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At least 793 records · Page 44Linked to original sources
USGS2022

Characterization of ambient groundwater quality within a statewide, fixed-station monitoring network in Pennsylvania, 2015–19

Pennsylvania leads the Nation in the number of individuals that use groundwater for private domestic water supply; more than 3 million rural and suburban Pennsylvania residents rely on private domestic supplies for drinking water. These supplies are not regulated nor routinely monitored; thus relevant groundwater-quality information is not widely available. The U.S. Geological Survey (USGS), in cooperation with the Pennsylvania Department of Environmental Protection (PaDEP) Safe Drinking Water Bureau, established a statewide, fixed-station ambient groundwater quality network in 2015. The goals for the Pennsylvania Groundwater Monitoring Network (GWMN) include characterizing ambient groundwater quality conditions in rural areas of the State and documenting potential changes in conditions over time. Seventeen wells were selected for monitoring at 6-month intervals beginning in 2015. Since then, several wells have been added to the GWMN, bringing the total number of wells sampled in the fall of 2019 to 28. Routinely monitored constituents included physical characteristics and chemical concentrations in filtered and unfiltered samples (major and trace elements, nutrients, and organic compounds). Samples for volatile organic compounds (VOCs), radionuclides, and dissolved hydrocarbon gases were collected during the first sampling event at each well. To offer insights on the quality of groundwater used for domestic supply in Pennsylvania, summary statistics for the 221 GWMN samples collected during 2015–19 are compared to U.S. Environmental Protection Agency (EPA) drinking-water standards, which are applicable to public water supplies. Results show that samples across the GWMN generally meet drinking-water standards for inorganic and organic constituents; however, a percentage of samples had concentrations that exceeded maximum contaminant level (MCL) thresholds for nitrate (3 percent) and secondary maximum contaminant level (SMCL) thresholds for iron (32 percent), manganese (36 percent), and aluminum (5 percent). Radon-222 activities, which were sampled only during the initial visit to a well, exceeded the lower proposed drinking water standard of 300 picocuries per liter (pCi/L) in 64 percent of wells in the GWMN; additionally, 7 percent of wells exceeded the higher proposed standard of 4,000 pCi/L. There were no exceedances for VOCs, but one well had a tribromomethane detection. Three wells had detectable concentrations of methane, with one sample exceeding the Pennsylvania action level of 7 milligrams per liter (mg/L). The pH and dissolved oxygen concentrations varied widely across the GWMN and were correlated with dissolved metal concentrations and other chemical characteristics of groundwater samples. Considering all samples collected for the study, the pH ranged from 4.2 to 8.3; 42 percent of pH values were either above or below the SMCL range of 6.5–8.5. The highest pH values resulted from contamination of loose grout used in the construction of one well and decreased to levels consistent with other wells in the vicinity after repeated sampling rounds. Dissolved oxygen (DO), which ranged from 0 to 13.9 mg/L, influences the mobility and prevalence of constituents with variable oxidation state, including iron, manganese, and nitrogen species. Samples with acidic pH (less than 6.5) and (or) low DO had the highest concentrations of manganese and iron, whereas those with neutral to alkaline pH values had the highest concentrations of calcium, magnesium, sodium, and other major ions. Analysis of major ions indicates that calcium/bicarbonate water types are the most common, with a few characterized as calcium/chloride or sodium/chloride, and most others as mixed water types including calcium-magnesium/bicarbonate, sodium-magnesium/bicarbonate, and sodium/bicarbonate-chloride. Nonparametric statistical methods were used to evaluate the data for spatial and temporal trends. A principal components analysis (PCA) model developed with ranked data values for the entire network resulted in three components, (1) dissolved solids, (2) redox, and (3) sodium-chloride, which explained 74.5 percent of variance in the dataset. On the basis of individual contributions to the PCA, certain wells were identified through hierarchical cluster analysis that shared relevant water-quality characteristics. The spatial distribution of sampling locations and the temporal trends of constituent concentrations indicate that hydrogeologic setting and topographic position as defined in the PCA model are important factors affecting the spatial and temporal patterns of groundwater quality in the GWMN.

Pennsylvania↗
USGS2022

Occurrence of per- and polyfluoroalkyl substances and inorganic analytes in groundwater and surface water used as sources for public water supply in West Virginia

Per- and polyfluoroalkyl substances (PFAS) are widely observed anthropogenic compounds found in water supplies worldwide and increasingly linked with adverse health effects in humans. In 2019, the West Virginia Legislature recognized the contamination risk to public source-water supplies posed by PFAS and passed a resolution that required a statewide PFAS study. The purpose of the resolution was to understand the occurrence and distribution of PFAS contamination throughout the State’s rivers, lakes, and groundwater aquifers. The U.S. Geological Survey has worked in cooperation with the West Virginia Department of Environmental Protection and West Virginia Department of Health and Human Resources to collect raw-water samples at 279 public-water systems across West Virginia. Public-water systems sampled for this study were identified by the West Virginia Department of Health and Human Resources and included all community water systems in the State and all daycares and schools that operate their own water systems. Raw source water was sampled for both groundwater and surface-water sites at the first available tap in the public-water system, prior to any treatment. One hundred and seventy-three samples were collected from groundwater sources and 106 samples were collected from surface-water sources. Parameters collected at the time of sampling included pH, specific conductance, water temperature, dissolved oxygen, turbidity, and alkalinity. PFAS was analyzed at all 279 sites, major ions and trace elements were analyzed at 272 sites, and nutrients were analyzed at 270 sites. The type of source water used for public supply in West Virginia is generally dependent on geology with more groundwater sites sampled in high-yield aquifers such as karst and alluvium. Surface-water sites were more evenly distributed throughout the State and are often the only source used in areas underlain by lower-yielding fractured-rock aquifers. Twenty-four percent of the sites sampled for this study had at least 1 PFAS detected, 47 of which were in groundwater sources and 20 in surface-water sources. Five sites exceeded the U.S. Environmental Protection Agency’s health advisory for combined perfluorooctanoate and perfluorooctanesulfonate concentrations of 70 nanograms per liter. These sites were located in highly susceptible karst and alluvial groundwater aquifers on the east and west sides of the State. Higher PFAS concentrations were more commonly found in groundwater than surface-water sources, and high concentrations and PFAS detections were generally concentrated in the Ohio River Valley and West Virginia’s eastern panhandle. PFAS was rarely detected in groundwater sites in fractured-rock aquifers and abandoned underground coal-mine aquifers in the Appalachian Plateaus Physiographic Province had very little PFAS detected. These data represent a baseline summary of source water in West Virginia. Additional studies may be needed to understand exposure to private homeowners with domestic-water sources, variability of PFAS concentrations over time, and PFAS in finished drinking water as evaluated by current and future drinking-water regulations.

West Virginia↗
USGS2023

Groundwater flow model investigation of the vulnerability of water resources at Chaco Culture National Historical Park related to unconventional oil and gas development

Chaco Culture National Historical Park (CCNHP), located in northwestern New Mexico, protects the greatest concentration of Chacoan historical sites in the American Southwest. Geologically, CCNHP is located within the San Juan structural basin, which consists in part of complex Cretaceous stratigraphy and hosts a variety of energy resources. As part of a larger study to investigate the vulnerability of water resources at CCNHP related to oil and natural gas extraction activities, a MODFLOW groundwater model of the Mancos Shale and Gallup Sandstone units was created by the U.S. Geological Survey, as a part of a cooperative study with the National Park Service, to assess advective groundwater flow paths and traveltimes. The model determined that groundwater flow directions currently trend from south-southeast to north-northwest within the vicinity of CCNHP, groundwater traveltime through the Gallup Sandstone ranges from thousands to tens of thousands of years, and traveltime through the Mancos Shale may range from millions to tens of millions of years. The capture zone of the main CCNHP well (referred to as the “Chaco well”) extends to the south-southeast and ranges in width from approximately 1 to 12 miles, depending on pumping rate. Eighteen inactive hydrocarbon related wells are located within the capture zone and within 10 kilometers of the Chaco well. Given model estimates of traveltimes of groundwater in the Gallup Sandstone aquifer, advective groundwater transport to the Chaco well would take approximately 430 years from the nearest inactive hydrocarbon related wells. Differencing of historical and modern-day potentiometric surfaces of the Gallup Sandstone indicate a drop in groundwater levels between 34 and 96 feet within the CCNHP boundaries. Hydraulic fracturing, simulated as increased hydraulic conductivity zones, decreased groundwater traveltimes (from millions to thousands of years) and acted as permeable pathways from the Mancos Shale to the Gallup Sandstone.

Arizona, New Mexico↗
USGS2023

Simulating groundwater flow in the Mississippi Alluvial Plain with a focus on the Mississippi Delta

The Mississippi Alluvial Plain has become one of the most important agricultural regions in the United States but relies heavily on groundwater for irrigation. On average, more than 12 billion gallons are withdrawn daily from the Mississippi River Valley alluvial aquifer. Declining groundwater levels, especially in the Delta region of northwest Mississippi and the Cache and Grand Prairie regions of eastern Arkansas, have led to concerns about future sustainability. The U.S. Geological Survey Mississippi Alluvial Plain Project is focused on quantifying the groundwater system in the alluvial plain and the response of groundwater resources to future development. A key objective of the project is to provide updated groundwater flow models supported by extensive data collection and analyses. MODFLOW 6, PEST++, and several open-source python packages were used to develop a simplified, faster running version of the Mississippi Embayment Regional Aquifer Study model that can provide boundary conditions for local inset models, including the Mississippi Delta model described in this report. An automated workflow was used for model construction, history matching, and development of baseline future climate scenarios. The models incorporate information from a Soil-Water-Balance code simulation of the terrestrial water balance, metering-based estimates of water use from thousands of wells, measured and estimated streamflow and stages, and the largest airborne electromagnetic survey flown to date in the United States. Baseline scenarios for the Mississippi Delta under potential future climates were constructed using recharge, surface runoff and irrigation pumping forcings from a future version of the Soil-Water-Balance model, driven by downscaled temperature and precipitation output from 10 general circulation model simulations, including high and moderate carbon emissions pathways. Results indicate a complex water balance that varies in time and space in terms of the terrestrial recharge, stream leakage, and regional groundwater flow components, which are affected by seasonal forcings, human activity, and alluvial geomorphology. The general circulation model outputs indicate a continued rise in average temperatures but no clear precipitation trend. Increased crop water demand is anticipated from the higher temperatures, resulting in increased irrigation withdrawals to sustain current levels of irrigated agriculture. Simulated drawdowns in groundwater levels at the mid-21st century vary greatly. Under moderate or wet climate scenarios, and in parts of the aquifer that are well connected to surface water, little to no additional drawdown is anticipated. Under dry or warm scenarios, drawdowns of as much as 10 meters or more are possible in parts of the aquifer that are relatively disconnected from surface water. Under dry or warm scenarios, the portion of the Delta with greater than 60 feet of saturated thickness could be reduced from near 100 percent currently (2018) to 80–90 percent by mid-century. Future simulations with the model could include alternative management scenarios to identify options for improving groundwater sustainability. The automated model construction workflows are designed to facilitate regular updating, making this a “living” framework that the Mississippi Department of Environmental Quality and other stakeholders can use for adaptive management going forward.

Arkansas, Louisiana, Mississippi↗
USGS2024

Streamflow, base flow, and precipitation trends and simulated effects of Rush Springs aquifer groundwater withdrawals on base flows upgradient from Fort Cobb Reservoir, western Oklahoma

To better understand the relation between groundwater use in the Rush Springs aquifer and inflows to the Fort Cobb Reservoir, the U.S. Geological Survey, in cooperation with the Bureau of Reclamation, used a previously published numerical groundwater-flow model and historical streamflow records to evaluate four scenarios to investigate how changing groundwater withdrawals could affect base flows in streams that flow into Fort Cobb Reservoir. These scenarios consisted of observing simulated base-flow response by (1) scaling the 20-year equal-proportionate-share groundwater-withdrawal rate by various percentages over a 50-year period; (2) scaling the historical groundwater-withdrawal rates by various percentages across the entire Rush Springs aquifer; (3) scaling the historical groundwater-withdrawal rates within various subareas (zones) of the Fort Cobb Reservoir surface watershed; and (4) simulating a base-flow-depletion scenario. Cobb, Lake, and Willow Creeks are the major streams upgradient from the Fort Cobb Reservoir (listed from highest to lowest mean annual base flow). The results of scenarios 1 and 2 indicated that Willow Creek is the most susceptible to drying, but Cobb Creek was the most likely to have reduced base flow. Scenarios 3 and 4 indicated that groundwater withdrawals affect Cobb Creek base flows over a broader watershed area compared to Lake and Willow Creeks. In scenario 4, Cobb Creek base-flow depletion was higher across a larger area than Lake Creek and Willow Creek. Groundwater withdrawals in the Cobb Creek watershed tended to affect total inflows into Fort Cobb Reservoir more than other areas in the extent of the Rush Springs aquifer.

Oklahoma↗
USGS2024

Simulation of groundwater and surface-water interaction and lake resiliency at Crystal Lake, City of Crystal Lake, Illinois

The U.S. Geological Survey, in cooperation with the City of Crystal Lake, Illinois, started a study to increase understanding of groundwater and surface-water interaction between the glacial aquifer and the city’s namesake lake, Crystal Lake, and the effect of higher and lower precipitation conditions on groundwater and lake levels. The results from this study could be used by the city and others to aid in lake management strategies. This report describes the hydrologic lake budget and each of the budget components, which are then used in the construction, calibration, and application of a regional groundwater flow model. The flow model is used to simulate the shallow groundwater flow system and the lake responses to increased and decreased precipitation under the current weir elevation and the proposed lowered weir elevation. Using the program groundwater flow analytic element model (GFLOW), a two-dimensional, steady-state model was constructed. The model was calibrated by matching target water levels and stream base flows by adjusting model input parameters. A sensitivity analysis was completed by adjusting the parameters within reasonable ranges and noting the magnitude of changes in model calibration targets. Potential effects of extended wet and dry periods (within historical ranges and published predicted ranges) were evaluated by adjusting precipitation, groundwater recharge, and discharge at Crystal Lake culvert outlet in the model and comparing the resulting simulated lake stage and water budgets to stages and water budgets from the calibrated model. Model results under average, wet, and dry conditions with a lowered weir of 1 foot at the Crystal Lake culvert outlet indicate minor changes in the simulated lake-water budgets and associated lake levels and groundwater elevation contours; however, simulations with an increased outflow at the Crystal Lake culvert outlet decreased the lake water levels by as much as 1.87 feet and also decreased the groundwater levels surrounding the lake by about 1–2 feet during average and wet conditions.

Illinois↗
USGS2024

Use of a numerical groundwater-flow model and projected climate scenarios to simulate the effects of future climate conditions on base flow for reach 1 of the Washita River alluvial aquifer and Foss Reservoir storage, western Oklahoma

To better understand the relation between climate variability and future groundwater resources in reach 1 of the Washita River alluvial aquifer and Foss Reservoir in western Oklahoma, the U.S. Geological Survey, in cooperation with the Bureau of Reclamation, used a previously published numerical groundwater-flow model and climate-model data to investigate changes in base flow and reservoir storage by evaluating three scenarios. The three projected climate scenarios were (1) a central-tendency scenario, (2) a warmer/drier scenario, and (3) a less-warm/wetter scenario. To estimate future base flow and groundwater availability in western Oklahoma, specifically in reach 1 of the Washita River alluvial aquifer, downscaled climate-model data from 231 Coupled Model Intercomparison Project phase 5 (CMIP5) projections coupled with a previously published numerical groundwater-flow model were used to compare the effects of different climate scenarios on the aquifer. Changes in base flow and groundwater-level elevations during a 30-year baseline scenario (1985–2014) and the three 30-year projected climate scenarios (2050–79) under central-tendency, warmer/drier, and less-warm/wetter climatic conditions were assessed by using the calibrated model. In the simulations, the amount of base flow and reservoir storage declined in the central-tendency and warmer/drier scenarios compared to the amount of base flow and reservoir storage under historical climatic conditions (baseline scenario). Mean annual change in reservoir storage decreased from the baseline scenario the most in the warmer/drier scenario, followed by the central-tendency scenario, but increased in the less-warm/wetter scenario compared to the baseline scenario. At the end of the simulation period (2079), the largest magnitude differences in groundwater-level elevations in all three projected climate scenarios relative to the baseline scenario occurred upstream from Foss Reservoir. Results from incorporating downscaled climate projections into localized numerical groundwater-flow models can highlight potential future changes in and implications for groundwater resources and availability.

Oklahoma↗
USGS2025

Groundwater-level elevations in the Denver Basin bedrock aquifers and Upper Black Squirrel Creek alluvial aquifer, El Paso County, Colorado, 2021–24

El Paso County is the second-most populous county in Colorado and is projected to grow another 15 percent by 2030. Within El Paso County is the Upper Black Squirrel Creek Designated Groundwater Basin (Black Squirrel Basin), an area where surface water is scarce and water users rely primarily on groundwater from five different aquifers (the Upper Black Squirrel Creek alluvial aquifer and four bedrock aquifers within the Denver Basin aquifer system: the lower Dawson, Denver, Arapahoe, and Laramie-Fox Hills aquifers) to meet their needs. Currently (2024), land within the Upper Black Squirrel Creek Basin is primarily used for rural grazing and agriculture; however, municipal development is ongoing. In 2021, the U.S. Geological Survey, in cooperation with the Upper Black Squirrel Creek Ground Water Management District, began a study to establish a baseline dataset and assess the groundwater resources of the aquifers within the Black Squirrel Basin. A network of 39 wells was established in 2021; discrete groundwater-level measurements were made bimonthly. Nine of the 39 wells were equipped with pressure transducers to record hourly groundwater-level data. Seven wells had statistically significant seasonal trends, and trends at 3 wells were negative. For the discrete data, 16 wells had a significant trend for the study period, and 4 wells had negative trends. For the time-series data, 8 wells had significant trends, and 3 wells had negative trends. Potentiometric surface maps were created for this study using discrete, static groundwater levels measured in April 2023. These maps showed the estimated groundwater flow direction from the north-northwest to the south-southeast in the alluvial aquifer and from the northwest to the east-southeast for the lower Dawson and Denver aquifer wells. This study indicates the potential benefit of monitoring wells in the areas near municipal pumping. Additional monitoring could lead to a better understanding of connectivity between aquifers and be an important tool for assessing long-term sustainability of groundwater use.

Colorado↗
USGS2025

Groundwater age estimates for the Mississippi River Valley alluvial aquifer based on tracer data collected during 2018–20

This study characterized groundwater age across the Mississippi River Valley alluvial aquifer (MRVA). Groundwater samples from 69 MRVA wells and 19 wells in Tertiary units of the Mississippi embayment aquifer system (MEAS) were analyzed for sulfur hexafluoride (SF 6 ), tritium ( 3 H), helium (He), and (or) carbon-14 of dissolved inorganic carbon ( 14 C). The age distributions of 89 samples were estimated by fitting lumped parameter models to processed tracer concentrations with the U.S. Geological Survey software TracerLPM. Mean ages of MRVA groundwater samples ranged from 12 to 22,000 years, with a median of 140 years. Mean ages of MEAS groundwater samples ranged from 230 to 52,000 or more years, with a median of 13,500 years. The spatial distribution of MRVA groundwater ages was found to be influenced by depth, inflow of groundwater from deeper units, and soil saturated hydraulic conductivity. In parts of the MRVA, the spatial distribution of MRVA groundwater ages was found to be influenced by annual recharge and (or) annual groundwater pumpage.

Alabama, Arkansas, Illinois, Kentucky, Louisiana, ↗
USGS2015

From submarine to lacustrine groundwater discharge

Submarine groundwater discharge (SGD) and its role in marine nutrient cycling are well known since the last decade. The freshwater equivalent, lacustrine groundwater discharge (LGD), is often still disregarded, although first reports of LGD are more than 50 years old. We identify nine different reasons why groundwater has long been disregarded in both freshwater and marine environments such as invisibility of groundwater discharge, the size of the interface and its difficult accessibility. Although there are some fundamental differences in the hydrology of SGD and LGD, caused primarily by seawater recirculation that occurs only in cases of SGD, there are also a lot of similarities such as a focusing of discharge to near-shore areas. Nutrient concentrations in groundwater near the groundwater–surface water interface might be anthropogenically enriched. Due to spatial heterogeneity of aquifer characteristics and biogeochemical processes, the quantification of groundwater-borne nutrient loads is challenging. Both nitrogen and phosphorus might be mobile in near-shore aquifers and in a lot of case studies large groundwater-borne nutrient loads have been reported.

Conference Paper↗
USGS2008

A simulation/optimization model for groundwater resources management in the Afram Plains area, Ghana

A groundwater flow simulation model was developed using available hydrogeo logical data to A groundwater flow simulation model was developed using available hydrogeological data to describe groundwater flow in the Afram Plains area. A nonlinear optimization model was then developed and solved for the management of groundwater resources to meet irrigation and household needs. The objective was to maximize groundwater extraction for irrigation activities from the shallow aquifers of the southern Voltaian Sedimentary Basin that underly the area This would improve food security, raise the standard of living and ultimately alleviate poverty in the Afram Plains. The calibrated flow model is in tandem with the general hydrochemical evolution of groundwater in the area and fits the observed data with about a 98% degree of confidence. Groundwater resources may not be the limiting factor in the development of irrigated agriculture. Groundwater has tremendous potential to meet current and future irrigation needs. It was determined from this study that profit from maize irrigation in the Afram Plains area could rise from US$301, 000 in 2007 to over US$3.5 million by the end of the last management period (2013) as irrigation practice is improved, and the economic strength to increase the acreage for irrigation improves. Even with these margins of profit, the drawdown constraint was not reached in any of the management periods. It is expected that rechargefrom the irrigation water would reclaim the lost hydraulic head. The single significant constraint was the amount of land area that could be developed for irrigation in the area. The profit obtained per unit cubic meter of water used also improved over the same management period.

Journal of Environmental Hydrology↗
USGS2011

Guidance manual for forensic analysis of perchlorate in groundwater using chlorine and oxygen isotopic analyses

Increased health concerns about perchlorate (ClO4-) during the past decade and subsequent regulatory considerations have generated appreciable interest in source identification. The key objective of the isotopic techniques described in this guidance manual is to provide evidence concerning the origin of ClO4- in soils and groundwater and, more specifically, whether that ClO4- is synthetic or natural. Chlorine and oxygen isotopic analyses of ClO4- provide the primary direct approach whereby different sources of ClO4- can be distinguished from each other. These techniques measure the relative abundances of the stable isotopes of chlorine (37Cl and 35Cl) and oxygen (18O, 17O, and 16O) in ClO4- using isotope-ratio mass spectrometry (IRMS). In addition, the relative abundance of the radioactive chlorine isotope 36Cl is measured using accelerator mass spectrometry (AMS). Taken together, these measurements provide four independent quantities that can be used to distinguish natural and synthetic ClO4- sources, to discriminate different types of natural ClO4-, and to detect ClO4- biodegradation in the environment. Other isotopic, chemical, and geochemical techniques that can be applied in conjunction with isotopic analyses of ClO4- to provide supporting data in forensic studies are also described. This guidance manual is intended to provide details of the methodology used to (1) collect ClO4- samples from the environment, particularly from groundwater, which is the main medium of interest for ClO4- source identification; (2) purify the collected ClO4- samples; (3) conduct oxygen (O) and chlorine (Cl) isotopic analyses on the purified samples; and (4) determine probable sources using the resulting isotope data. Current practices for groundwater sampling and quality assurance for sample collection, purification, and measurement of Cl and O isotopes in ClO4- are provided. A detailed case study of source evaluation in groundwater on Long Island is given along with the current literature on the subject of ClO4- source discrimination. ClO4- in the environment is derived from both synthetic and natural sources. Synthetic ClO4- salts, including ammonium perchlorate (NH4ClO4) and potassium perchlorate (KClO4), have been widely used as oxidants by the military and aerospace industry. A variety of commercial products also contain synthetic ClO4-,including fireworks, matches, air bags, chlorine bleach, safety flares, perchloric acid, and chlorate herbicides. Historical disposal practices by the military, aerospace industry, and chemical manufacturers have resulted in groundwater and drinking water contamination with ClO4- in the United States. Isolated contamination from fireworks, road flares, explosives, and perchloric acid has also been reported. However, ClO4- is also a naturally occurring anion. It is present with sodium nitrate (NaNO3) in surficial deposits in the Atacama Desert of Chile at an average concentration of around 0.1% (by mass) of the total soluble salt, and these deposits (sometimes referred to as “Chilean caliche”) were widely used in the United States during the first half of the 20th century as a source of inorganic nitrogen fertilizer. Natural ClO4- that is not associated with Chilean fertilizers has also recently been detected in the vadose zone, groundwaters, and mineral deposits collected from the arid southwestern United States, including 155,000 km2 of groundwater in the Southern High Plains (SHP) of Texas and New Mexico. In addition to synthetic sources, natural ClO4- from both Chilean fertilizers and indigenous sources represents a potentially large source of ClO4- in groundwater and drinking water in the United States.

Report↗
USGS1998

Hydrogeology and groundwater quality of the glaciated valleys of Bradford, Tioga, and Potter Counties, Pennsylvania

The most important sources of groundwater in Bradford, Tioga, and Potter Counties are the stratified-drift aquifers. Saturated sand and gravel primarily of outwash origin forms extensive unconfined aquifers in the valleys. Outwash is underlain in most major valleys by silt, clay, and very fine sand of lacustrine origin that comprise extensive confining units. The lacustrine confining units locally exceed 100 feet in thickness. Confined aquifers of ice-contact sand and gravel are buried locally beneath the lacustrine deposits. Bedrock and till are the basal confining units of the stratifies-drift aquifer systems. Recharge to the stratified-drift aquifers if by direct infiltration of precipitation, tributary-stream infiltration, infiltration of unchanneled runoff at the valley walls, and groundwater inflow from the bedrock and till uplands. Valley areas underlain by superficial sand and gravel contribute about 1 million gallons per day per square mile of water from precipitation to the aquifers. Tributary streams provide recharge of nearly 590 gallons per day per foot of stream reach. Water is added at the rate of 1 million gallons per day per square mile of bordering uplands not drained by tributary streams to the stratified-drift aquifers from unchanneled runoff and groundwater inflow. Induced infiltration can be a major source of recharge to well fields completed in unconfined stratified-drift aquifers that are in good hydraulic connection with surface water. The well fields of an industrial site in North Towanda, a public-water supplier at Tioga Point, and the U.S. Fish and Wildlife Service at Asaph accounted for 75 percent of the 10.8 million gallons per day pf groundwater withdrawn by public suppliers and other selected users in 1985. The well fields tap stratified-drift aquifers that are substantially recharged by induced infiltration or tributary-stream infiltration. Specific-capacity data from 95 wells indicate that most wells completed in stratified-drift aquifers have specific capacities an order of magnitude greater than those completed in till and bedrock, Wells completed in unconfined stratified-drift aquifers and in bedrock aquifers have the highest and lowest median specific capacities -- 24 and 0.80 gallons per minute per foot of drawdown, respectively. Wells completed in confined stratified-drift aquifers and in till have median specific capacties of 11 and 0.87 gallons per minute per foot of drawdown, respectively. The results of 223 groundwater-quality analyses indicate two major hydrogeochemical zones: (1) a zone of unrestricted groundwater flow that contains water of the calcium bicarbonate type (this zone is found in almost all of the stratified-drift aquifers, till, and shallow bedrock systems); and (2) a zone of restricted groundwater slow that contains water of the sodium chloride type (this zone is found in the bedrock, and, in some areas, in till and confined stratified-drift aquifers). Samples pumped from wells that penetrate restricted-flow zones have median concentrations of total dissolved solids, dissolved chloride, and dissolved barium of 840 and 350 milligrams per liter, and 2,100 micrograms per liter, respectively. Excessive concentrations of iron and manganese are common in the groundwater of the study area; about 50 percent of the wells sampled contain water that has iron and manganese concentrations that exceed the U.S. Environmental Protection Agency secondary maximum contaminant levels of 300 and 50 micrograms per liter, respectively. Only water in the unconfined stratified-drift aquifers and the Catskill Formation has median concentrations lower than these limits.

Pennsylvania↗
USGS2011

Groundwater conditions in Utah, spring of 2011

This is the forty-eighth in a series of annual reports that describe groundwater conditions in Utah. Reports in this series, published cooperatively by the U.S. Geological Survey and the Utah Department of Natural Resources, Division of Water Rights, and the Utah Department of Environmental Quality, Division of Water Quality, provide data to enable interested parties to maintain awareness of changing groundwater conditions. This report, like the others in the series, contains information on well construction, groundwater withdrawal from wells, water-level changes, precipitation, streamflow, and chemical quality of water. Information on well construction included in this report refers only to wells constructed for new appropriations of groundwater. Supplementary data are included in reports of this series only for those years or areas that are important to a discussion of changing groundwater conditions and for which applicable data are available. This report includes individual discussions of selected significant areas of groundwater development in the State for calendar year 2010. Most of the reported data were collected by the U.S. Geological Survey in cooperation with the Utah Department of Natural Resources, Division of Water Rights, and the Utah Department of Environmental Quality, Division of Water Quality. This report is also available online at http:// www.waterrights.utah.gov/techinfo/ and http://ut.water.usgs. gov/publications/GW2011.pdf. Groundwater conditions in Utah for calendar year 2009 are reported in Burden and others (2010) and available online at http://ut.water.usgs.gov/ publications/GW2010.pdf. Analytical results associated with water samples collected from each area of groundwater development were compared to State of Utah Maximum Contaminant Levels (MCLs) and secondary drinking-water standards of routinely measureable substances present in water supplies. The MCLs and secondary drinking-water standards can be accessed online at http://www.rules.utah.gov/publicat/code/r309/r309-200. htm#T5. The U.S. Environmental Protection Agency (EPA) drinking-water standards can be accessed at http://www.epa. gov/safewater/mcl.html#mcls. Maximum Contaminant Levels and secondary drinking-water standards were developed for public water systems and do not apply to the majority of wells sampled during this study.

Utah↗
USGS2020

Inventory and analysis of groundwater resources: Theodore Roosevelt National Park, North Dakota

Industrial and commercial developments in western North Dakota potentially could affect the sources of water that contribute to wells, spring flow, and seeps within Theodore Roosevelt National Park. Without basic water resources data, accurately predicting the effects of water withdrawals and water quality concerns related to industrial and commercial developments near the park would be challenging. Water resources in the park include surface water and groundwater. The Little Missouri River and its tributaries cross all three Theodore Roosevelt National Park units and are the primary surface-water features in the park. Groundwater resources include well discharges, springs, and seeps. The geology and hydrogeology of Theodore Roosevelt National Park are defined by the surrounding Williston Basin. Four aquifers are sources of groundwater to the park: unconsolidated aquifers including alluvial systems, the upper Fort Union aquifer, the lower Fort Union aquifer, and the Fox Hills-lower Hell Creek aquifer. Data used for wells, springs, seeps, and water quality in this report were compiled from the U.S. Geological Survey National Water Information System or from the North Dakota State Water Commission. An inventory of 16 wells was completed for sites within the boundaries of the park. In addition to well data, an inventory of 11 springs and seeps was completed. The groundwater-quality analysis had two objectives: (1) to characterize the groundwater chemistry in aquifers underlying the park and (2) to spatially map selected physical properties and chemical constituents of interest. Groundwater-quality data from the North Dakota State Water Commission were summarized, mapped, and used to characterize groundwater for each aquifer in the study area. Spatial concentration distribution maps were constructed for selected physical properties and chemical constituents using summary statistics and exceedances. Piper diagrams were used to classify and characterize groundwater for each aquifer. Future research to help fill data gaps in water resources information for Theodore Roosevelt National Park, including recommendations from previous studies, consists of the following: (1) evaluating the variability in discharge from springs and seeps in comparison to changes in precipitation or other recharge sources, (2) evaluating flow control measures for flowing artesian wells, (3) completing a water rights review, and (4) performing routine water-quality monitoring for wells and springs.

North Dakota↗
USGS2013

A comparison of data-driven groundwater vulnerability assessment methods

Increasing availability of geo-environmental data has promoted the use of statistical methods to assess groundwater vulnerability. Nitrate is a widespread anthropogenic contaminant in groundwater and its occurrence can be used to identify aquifer settings vulnerable to contamination. In this study, multivariate Weights of Evidence (WofE) and Logistic Regression (LR) methods, where the response variable is binary, were used to evaluate the role and importance of a number of explanatory variables associated with nitrate sources and occurrence in groundwater in the Milan District (central part of the Po Plain, Italy). The results of these models have been used to map the spatial variation of groundwater vulnerability to nitrate in the region, and we compare the similarities and differences of their spatial patterns and associated explanatory variables. We modify the standard WofE method used in previous groundwater vulnerability studies to a form analogous to that used in LR; this provides a framework to compare the results of both models and reduces the effect of sampling bias on the results of the standard WofE model. In addition, a nonlinear Generalized Additive Model has been used to extend the LR analysis. Both approaches improved discrimination of the standard WofE and LR models, as measured by the c -statistic. Groundwater vulnerability probability outputs, based on rank-order classification of the respective model results, were similar in spatial patterns and identified similar strong explanatory variables associated with nitrate source (population density as a proxy for sewage systems and septic sources) and nitrate occurrence (groundwater depth).

Milan District↗
USGS2017

Groundwater development stress: Global-scale indices compared to regional modeling

The increased availability of global datasets and technologies such as global hydrologic models and the Gravity Recovery and Climate Experiment (GRACE) satellites have resulted in a growing number of global-scale assessments of water availability using simple indices of water stress. Developed initially for surface water, such indices are increasingly used to evaluate global groundwater resources. We compare indices of groundwater development stress for three major agricultural areas of the United States to information available from regional water budgets developed from detailed groundwater modeling. These comparisons illustrate the potential value of regional-scale analyses to supplement global hydrological models and GRACE analyses of groundwater depletion. Regional-scale analyses allow assessments of water stress that better account for scale effects, the dynamics of groundwater flow systems, the complexities of irrigated agricultural systems, and the laws, regulations, engineering, and socioeconomic factors that govern groundwater use. Strategic use of regional-scale models with global-scale analyses would greatly enhance knowledge of the global groundwater depletion problem.

Groundwater↗
USGS2019

Wetland-scale mapping of preferential fresh groundwater discharge to the Colorado River

Quantitative evaluation of groundwater/surface water exchange dynamics is universally challenging in large river systems, because existing methodology often does not yield spatially‐distributed data and is difficult to apply in deeper water. Here we apply a combined near‐surface geophysical and direct groundwater chemical toolkit to refine fresh groundwater discharge estimates to the Colorado River through a 4‐km 2 wetland that borders the town of Moab, Utah, USA. Preliminary characterization of raw electromagnetic imaging (EMI) data, collected by kayak and by walking, was used to guide additional direct‐contact electrical measurements and installation of new monitoring wells. Chemical data from the wells strongly supported the EMI spatial characterization of preferential fresh groundwater discharge embedded in natural brine groundwaters and weighted to the southern wetland section. Inversion of the EMI data revealed sub‐meter scale detail regarding bulk electrical conductivity zonation across approximately 15.5 km of transects, collected in only 3 days. This electrical detail indicates processes such as salinization of the unsaturated zone and direct discharge through the Colorado River sediments and a tributary creek bed. Overall, the study contributed to a substantial reduction in fresh groundwater discharge estimates previously made using sparse existing well data and a simplified assumption of diffuse fresh groundwater discharge below the entire wetland. EMI will likely become a widely used tool in systems with natural electrical contrast as groundwater/surface water hydrogeologists continue to recognize the prevalence of preferential groundwater discharge processes.

Groundwater↗