specific nitrate sources; (9) strontium isotopes; (10) stable isotopes of hydrogen and oxygen; (11) organic contaminant concentrations, including pesticides and volatile organic compounds; (12) age tracers, apparent-age distribution, and dissolved gas data used in age interpretations; (13) depth-dependent water chemistry collected from the PSW under different pumping conditions to assess zones of contribution; and (14) temporal variability in groundwater composition from the PSW and selected monitoring wells in response to an aquifer recharge event. Geochemical results indicate that the well-field and monitoring well samples were largely representative of groundwater in the regional confined aquifer. Constituents of concern in the Edwards aquifer for the long-term sustainability of the groundwater resource include the nutrient nitrate and anthropogenic organic contaminants. Nitrate concentrations (as nitrogen) for regional aquifer PSWs had a median value of 1.9 milligrams per liter, which is similar to previously reported values for the regional aquifer. Nitrate-isotope compositions for groundwater samples collected from the well-field PSWs and monitoring wells had a narrow range, with values indicative of natural soil organic values. A comparison with historical nitrate-isotope values, however, suggests that a component of nitrate in groundwater from biogenic sources might have increased over the last 30 years. Several organic contaminants (the pesticide atrazine, its degradate deethylatrazine, trichloromethane (chloroform; a drinking-water disinfection byproduct), and the solvent tetrachloroethene (PCE)) were widely distributed throughout the regional aquifer and in the local-scale TANC study area at low concentrations (less than 1 microgram per liter). Higher concentrations of PCE were detected in samples from the well-field PSWs and Zarzamora monitoring wells relative to the regional aquifer PSWs. The urban environment is a likely source of contaminants to the aquifer, and these results indicate that one or more local urban sources might be supplying PCE to the Zarzamora monitoring wells and the well-field wells. Samples from the well field also had high concentrations of chloroform relative to the monitoring wells and regional aquifer PSWs. For samples from the regional aquifer PSWs, the most frequently detected organic contaminants generally decreased in concentration with increasing well depth. Deeper wells might intercept longer regional flow paths with higher fractions of older water or water recharged in rural recharge areas in the western part of the aquifer that have been less affected by anthropogenic contaminants. A scenario of hypothetical contaminant loading was evaluated by using results from groundwater-flow-model particle tracking to assess the response of the aquifer to potential contamination. Results indicate that the aquifer responds quickly (less than 1 year to several years) to contaminant loading; however, it takes a relatively long time (decades) for concentrations to reach peak values. The aquifer also responds quickly (less than 1 year to several years) to the removal of contaminant loading; however, it also takes a relatively long time (decades) to reach near background concentrations. Interpretation of geochemical age tracers in this well-mixed karst system was complicated by contamination of a majority of measured tracers and complexities of extensive mixing. Age-tracer results generally indicated that groundwater samples were composed of young, recently recharged water with piston-flow model ages ranging from less than 1 to 41 years, with a median of 17 years. Although a piston-flow model is typically not valid for karst aquifers, the model ages provide a basis for comparing relative ages of different samples and a reference point for more complex hydrogeologic models for apparent-age interpretations. Young groundwater ages are consistent with particle-tracking results from hydrogeologic modeling for the local-scale TANC study area. Age-tracer results compared poorly with other geochemical indicators of groundwater residence time and anthropogenic effects on water quality, indicating that hydrogeologic conceptual models used in groundwater age interpretations might not adequately account for mixing in this karst system. Groundwater samples collected from the well field under a variety of pumping conditions were relatively homogeneous and well mixed for numerous geochemical constituents (with the notable exception of age tracers). Groundwater contributions to the PSW were dominated by well-mixed, relatively homogeneous groundwater, typical of the regional confined aquifer. Zones of preferential flow were determined for the PSW, but groundwater samples from different stratigraphic units were not geochemically distinct. Variations in chemical constituents in response to a rainfall and aquifer recharge event occurred but were relatively minor in the PSW and monitoring wells. This observation is consistent with the hypothesis that the response to individual recharge events in the confined aquifer, unless intersecting conduit flow paths, might be attenuated by mixing processes along regional flow paths. Results of this study are consistent with the existing conceptual understanding of aquifer processes in this karst system and are useful for water-resource development and management practices.

Texas↗

USGS2006

Physical and hydrochemical evidence of lake leakage near Jim Woodruff Lock and Dam and of ground-water inflow to Lake Seminole, and an assessment of karst features in and near the lake, southwestern Georgia and northwestern Florida

Hydrogeologic data and water-chemistry analyses indicate that Lake Seminole leaks into the Upper Floridan aquifer near Jim Woodruff Lock and Dam, southwestern Georgia and northwestern Florida, and that ground water enters Lake Seminole along upstream reaches of the lake’s four impoundment arms (Chattahoochee and Flint Rivers, Spring Creek, and Fishpond Drain). Written accounts by U.S. Army Corps of Engineers geologists during dam construction in the late 1940s and early 1950s, and construction-era photographs, document karst-solution features in the limestone that comprise the lake bottom and foundation rock to the dam, and confirm the hydraulic connection of the lake and aquifer. More than 250 karst features having the potential to connect the lake and aquifer were identified from preimpoundment aerial photographs taken during construction. An interactive map containing a photomosaic of 53 photographic negatives was orthorectfied to digital images of 1:24,000-scale topographic maps to aid in identifying karst features that function or have the potential to function as locations of water exchange between Lake Seminole and the Upper Floridan aquifer. Some identified karst features coincide with locations of mapped springs, spring runs, and depressions that are consistent with sinkholes and sinkhole ponds. Hydrographic surveys using a multibeam echosounder (sonar) with sidescan sonar identified sinkholes in the lake bottom along the western lakeshore and in front of the dam. Dye-tracing experiments indicate that lake water enters these sinkholes and is transported through the Upper Floridan aquifer around the west side of the dam at velocities of about 500 feet per hour to locations where water "boils up" on land (at Polk Lake Spring) and in the channel bottom of the Apalachicola River (at the "River Boil"). Water discharging from Polk Lake Spring joins flow from a spring-fed ground-water discharge zone located downstream of the dam; the combined flow disappears into a sinkhole located on the western floodplain of the river and is transmitted through the Upper Floridan aquifer, eventually discharging to the Apalachicola River at the River Boil. Acoustic Doppler current profiling yielded flow estimates from the River Boil in the range from about 140 to 220 cubic feet per second, which represents from about 1 to 3 percent of the average daily flow in the river. Binary mixing-model analysis using naturally occurring isotopes of oxygen and hydrogen (oxygen-18 and deuterium) indicates that discharge from the River Boil consists of a 13-to-1 ratio of lake water to ground water and that other sources of lake leakage and discharge to the boil probably exist. Analyses of major ions, nutrients, radon-222, and stable isotopes of hydrogen and oxygen contained in water samples collected from 29 wells, 7 lake locations, and 5 springs in the Lake Seminole area during 2000 indicate distinct chemical signatures for ground water and surface water. Ground-water samples contained higher concentrations of calcium and magnesium, and higher alkalinity and specific conductance than surface-water samples, which contained relatively high concentrations of total organic carbon and sulfate. Solute and isotopic tracers indicate that, from May to October 2000, springflow exhibited more ground-water qualities (high specific conductance, low dissolved oxygen, and low temperature) than surface water; however, the ratio of ground water to surface water of the springs was difficult to quantify from November to April because of reduced springflow and rapid mixing of springflow and lake water during sampling. The saturation index of calcite in surface-water samples indicates that while surface water is predominately undersaturated with regard to calcite year-round, a higher potential for dissolution of the limestone matrix exists from late fall through early spring than during summer. The relatively short residence time (5–7 hours) and rapid flow velocity (nearly 500 feet per hour) of lake water leaking into the Upper Floridan aquifer and exiting at the River Boil in the Apalachicola River implies that calcite-undersaturated water is in constant contact with the limestone, increasing the potential for limestone dissolution and enlargement of flow pathways by erosion. A relatively low potential exists, however, for limestone dissolution to cause sudden sinkhole collapse followed by catastrophic lake drainage because ground-water levels close to the lake, except near the dam, are nearly the same as lake stage, resulting in low vertical and lateral hydraulic gradients and low flow between the lake and aquifer. An increased potential for lake leakage and sinkhole formation and collapse exists near some in-lake springs during colder months of the year, as density differences and the hydraulic potential between lake water and ground water establish the conditions for calcite-undersaturated lake water to enter nonflowing springs and contact limestone.

Florida, Georgia↗

USGS2014

Assessment of ethylene dibromide, dibromochloropropane, other volatile organic compounds, radium isotopes, radon, and inorganic compounds in groundwater and spring water from the Crouch Branch and McQueen Branch aquifers near McBee, South Carolina, 2010-2012

Public-supply wells near the rural town of McBee, in southwestern Chesterfield County, South Carolina, have provided potable water to more than 35,000 residents throughout Chesterfield County since the early 1990s. Groundwater samples collected between 2002 and 2008 in the McBee area by South Carolina Department of Health and Environmental Control (DHEC) officials indicated that groundwater from two public-supply wells was characterized by the anthropogenic compounds ethylene dibromide (EDB) and dibromochloropropane (DBCP) at concentrations that exceeded their respective maximum contaminant levels (MCLs) established by the U.S. Environmental Protection Agency’s (EPA) National Primary Drinking Water Regulations (NPDWR). Groundwater samples from all public-supply wells in the McBee area were characterized by the naturally occurring isotopes of radium-226 and radium-228 at concentrations that approached, and in one well exceeded, the MCL for the combined isotopes. The local water utility installed granulated activated carbon filtration units at the two EDB- and DBCP-contaminated wells and has, since 2011, shut down these two wells. Groundwater pumped by the remaining public-supply wells is currently (2014) centrally treated at a water-filtration plant. To assess the occurrence, distribution, and potential sources of the anthropogenic and naturally occurring compounds detected in groundwater in the McBee area, samples of groundwater and spring water were collected from public-supply, domestic-supply, agricultural-supply, and monitoring wells and springs, respectively, between 2010 and 2012 by the U.S. Geological Survey. The water samples were analyzed for concentrations of EDB, DBCP, other volatile organic compounds (VOCs), radium-226 and radium-228, radon, and inorganic compounds. All wells sampled were screened in the shallow Crouch Branch aquifer, the deeper McQueen Branch aquifer, or, for most public-supply wells, both aquifers. In areas where no wells existed or wells could not be installed, passive samplers that adsorb EDB, DBCP, and various VOCs, were installed in the shallow subsurface. A representative groundwater flow pathway to each public supply well and selected other wells was determined by using a calibrated three-dimensional groundwater-flow model of the Atlantic Coastal Plain in Chesterfield County and particle-tracking analysis. The aerial extent of the groundwater flow pathway to public-supply wells was mapped by using chlorofluorocarbon-concentration based, apparent-age dates of the groundwater. The water-quality data collected between 2010 and 2012, in conjunction with groundwater flow pathways and historical aerial photographs of land uses near McBee, indicate an area where EDB-, DBCP-, 1,2-dichloropropane-, 1,3-dichloropropane-, and carbon disulfide-contaminated groundwater exists in the Crouch Branch aquifer in the Cedar Creek Basin and north of McBee and is most likely related to the past use of these compounds between the early 1900s and the 1980s as soil fumigants in predominately agricultural areas north of McBee. The highest EDB concentration detected (18.6 micrograms per liter) during the 3-year study was in a groundwater sample from an agricultural-supply well located north of McBee. Other VOCs, such as dichloromethane and 1,1,2-trichloroethane, also were detected in groundwater samples from this EDB-contaminated agricultural-supply well but are from unknown source(s). The fact that the agricultural area north of McBee is located in a recharge area for the Crouch Branch aquifer most likely facilitated the groundwater contamination in this area. DBCP-contaminated groundwater detected in three public-supply wells south of McBee in the deeper McQueen Branch aquifer appears to be related to past soil fumigation practices that used DBCP in agricultural areas located south of McBee. One of the three DBCP-contaminated public-supply wells also contained EDB, most likely present in groundwater due to the release of leaded gasolines that contained EDB as a fuel additive between the 1940s and 1970s. A gasoline-source of EDB, rather than a soil-fumigation source, is supported by the co-detection in groundwater from the well of 1,2-dichloroethane, a lead scavenger compound also added to leaded gasoline. Groundwater pumped from two public-supply wells located within and to the east of the McBee town limits and one domestic-supply well east of McBee was characterized by the detection of 1,1-dichloroethane, trichloroethylene, 1,1-dichloroethylene, and perchloroethylene. Groundwater flow pathways determined for these wells indicate that the potential source(s) of these compounds detected in one public-supply well and the domestic-supply well may be located within the McBee town limits, and that the potential source(s) of these compounds detected in the public-supply well to the east of McBee may be located in an area north of McBee formerly used for agriculture, but used for industry since at least the 1970s. Radium isotopes (defined in this study as the sum of radium-226 and radium-228 concentrations) and radon were detected in all wells sampled in the McBee area between 2010 and 2012. Wells characterized by radium isotope concentrations in groundwater that exceeded the MCL of 5.0 picocuries per liter were also characterized by specific conductance values greater than 30 microsiemens per centimeter and clustered north of McBee in a predominately agricultural area, and in agricultural and urban areas located within and east of McBee. The elevated specific conductance values measured in groundwater from these wells most likely are due to recharge by water mineralized by fertilizer application in agricultural areas, or due to the recharge by water mineralized by septic-tank drain-field effluent near urban areas. Radon was detected in groundwater from all wells sampled, and radon concentrations in groundwater from three monitoring wells exceeded the proposed MCL of 300 picocuries per liter. Concentrations of uranium in groundwater in the McBee area increased with increased groundwater-sample depth, most likely due to the proximity of the sample-collection location to basement rock that contains uranium-bearing minerals.

South Carolina↗

USGS2021

Evaluation of hydrologic simulation models for fields with subsurface drainage to mitigated wetlands in Barnes, Dickey, and Sargent Counties, North Dakota

Proper identification of wetlands, along with a better understanding of the hydrology of mitigated wetlands, is needed to assist with conservation efforts aimed at maintaining the productivity and ecological function (wetland mitigation) of agricultural lands. The U.S. Geological Survey, in cooperation with the U.S. Department of Agriculture Natural Resources Conservation Service, completed a study to evaluate two models for simulating hydrologic conditions in fields with subsurface drainage to mitigated wetlands at several sites in North Dakota. These two models were evaluated as possible tools for water resource managers to use for designing wetland mitigation projects in the area in the future. The Soil-Plant-Atmosphere-Water (SPAW) model simulates the daily hydrologic water budgets of agricultural landscapes by two linked routines, one for farm fields (field hydrology) and one for impoundments such as wetlands and ponds (pond model). The DRAINMOD model was used in conjunction with the SPAW model because although the SPAW model can be used to simulate the hydrology of small drainage basins containing wetlands, the SPAW model does not contain routines to simulate drainage, either subsurface drainage or surface (drainage ditches), that can directly affect the wetland hydrology. The wetlands in the study areas in this report are all downstream from and adjacent to drained agricultural fields. SPAW and DRAINMOD models were developed and calibrated at three study areas (study areas B, D, and S) to evaluate how the models simulated field-scale hydrologic characteristics and the water balance in wetlands from January 1, 2003, through December 31, 2018. The SPAW model developed for study area B included five modeled fields in the field hydrology portion of SPAW that contributed inflow to one wetland simulated in the pond model portion of SPAW. Simulated wetland water depths were most similar to water depths measured at site BWET1, with an absolute mean error of 0.10 foot and a root mean square error of 0.14 foot. Site BWET2 had slightly larger errors, with an absolute mean error of 0.22 foot and a root mean square error of 0.28 foot. Simulated water depths were similar to the pattern of measured water depths at BWET1 and BWET2 from about mid-April 2018 through about mid-September 2018, but overpredicted water depths in the fall from about mid-September 2018 through about mid-October 2018. The SPAW model developed for study area D included six modeled fields in the field hydrology portion of SPAW that contributed inflow to five wetlands connected in series in the pond model portion of SPAW. Simulated water depths compared relatively well to water depths in the five wetlands, with the absolute mean error ranging from 0.17 foot (DWET1) to 0.39 foot (DWET2), and the root mean square error ranging from 0.28 foot (DWET1) to 0.56 foot (DWET5). The SPAW model developed for study area S included one modeled field in the field hydrology portion of SPAW that contributed inflow to one wetland in the pond model portion of SPAW. Among the SPAW models developed for the three study areas, the model for study area S had the best comparison between simulated and measured water depths, with an absolute mean error of 0.06 foot and a root mean square error of 0.10 foot. DRAINMOD models were developed and calibrated at the three study areas and provided inflow from subsurface drainage discharge to the SPAW models for simulating water levels in wetlands in the study areas. The calibrated DRAINMOD model for study area B showed the variability of hydrologic processes in the modeled field throughout the wide range of hydrologic conditions from January 1, 2003, through December 31, 2018. In general, the discharge through the modeled subsurface drainage system was in the spring and early summer (April through June) most years, with little to no discharge later in the year. Although the subsurface drainage system in study area D was the most complex among the three study areas and was simplified into a uniform system within DRAINMOD, simulated water table depths at study area D correlated better to measured water table depths compared to results from the model applications at the other two study areas. Simulated water table depths had an absolute mean error of 0.30 foot and root mean square error of 0.37 foot at site DGW1 and an absolute mean error of 0.29 foot and a root mean square error of 0.34 foot at site DGW2. Although the subsurface drainage system in study area S was the simplest and the modeled field was the smallest among the three study areas, simulated water table depths at study area S did not correlate as well to measured water table depths compared to results from the model applications at the other two study areas. The SPAW and DRAINMOD model applications at the three study areas in southeast North Dakota adequately simulated the hydrologic processes for fields with subsurface drainage that are connected to adjacent wetlands. However, more measured data would be needed to fully evaluate the models throughout the range of possible climatic conditions.

North Dakota↗

USGS2013

Status and understanding of groundwater quality in the South Coast Range-Coastal study unit, 2008: California GAMA Priority Basin Project

Groundwater quality in the South Coast Range–Coastal (SCRC) study unit was investigated from May through November 2008 as part of the Priority Basin Project of the Groundwater Ambient Monitoring and Assessment (GAMA) Program. The study unit is located in the Southern Coast Range hydrologic province and includes parts of Santa Barbara and San Luis Obispo Counties. The GAMA Priority Basin Project is conducted by the U.S. Geological Survey (USGS) in collaboration with the California State Water Resources Control Board and the Lawrence Livermore National Laboratory. The GAMA Priority Basin Project was designed to provide a statistically unbiased, spatially distributed assessment of untreated groundwater quality within the primary aquifer system. The primary aquifer system is defined as that part of the aquifer corresponding to the perforation interval of wells listed in the California Department of Public Health (CDPH) database for the SCRC study unit. The assessments for the SCRC study unit were based on water-quality and ancillary data collected in 2008 by the USGS from 55 wells on a spatially distributed grid, and water-quality data from the CDPH database. Two types of assessments were made: (1) status, assessment of the current quality of the groundwater resource, and (2) understanding, identification of the natural and human factors affecting groundwater quality. Water-quality and ancillary data were collected from an additional 15 wells for the understanding assessment. The assessments characterize untreated groundwater quality, not the quality of treated drinking water delivered to consumers by water purveyors. The first component of this study, the status assessment of groundwater quality, used data from samples analyzed for anthropogenic constituents such as volatile organic compounds (VOCs) and pesticides, as well as naturally occurring inorganic constituents such as major ions and trace elements. Although the status assessment applies to untreated groundwater, Federal and California regulatory and non-regulatory water-quality benchmarks that apply to drinking water are used to provide context for the results. Relative-concentrations (sample concentration divided by benchmark concentration) were used for evaluating groundwater. A relative-concentration greater than (>) 1.0 indicates a concentration greater than the benchmark and is classified as high. Inorganic constituents are classified as moderate if relative-concentrations are >0.5 and less than or equal to (≤) 1.0, or low if relative-concentrations are ≤0.5. For organic constituents, the boundary between moderate and low relative-concentrations was set at 0.1. Aquifer-scale proportion was used in the status assessment as the primary metric for evaluating regional-scale groundwater quality. High aquifer-scale proportion is defined as the areal percentage of the primary aquifer system with a high relative-concentration for a particular constituent or class of constituents. Moderate and low aquifer-scale proportions were defined as the areal percentage of the primary aquifer system with moderate and low relative-concentrations, respectively. Two statistical approaches—grid-based and spatially weighted—were used to evaluate aquifer-scale proportions for individual constituents and classes of constituents. Grid-based and spatially weighted estimates were comparable for the study (within 90 percent confidence intervals). For inorganic constituents with human-health benchmarks, relative-concentrations were high for at least one constituent for 33 percent of the primary aquifer system in the SCRC study unit. Arsenic, molybdenum, and nitrate were the primary inorganic constituents with human-health benchmarks that were detected at high relative-concentrations. Inorganic constituents with aesthetic benchmarks, referred to as secondary maximum contaminant levels (SMCLs), had high relative-concentrations for 35 percent of the primary aquifer system. Iron, manganese, total dissolved solids (TDS), and sulfate were the inorganic constituents with SMCLs detected at high relative-concentrations. In contrast to inorganic constituents, organic constituents with human-health benchmarks were not detected at high relative-concentrations in the primary aquifer system in the SCRC study unit. Of the 205 organic constituents analyzed, 21 were detected—13 with human-health benchmarks. Perchloroethene (PCE) was the only VOC detected at moderate relative-concentrations. PCE, dichlorodifluoromethane (CFC-12), and chloroform were detected in more than 10 percent of the primary aquifer system. Of the two special-interest constituents, one was detected; perchlorate, which has a human-health benchmark, was detected at moderate relative-concentrations in 29 percent of the primary aquifer system and had a detection frequency of 60 percent in the SCRC study unit. The second component of this study, the understanding assessment, identified the natural and human factors that may have affected groundwater quality in the SCRC study unit by evaluating statistical correlations between water-quality constituents and potential explanatory factors. The potential explanatory factors evaluated were land use, septic tank density, well depth and depth to top-of-perforations, groundwater age, density and distance to the nearest formerly leaking underground fuel tank (LUFT), pH, and dissolved oxygen (DO) concentration. Results of the statistical evaluations were used to explain the occurrence and distribution of constituents in the study unit. DO was the primary explanatory factor influencing the concentrations of many inorganic constituents. Arsenic, iron, and manganese concentrations increased as DO concentrations decreased, consistent with patterns expected as a result of reductive dissolution of iron and (or) manganese oxides in aquifer sediments. Molybdenum concentrations increased in anoxic conditions and in oxic conditions with high pH, reflecting two mechanisms for the mobilization of molybdenum—reductive dissolution and pH-dependent desorption under oxic conditions from aquifer sediments. Nitrate concentrations decreased as DO concentrations decreased which would be consistent with degradation of nitrate under anoxic conditions (denitrification). It also is possible that nitrate concentrations decreased in relation to increasing depth and groundwater age and not as a result of denitrification. Groundwater age was another explanatory factor frequently correlated to several inorganic constituents. Iron and manganese concentrations were higher in pre-modern (water recharged before 1952) or mixed-age groundwater. This correlation is one indication that iron and manganese are from natural sources. Nitrate, TDS, and sulfate concentrations were higher in modern groundwater (water recharged since 1952) and may indicate that human activities increase concentrations of nitrate, TDS, and sulfate. Land use was a third explanatory factor frequently correlated with inorganic constituents. Nitrate, TDS, and sulfate concentrations were higher in agricultural land-use areas than in natural land-use areas, indicating that increased concentrations may be a result of agricultural practices. Organic constituents usually were detected at low relative-concentrations; therefore, statistical analyses of relations to explanatory factors usually were done for classes of constituents (for example, pesticides or solvents) as well as for selected constituents. The number of VOCs detected in a well was not correlated to any of the explanatory factors evaluated. The number of pesticide and solvent detections and PCE and CFC-12 concentrations were higher in modern groundwater than in pre-modern groundwater. PCE and CFC-12 also were positively correlated to the density of LUFTs. PCE was negatively correlated to natural land use. Chloroform concentrations were positively correlated to the density of septic systems. Perchlorate concentrations were greater in agricultural areas than in urban or natural areas. Correlation of perchlorate with DO may indicate that perchlorate biodegradation under anoxic conditions may occur. Anthropogenic sources have contributed perchlorate to groundwater in the SCRC study unit, although low levels of perchlorate may occur naturally.

California↗

USGS2014

Aquifers of Arkansas: protection, management, and hydrologic and geochemical characteristics of groundwater resources in Arkansas

Sixteen aquifers in Arkansas that currently serve or have served as sources of water supply are described with respect to existing groundwater protection and management programs, geology, hydrologic characteristics, water use, water levels, deductive analysis, projections of hydrologic conditions, and water quality. State and Federal protection and management programs are described according to regulatory oversight, management strategies, and ambient groundwater-monitoring programs that currently (2013) are in place for assessing and protecting groundwater resources throughout the State. Physical attributes, groundwater geochemistry, and groundwater quality are described for each of the 16 aquifers of the State. Information in regard to the hydrology and geochemistry of each of the aquifers is summarized from about 550 historical and recent publications. Additionally, more than 8,000 sites with groundwater-quality data were obtained from the U.S. Geological Survey National Water Information System and the Arkansas Department of Environmental Quality databases and entered into a spatial database to investigate distribution and trends in chemical constituents for each of the aquifers. The 16 aquifers of the State were divided into two major physiographic regions of the State: the Coastal Plain Province (referred to as Coastal Plain) of eastern and southern Arkansas, which includes 11 of the 16 aquifers, and the Interior Highlands Division (referred to as Interior Highlands) of western Arkansas, which includes the remaining 5 aquifers. The 11 aquifers in the Coastal Plain consist of various geologic units that are Cenozoic in age and consist primarily of Cretaceous, Tertiary, and Quaternary sands, gravels, silts, and clays. Groundwater in the Coastal Plain represents one of the most valuable natural resources in the State, driving the economic engines of agriculture, while also supplying abundant water for commercial, industrial, and public-supply use. In terms of age from youngest to oldest, the aquifers of the Coastal Plain include Quaternary alluvial aquifers, including the Mississippi River Valley alluvial aquifer (the most important aquifer in Arkansas in terms of volume of use and economic benefits), the Jackson Group (a regional confining unit that served for decades as an important source of domestic supply), and the Cockfield, Sparta, Cane River, Carrizo, Wilcox, Nacatoch, Ozan, Tokio, and Trinity aquifers. The Mississippi River Valley alluvial aquifer accounts for approximately 94 percent of all groundwater used in the State, and the aquifer is used primarily for irrigation purposes. The Sparta aquifer is the second most important aquifer in terms of use, and the aquifer was used in the past dominantly as a source of public and industrial supply, although increasing irrigation use is occurring because of critically declining water levels in the Mississippi River Valley alluvial aquifer. Other aquifers of the Coastal Plain generally are used as important local sources of domestic, industrial, and public supply, in addition to other minor uses. Water quality generally is good for all aquifers of the Coastal Plain, except for elevated iron concentrations and localized areas of high salinity. The high salinity results from intrusion from underlying formations, evapotranspiration processes in areas of low recharge, and inadequate flushing in downgradient areas of residual salinity from deposition in marine environments. Trends in the spatial distribution of individual chemical constituents are related to position along the flow path for most aquifers of the Coastal Plain. These trends include elevated iron and nitrate concentrations with lower pH values and dissolved solids in groundwater from the outcrop areas, transitioning to lower iron and nitrate (related to changes in redox) and higher pH and dissolved solids (dominantly from the dissolution of carbonate minerals) in groundwater downgradient from outcrop areas. Groundwater generally trended from a calcium- to a sodium-bicarbonate water type with increasing cation exchange along the flow path. The Interior Highlands of western Arkansas has less reported groundwater use than other areas of the State, reflecting a combination of factors. These factors include prevalent and increasing use of surface water, less intensive agricultural uses, lower population and industry densities, lesser potential yield of the resource, and lack of detailed reporting. The overall low yields of aquifers of the Interior Highlands result in domestic supply as the dominant use, with minor industrial, public, and commercial-supply use. Where greater volumes are required for growth of population and industry, surface water is the greatest supplier of water needs in the Interior Highlands. The various aquifers of the Interior Highlands generally occur in shallow, fractured, well-indurated, structurally modified bedrock of this mountainous region of the State, as compared to the relatively flat-lying, unconsolidated sediments of the Coastal Plain. In terms of age from youngest to oldest, the aquifers of the Interior Highlands include: the Arkansas River Valley alluvial aquifer, the Ouachita Mountains aquifer, the Western Interior Plains confining system, the Springfield Plateau aquifer, and the Ozark aquifer. Spatial trends in groundwater geochemistry in the Interior Highlands differ greatly from trends noted for aquifers of the Coastal Plain. In the Coastal Plain, the prevalence of long regional flow paths results in regionally predictable and mappable geochemical changes along the flow paths. In the Interior Highlands, short, topographically controlled flow paths (from hilltops to valleys) within small watersheds represent the predominant groundwater-flow system. As such, dense data coverage from numerous wells would be required to effectively characterize these groundwater basins and define small-scale geochemical changes along any given flow path for aquifers of the Interior Highlands. Changes in geochemistry generally were related to rock type and residence time along individual flow paths. Dominant changes in geochemistry for the Ouachita Mountains aquifer and the Western Interior Plains confining system are attributed to rock/water interaction and changes in redox zonation along the flow path. In these areas, groundwater evolves along flow paths from a calcium- to a sodium-bicarbonate water type with increasing reducing conditions resulting in denitrification, elevated iron and manganese concentrations, and production of methane in the more geochemically evolved and strongest reducing conditions. In the Ozark and Springfield Plateau aquifers, rapid influx of surface-derived contaminants, especially nitrogen, coupled with few to no attenuation processes was attributed to the karst landscape developed on Mississippian- and Ordovician-age carbonate rocks of the Ozark Plateaus. Increasing nitrate concentrations are related to increasing agricultural land use, and areas of mature karst development result in higher nitrate concentrations than areas with less karst features.

Arkasas↗

USGS2013

Hydrogeology, groundwater seepage, nitrate distribution, and flux at the Raleigh hydrologic research station, Wake County, North Carolina, 2005-2007

rom 2005 to 2007, the U.S. Geological Survey and the North Carolina Department of Environment and Natural Resources, Division of Water Quality, conducted a study to describe the geologic framework, measure groundwater quality, characterize the groundwater-flow system, and describe the groundwater/surface-water interaction at the 60-acre Raleigh hydrogeologic research station (RHRS) located at the Neuse River Waste Water Treatment Plant in eastern Wake County, North Carolina. Previous studies have shown that the local groundwater quality of the surficial and bedrock aquifers at the RHRS had been affected by high levels of nutrients. Geologic, hydrologic, and water-quality data were collected from 3 coreholes, 12 wells, and 4 piezometers at 3 well clusters, as well as from 2 surface-water sites, 2 multiport piezometers, and 80 discrete locations in the streambed of the Neuse River. Data collected were used to evaluate the three primary zones of the Piedmont aquifer (regolith, transition zone, and fractured bedrock) and characterize the interaction of groundwater and surface water as a mechanism of nutrient transport to the Neuse River. A conceptual hydrogeologic cross section across the RHRS was constructed using new and existing data. Two previously unmapped north striking, nearly vertical diabase dikes intrude the granite beneath the site. Groundwater within the diabase dike appeared to be hydraulically isolated from the surrounding granite bedrock and regolith. A correlation exists between foliation and fracture orientation, with most fractures striking parallel to foliation. Flowmeter logging in two of the bedrock wells indicated that not all of the water-bearing fractures labeled as water bearing were hydraulically active, even when stressed by pumping. Groundwater levels measured in wells at the RHRS displayed climatic and seasonal trends, with elevated groundwater levels occurring during the late spring and declining to a low in the late fall. Vertical gradients in the groundwater discharge area near the Neuse River were complex and were affected by fluctuations in river stage, with the exception of a well completed in a diabase dike. Water-quality data from the wells and surface-water sites at the RHRS were collected continuously as well as during periodic sampling events. Surface-water samples collected from a tributary were most similar in chemical composition to groundwater found in the regolith and transition zone. Nitrate (measured as nitrite plus nitrate, as nitrogen) concentrations in the sampled wells and tributary ranged from about 5 to more than 120 milligrams per liter as nitrogen. Waterborne continuous resistivity profiling conducted on the Neuse River in the area of the RHRS measured areas of low apparent resistivity that likely represent groundwater contaminated by high concentrations of nitrate. These areas were located on either side of a diabase dike and at the outfall of two unnamed tributaries. The diabase dike preferentially directed the discharge of groundwater to the Neuse River and may isolate groundwater movement laterally. Discrete temperature measurements made within the pore water beneath the Neuse River revealed seeps of colder groundwater discharging into warmer surface water near a diabase dike. Water-quality samples collected from the pore water beneath the Neuse River indicated that nitrate was present at concentrations as high as 80 milligrams per liter as nitrogen on the RHRS side of the river. The highest concentrations of nitrate were located within pore water collected from an area near a diabase dike that was identified as a suspected seepage area. Hydraulic head was measured and pore water samples were collected from two 140-centimeter-deep (55.1-inch-deep) multiport piezometers that were installed in bed sediments on opposite sides of a diabase dike. The concentration of nitrate in pore water at a suspected seepage area ranged from 42 to 82 milligrams per liter as nitrogen with a median concentration of 79 milligrams per liter as nitrogen. On the opposite side of the dike, concentrations of nitrate in pore water samples ranged from 3 to 91 milligrams per liter as nitrogen with a median concentration of 52 milligrams per liter. At one of the multiport piezometers the vertical gradient of hydraulic head between the Neuse River and the groundwater was too small to measure. At the multiport piezometer located in the suspected seepage area, an upward gradient of about 0.1 was present and explains the occurrence of higher concentrations of nitrate near the sediment/water interface. Horizontal seepage flux from the surficial aquifer to the edge of the Neuse River was estimated for 2006. Along a 130-foot flow path, the estimated seepage flux ranged from –0.52 to 0.2 foot per day with a median of 0.09 foot per day. The estimated advective horizontal mass flux of nitrate along a 300-foot reach of the Neuse River ranged from –10.9 to 5 pounds per day with a median of 2.2 pounds per day. The total horizontal mass flux of nitrate from the surficial aquifer to the Neuse River along the 130-foot flow path was estimated to be about 750 pounds for all of 2006. Seepage meters were deployed on the bed of the Neuse River in the areas of the multiport piezometers on either side of the diabase dike to estimate rates of vertical groundwater discharge and flux of nitrate. The average estimated daily seepage flux differed by two orders of magnitude between seepage areas. The potential vertical flux of nitrate from groundwater to the Neuse River was estimated at an average of 2.5 grams per day near one of the multiport piezometers and an average of 784 grams per day at the other. These approximations suggest that under some hydrologic conditions there is the potential for substantial quantities of nitrate to discharge from the groundwater to the Neuse River.

North Carolina↗

USGS2021

Status and understanding of groundwater quality in the northern Sierra Nevada foothills domestic-supply aquifer study units, 2015–17—California GAMA Priority Basin Project

Groundwater quality in the northern Sierra Nevada foothills region of California was investigated as part of California State Water Resources Control Board (SWRCB) Groundwater Ambient Monitoring Assessment Priority Basin Project (GAMA-PBP). The region was divided into two study units: the Yuba-Bear watersheds (YBW) study unit and the American-Cosumnes-Mokelumne watersheds (ACMW) study unit. The GAMA-PBP made a spatially unbiased assessment of aquifer systems used for domestic drinking-water supply in the study region, which are predominantly composed of fractured, hard-rock aquifers of varying lithology. These assessments characterized the quality of raw groundwater to evaluate ambient conditions in the domestic-supply aquifer and not the quality of treated drinking water. The study included three components: (1) a status assessment , which characterized the quality of groundwater resources used for domestic drinking-water supply in the YBW and ACMW study units; (2) an understanding assessment , which evaluated natural and anthropogenic explanatory factors that could potentially affect groundwater quality in the study region; and (3) a comparative assessment between the groundwater resources used for domestic and public drinking-water supply in the study region. The status assessment was based on data collected by the GAMA-PBP from 74 sites in the YBW study unit during 2015–16 and 67 sites in the ACMW study unit from 2016 to 2017. To contextualize water-quality results, concentrations of water-quality constituents in ambient groundwater were compared to regulatory and non-regulatory benchmarks typically used by the State of California and Federal agencies as health-based or aesthetic standards for public drinking water. The status assessment used a grid-based method to estimate proportions of groundwater resources with concentrations approaching or exceeding benchmark thresholds. This method provides spatially unbiased results and allows inter-comparability with similar groundwater-quality assessments. Inorganic constituents with health-based benchmarks were present at high relative concentration (RC), meaning they exceeded the benchmark threshold, in 5.4 and 10 percent of domestic-supply aquifer systems in the YBW and ACMW study units, respectively. Inorganic constituents with aesthetic-based benchmarks were detected at high-RCs in 20 and 28 percent of the YBW and ACMW study units, respectively. The inorganic constituents present at high RC were arsenic, barium, boron, molybdenum, strontium, nitrate, adjusted gross-alpha particle activity, chloride, total dissolved solids, specific conductance, iron, manganese, and hardness. Groundwater samples were tested for presence or absence of three microbial indicators (total coliform, Escherichia coli , and Enterococci ). At least one microbial indicator was present in 26 and 28 percent of the YBW and ACMW study units, respectively. At least one organic constituent was detected in 30 and 42 percent of the YBW and ACMW study units, respectively. Organic constituents were not present at high RC, but tetrachloroethene (PCE), trichloroethene (TCE), and toluene were detected in the YBW study unit at moderate RC (between the benchmark concentration and one-tenth of the benchmark concentration). Methyl tert -butyl ether (MTBE) and chloroform were present at low RC (less than one-tenth of the benchmark concentration) in the YBW and ACMW study units with detection frequencies greater than 10 percent. Perchlorate, a constituent of special interest, was detected in 31 and 41 percent of the YBW and ACMW study units, respectively, at either low or moderate RCs. Relations among select water-quality constituents and potential explanatory factors were evaluated using statistical and graphical approaches. Nitrate, microbial indicators, and perchlorate were all correlated to elevation-dependent variables relating to climate, land use, and recharge condition. Isotopic and dissolved noble-gas tracers indicated these water-quality constituents are associated with recharge conditions associated with irrigation during the summer dry-season, which is common in areas of rural-residential or agricultural land uses. Higher concentrations of iron and manganese were primarily associated with anoxic groundwater in aquifers of metasedimentary lithology. Increased hardness was primarily associated with anoxic groundwater in aquifers of mafic-ultramafic or metavolcanics lithologies at lower elevations in the study region in the Melones fault zone. Chloroform and MTBE were associated with shallow groundwater (wells depths less than 130 m) under oxic and anoxic redox conditions, respectively. The comparative assessment evaluated differences between the aquifer systems used for domestic- and public-supply in study region based on (1) well-construction characteristics, and (2) water quality. Analysis of over 60,000 well-completion reports in the study region showed that although domestic-supply wells span the deepest depth zones in regional aquifers, median depths for public-supply wells were significantly greater than those of domestic-supply wells in both study units. Water-quality data from more than 300 public-supply wells in the study region were assessed using a spatially weighted method for calculation aquifer-scale proportions and compared with the domestic-supply assessment results. Detections of inorganic constituents at high RC and detection frequencies for organic constituents were generally similar between the domestic- and public-supply aquifer systems in both study units, with a few notable exceptions in the ACMW study unit: nitrate was greater for the public- compared to domestic-supply aquifer system and both manganese, hardness, and MTBE were greater in the domestic- compared to public-supply aquifer system. These differences are likely related to contrasting land uses, aquifer lithologies, landscape positions, and depths characterizing domestic- and public-supply wells in the ACMW study unit. Overall, fewer samples from domestic-supply wells in the northern Sierra Nevada foothills exceeded health-based benchmarks compared to aesthetic-based benchmarks for groundwater quality. Exceedences of health-based benchmarks were primarily caused by nitrate and coliform bacteria, which were associated with recharge from diverted surface water used primarily for irrigation. Exceedences of aesthetic-based benchmarks were primarily caused by iron, managanese, and hardness, which were associated with geologic factors. Regional irrigation practices and aquifer lithology can affect groundwater quality in fractured-rock aquifers in the northern Sierra Nevada foothills used for domestic drinking-water supply.

California↗

USGS2014

A reconnaissance spatial and temporal assessment of methane and inorganic constituents in groundwater in bedrock aquifers, Pike County, Pennsylvania, 2012-13

Pike County in northeastern Pennsylvania is underlain by the Devonian-age Marcellus Shale and other shales, formations that have potential for natural gas development. During 2012–13, the U.S. Geological Survey in cooperation with the Pike County Conservation District conducted a reconnaissance study to assess baseline shallow groundwater quality in bedrock aquifers prior to possible shale-gas development in the county. For the spatial component of the assessment, 20 wells were sampled in summer 2012 to provide data on the occurrence of methane and other aspects of existing groundwater quality throughout the county, including concentrations of inorganic constituents commonly present at low levels in shallow, fresh groundwater but elevated in brines. For the temporal component of the assessment, 4 of the 20 wells sampled in summer 2012 were sampled monthly from July 2012 through June 2013 to provide data on seasonal variability in groundwater quality. All water samples were analyzed for major ions, nutrients, selected inorganic trace constituents (including metals and other elements), stable isotopes of water, radon-222, gross alpha- and gross beta-particle activity, dissolved gases (methane, ethane, and ethene), and, if possible, isotopic composition of methane. Additional analyses for boron and strontium isotopes, age-dating of water, and radium-226 were done on water samples collected from six wells in June 2013. Results of the summer 2012 sampling show that water from 16 (80 percent) of 20 wells had detectable concentrations of methane, but concentrations were less than 0.1 milligram per liter (mg/L) in most well-water samples; only two well-water samples had concentrations greater than 1 mg/L. The groundwater with elevated methane also had a chemical composition that differed in some respects (pH, selected major ions, and inorganic trace constituents) from groundwater with low methane concentrations. The two well-water samples with the highest methane concentrations (about 3.7 and 5.8 mg/L) also had the highest pH values (8.7 and 8.3, respectively) and the highest concentrations of sodium, lithium, boron, fluoride, and bromide. Elevated concentrations of some other constituents, such as barium, strontium, and chloride, were not limited to well-water samples with elevated methane, although the two samples with elevated methane also had among the highest concentrations of these constituents. One sample with elevated methane concentrations also had elevated arsenic concentrations, with the arsenic concentration of 30 micrograms per liter (μg/L) exceeding the drinking-water standard of 10 µg/L for arsenic. No other sample from the 20 wells sampled in summer 2012 had concentrations of constituents that exceeded any established primary drinking-water standards. However, radon-222 activities ranging up to 4,500 picocuries per liter (pCi/L) exceeded the proposed drinking-water standard of 300 pCi/L in 85 percent of the 20 well-water samples. The isotopic composition methane in the two high-methane samples (δC CH4 values of -64.55 and -64.41 per mil and δD CH4 values of -216.9 and -201.8 per mil, respectively) indicates a predominantly microbial source for the methane formed by a carbon dioxide reduction process. The stable isotopic composition of water (δD H20 and δ 18 O H20 ) in samples from all 20 wells falls on the local meteoric line, indicating water in the wells was of relatively recent meteoric origin (modern precipitation), including samples with elevated methane concentrations. Analytical results for 4 of the 20 wells sampled monthly for 1 year ending June 2013 in order to assess temporal variability in groundwater quality show that concentrations of major ions generally varied by less than 20 percent, with most differences less than 4 mg/L. Concentrations of methane varied by less than 1 μg/L (0.001 mg/L) in samples from three wells with low methane and by as much as 1 mg/L (1,000 μg/L) in samples from one well with relatively high methane. The isotopic composition of methane in the one well with relatively high methane varied slightly in the monthly samples, ranging from about -64.5 to -64.8 per mil for δ 13 C CH4 and from about -217 to -228 per mil for δD CH4 . The δ 13 C values for dissolved inorganic carbon (DIC) in water from this well were consistent with microbial methane formation by carbon dioxide reduction (drift-type methane) and varied little in the temporal samples, ranging from -10.5 to -10.1 per mil. Additional analyses of samples collected in late June 2013 from six wells with a range of methane and trace constituent concentrations provided baseline data on strontium and boron isotopic compositions ( 87 Sr/ 86 Sr ratios and δ 11 B, respectively) that potentially may be used to differentiate among sources of these constituents. The strontium and boron isotopic composition determined in the six shallow Pike County groundwater samples had 87 Sr/ 86 Sr ratios of 0.71426 to 0.71531 and δ 11 B values of 11.7 to 27.0 per mil, which differ from those reported for brines in Devonian-age formations in Pennsylvania. The June 2013 samples were also analyzed for radium-226 and age-dating dissolved gases. Activities of radium-226 ranged from 0.041 to 0.29 pCi/L in water samples from the six wells and were less than the drinking-water standard of 5 pCi/L for combined radium-226 and radium-228. Age-dating of groundwater using a method based on the presence of anthropogenic gases (chlorofluorocarbons and sulfur hexafluoride) released into the atmosphere yielded estimated recharge dates for water from these six wells that ranged from the 1940s to early 2000s. The oldest water was in samples from wells that had the highest methane concentrations and the youngest water was in samples from a continuously pumped 300-foot deep production well.

Pennsylvania↗