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Groundwater & aquifers

Find USGS groundwater and aquifer publications. Search an aquifer name or place, compare source-supplied dates and open reports for hydrogeology, recharge and water-level studies.

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At least 1,405 records · Page 78Linked to original sources

USGS2010

Effects of groundwater-flow paths on nitrate concentrations across two riparian forest corridors

Groundwater levels, apparent age, and chemistry from field sites and groundwater-flow modeling of hypothetical aquifers collectively indicate that groundwater-flow paths contribute to differences in nitrate concentrations across riparian corridors. At sites in Virginia (one coastal and one Piedmont), lowland forested wetlands separate upland fields from nearby surface waters (an estuary and a stream). At the coastal site, nitrate concentrations near the water table decreased from more than 10 mg/L beneath fields to 2 mg/L beneath a riparian forest buffer because recharge through the buffer forced water with concentrations greater than 5 mg/L to flow deeper beneath the buffer. Diurnal changes in groundwater levels up to 0.25 meters at the coastal site reflect flow from the water table into unsaturated soil where roots remove water and nitrate dissolved in it. Decreases in aquifer thickness caused by declines in the water table and decreases in horizontal hydraulic gradients from the uplands to the wetlands indicate that more than 95% of the groundwater discharged to the wetlands. Such discharge through organic soil can reduce nitrate concentrations by denitrification. Model simulations are consistent with field results, showing downward flow approaching toe slopes and surface waters to which groundwater discharges. These effects show the importance of buffer placement over use of fixed-width, streamside buffers to control nitrate concentrations.

Virginia↗

USGS2015

Large fractions of CO 2 -fixing microorganisms in pristine limestone aquifers appear to be involved in the oxidation of reduced sulfur and nitrogen compounds

The traditional view of the dependency of subsurface environments on surface-derived allochthonous carbon inputs is challenged by increasing evidence for the role of lithoautotrophy in aquifer carbon flow. We linked information on autotrophy (Calvin-Benson-Bassham cycle) with that from total microbial community analysis in groundwater at two superimposed—upper and lower—limestone groundwater reservoirs (aquifers). Quantitative PCR revealed that up to 17% of the microbial population had the genetic potential to fix CO 2 via the Calvin cycle, with abundances of cbbM and cbbL genes, encoding RubisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) forms I and II, ranging from 1.14 × 10 3 to 6 × 10 6 genes liter −1 over a 2-year period. The structure of the active microbial communities based on 16S rRNA transcripts differed between the two aquifers, with a larger fraction of heterotrophic, facultative anaerobic, soil-related groups in the oxygen-deficient upper aquifer. Most identified CO 2 -assimilating phylogenetic groups appeared to be involved in the oxidation of sulfur or nitrogen compounds and harbored both RubisCO forms I and II, allowing efficient CO 2 fixation in environments with strong oxygen and CO 2 fluctuations. The genera Sulfuricella and Nitrosomonas were represented by read fractions of up to 78 and 33%, respectively, within the cbbM and cbbL transcript pool and accounted for 5.6 and 3.8% of 16S rRNA sequence reads, respectively, in the lower aquifer. Our results indicate that a large fraction of bacteria in pristine limestone aquifers has the genetic potential for autotrophic CO 2 fixation, with energy most likely provided by the oxidation of reduced sulfur and nitrogen compounds.

Applied and Environmental Microbiology↗

USGS1999

Tributary stream infiltration as a source of herbicides in an alluvial aquifer

Where Walnut Creek flows across the South Skunk River alluvial aquifer, it provides a potential source of herbicides and herbicide metabolites. This straightened reach of the creek loses water and dissolved contaminants to the alluvial aquifer through a layer of fine-grained flood plain deposits. Estimates of potential flux of chemicals were based on measurements taken during baseflow in April 1994 before herbicides were applied to the watershed and in June 1994 after chemical application and when stream discharge included runoff and tile-drainage water. Hydraulic head measurements between the creek and flood plain deposits and between the creek and aquifer confirmed the potential for downward groundwater flow during both sampling periods. Hydraulic conductivity estimates from slug tests were used to calculate an average linear groundwater velocity of 0.5 m d −1 in the fine-grained flood plain deposits. At this velocity, contaminants could be advectively transported to the aquifer within 6 d. The potential for atrazine (2-chloro-4-ethylamino-6-isopropylamino- s -triazine) flux to the aquifer from the creek was estimated to be between 60 and 3000 µg d −1 m −2 . This rate is one to three orders of magnitude greater than the estimated flux via leaching beneath a typical field. If the process of vertical stream leakage occurs in many hydrologic settings, it may constitute a substantial source of herbicides to shallow alluvial aquifers in many areas of the Midwest.

Iowa↗

USGS2012

Sampling history and 2009--2010 results for pesticides and inorganic constituents monitored by the Lake Wales Ridge Groundwater Network, central Florida

The Lake Wales Ridge Monitoring (LWRM) Network was established to provide a long-term record of water quality of the surficial aquifer in one of the principal citrus-production areas of Florida. This region is underlain by sandy soils that contain minimal organic matter and are highly vulnerable to leaching of chemicals into the subsurface. This report documents the 1989 through May 2010 sampling history of the LWRM Network and summarizes monitoring results for 38 Network wells that were sampled during the period January 2009 through May 2010. During 1989 through May 2010, the Network’s citrus land-use wells were sampled intermittently to 1999, quarterly from April 1999 to October 2009, and thereafter quarterly to semiannually. The water-quality summaries in this report focus on the period January 2009 through May 2010, during which the Network’s citrus land-use wells were sampled six times and the non-citrus land-use wells were sampled two times. Within the citrus land-use wells sampled, a total of 13 pesticide compounds (8 parent pesticides and 5 degradates) were detected of the 37 pesticide compounds analyzed during this period. The most frequently detected compounds included demethyl norflurazon (83 percent of wells), norflurazon (79 percent), aldicarb sulfoxide (41 percent), aldicarb sulfone (38 percent), imidacloprid (38 percent), and diuron (28 percent). Agrichemical concentrations in samples from the citrus land-use wells during the 2009 through May 2010 period exceeded Federal drinking-water standards (maximum contaminant levels, MCLs) in 1.5 to 24 percent of samples for aldicarb and its degradates (sulfone and sulfoxide), and in 68 percent of the samples for nitrate. Florida statutes restrict the distance of aldicarb applications to drinking-water wells; however, these statutes do not apply to monitoring wells. Health-screening benchmark levels that identify unregulated chemicals of potential concern were exceeded for norflurazon and diuron in 29 and 7 percent, respectively, of the 2009–2010 samples. A comparison of agrichemical land-use effects on groundwater quality, determined on the basis of samples from LWRM Network wells in citrus and in non-citrus land-use areas, indicated significantly higher (p<0.05) concentrations of inorganic constituents in samples from citrus land-use areas compared to samples from non-citrus areas. These inorganic constituents include calcium, magnesium, chloride, sulfate, potassium, nitrate, aluminum, manganese, strontium, and total nitrogen, and also specific conductance, an indicator of total dissolved solutes in water. In addition to land use, including irrigation, site differences such as soils and groundwater reduction/oxidation conditions might have contributed to the differences in some of these constituents. Pesticide detections were primarily restricted to the citrus land-use wells, where 22 of 23 wells yielded pesticide detections, with a median of four detected pesticide compounds per well. For the non-citrus land-use wells, typically surrounded by mixed land use including developed and undeveloped land, one of the eight sampled wells yielded pesticide detections consisting of norflurazon and its degradate, and the source(s) of these detections might have been active or recently active citrus orchards in the vicinity of this well. Results from the LWRM Network during the 1989 through May 2010 period have provided early warning of chemicals prone to leaching, guidance for developing or modifying chemical usage practices to minimize impacts to groundwater, and a mechanism for prioritizing State sampling of domestic wells to assure safe drinking-water supplies. Given the typically long time period (years to tens of years or longer) required to remove chemical contamination once it enters the groundwater system, groundwater monitoring is important to protect drinking-water sources as well as the numerous lakes in this region, which are closely connected with the surficial aquifer. Long-term monitoring of the LWRM Network is planned to continue providing early warning of potential for groundwater contamination, and to assess spatial and temporal trends in water quality resulting from changes in pesticide-use patterns and in land use.

Florida↗

USGS1987

Potential for aquifer compaction, land subsidence, and earth fissures in the Tucson basin, Pima County, Arizona

In 1979 the U.S. Geological Survey, in cooperation with the city of Tucson, began an investigation of aquifer compaction and its effects in the Tucson basin, including the potential hazards of aquifer compaction and subsequent damage to engineered structures, roads, water supply systems, gaslines, and sewage systems. Groundwater is the main source of water for irrigation, public supply, and industry. Pumping has exceeded recharge for several decades, and declines in groundwater levels have occurred in parts of the basin. In places, water level declines have resulted in small amounts of aquifer compaction and land subsidence. Further water level declines in parts of the Tucson basin may result in large amounts of aquifer compaction and land subsidence. Differential land subsidence may, in places, be accompanied by the formation of earth fissures. Aquifer compaction, land subsidence, and earth fissures can damage or affect the functional capability of manmade features. Specifically, aquifer compaction may damage wells, fissures may damage engineered structures, and differential land subsidence may affect structures such as sewers and water-supply systems that are dependent on gravity for their operation. (USGS)

Open-File Report↗

USGS2012

Groundwater availability in the Lahaina District, west Maui, Hawai'i

Most of the public water supply in the Lahaina District, west Maui, Hawai'i, is pumped from a freshwater lens in volcanic rocks. Because of population growth, groundwater withdrawals from wells in this area are expected to increase from about 5.8 million gallons per day in 2007 to more than 11 million gallons per day by 2030. Currently (2011), the salinity of water pumped from some of the wells in the area exceeds acceptable limits for drinking water. The expected increasing demand for water in an area in which the salinity of water is already unacceptable has led to concern over the long-term sustainability of withdrawals from existing and proposed wells. A three-dimensional numerical groundwater flow and transport model was developed to simulate the effects of hypothetical withdrawal and recharge scenarios on water levels and on the transition zone between freshwater and saltwater. The model was constructed using time-varying recharge, withdrawals, and ocean levels. Hydraulic characteristics used to construct the model were initially based on published estimates but ultimately were varied to obtain better agreement between simulated and measured water levels and salinity profiles in the modeled area during 1926–2008. Scenarios included groundwater withdrawal at 2008–09 rates and locations with projected recharge (based on 2000–04 land use, no agricultural irrigation, and the rainfall record for the period 1926–2004) and withdrawal at redistributed rates and locations with several different recharge scenarios. Simulation results indicate that continuing the 2008–09 withdrawal rates and distribution (6.3 million gallons per day from 21 wells) into the future would result in decreased water levels, a thinner freshwater lens, and increased salinity of water pumped from wells. Groundwater demand projections and proposed new well sites were used to produce a projected withdrawal rate and distribution during 2010–39. Simulation results from this projected withdrawal scenario (11.2 million gallons per day from 28 wells, including 10 proposed wells) also indicate decreased water levels, a thinner freshwater lens, increased water salinity, and unacceptable salinity at several current withdrawal sites, mainly in the Honokōwai Aquifer System; however, more groundwater is available than in the previous scenario. A simulation in which injection of treated wastewater is stopped indicates that several wells will have increased salinities compared to the scenario in which injection continues. A scenario in which increased groundwater withdrawal was redistributed in an attempt to maximize withdrawal while maintaining acceptable salinities in the withdrawn water was simulated. The redistributed withdrawal simulates 20.7 million gallons per day of withdrawal from 26 wells or well fields in the Lahaina District. Simulation results indicate the following: (1) average water levels decrease by about 0.5–1 feet and the transition zone rises 20–50 feet in some areas after 30 years, mainly in the Launiupoko Aquifer System near the proposed wells, and (2), all wells produce water with salinities in the acceptable class (less than one-percent seawater salinity) after 30 years.

Hawai'i↗

USGS2016

Ambient water quality in aquifers used for drinking-water supplies, Gem County, southwestern Idaho, 2015

In recent years, the rapid population growth in Gem County, Idaho, has been similar to other counties in southwestern Idaho, increasing about 54 percent from 1990 to 2015. Because the entire population of the study area depends on groundwater for drinking water supply (either from self-supplied domestic, community, or municipal-supply wells), this population growth, along with changes in land use (including potential petroleum exploration and development), indicated to the public and local officials the need to assess the quality of groundwater used for human consumption. To this end, the U.S. Geological Survey, in cooperation with Gem County and the Idaho Department of Environmental Quality, assessed the quality of groundwater from freshwater aquifers used for domestic supply in Gem County. A total of 47 domestic or municipal wells, 1 spring, and 2 surface-water sites on the Payette River were sampled during September 8–November 19, 2015. The sampled water was analyzed for a variety of constituents, including major ions, trace elements, nutrients, bacteria, radionuclides, dissolved gasses, stable isotopes of water and methane, and either volatile organic compounds (VOCs) or pesticides. To better understand analytical results, a conceptual hydrogeologic framework was developed in which three hydrogeologic units were described: Quaternary-Tertiary deposits (QTd), Tertiary Idaho Group rocks (Tig), and Tertiary-Cretaceous igneous rocks (TKi). Water levels were measured in 30 wells during sampling, and a groundwater-level altitude map was constructed for the QTd and Tig units showing groundwater flow toward the Emmett Valley and Payette River. Analytical results indicate that groundwater in Gem County is generally of good quality. Samples collected from two wells contained water with fluoride concentrations greater than the U.S. Environmental Protection Agency (EPA) Maximum Contaminant Level (MCL) of 4 milligrams per liter (mg/L), six wells contained arsenic at concentrations greater than the EPA MCL of 10 micrograms per liter, and a sample from one well exceeded the MCL of 15 picocuries per liter for alpha particles. Although previous samples collected from some wells in Gem County contained nitrate concentrations greater than the MCL of 10 mg/L, the largest concentration detected in the current study was 5.2 mg/L. Total coliform bacteria was detected in four groundwater samples. Three volatile organic compounds (VOCs) were detected in samples collected from five wells, and five compounds of the triazine class of herbicides were detected in samples from five wells; no concentrations were greater than applicable EPA MCLs. Methane was detected in samples from 36 wells, with the concentration in 1 well large enough to be considered an explosion hazard by U.S. Office of Surface Mining guidelines. Stable isotope signatures of methane in six samples suggest that naturally occurring methane in Gem County is probably of both thermogenic and biogenic origin.

Idaho↗

USGS2022

Trends in groundwater levels, and orthophosphate and nitrate concentrations in the Middle Snake River Region, south-central Idaho

The U.S. Geological Survey (USGS) evaluated nitrate and orthophosphate concentrations in groundwater for temporal trends (monotonic and step trends) for the middle Snake River region (Cassia, Gooding, Jerome, Lincoln, Minidoka, and Twin Falls Counties) in south-central Idaho using the Regional Kendall test (monotonic trends) and the Wilcoxon signed rank test (step trends). The study evaluated two trend periods: 2000–09 and 2010–19/20. The study area was divided into six hydrogeologic zones (HZs) that had similar geologic and hydrologic characteristics and that correlated with county boundaries where possible. Two well networks sampled by the USGS National Water Quality Program within the HZs were also evaluated. The northern Gooding County HZ had statistically significant increasing nitrate concentration trends for both the monotonic and step trends in the early trend period, while the Cassia and Jerome/Southern Gooding County HZs only had one of the statistical tests with statistically significant increasing nitrate concentrations. The Minidoka County HZ had conflicting results between the two statistical tests for the early time period with a statistically significant increasing monotonic trend in nitrate concentration and a statistically significant decreasing step trend. The differing results between these two statistical tests indicates the significance of concentration data during the middle of the time period. Both the Lincoln and Twin Falls County HZs did not have statistically significant trends for either test during either time period as well as the Northern Gooding County HZ for the latter time period. The Minidoka County HZ had statistically significant nitrate trends for both tests in the latter time period along with one of the trend tests for the Cassia and Jerome/Southern Gooding County HZ. Most of the nitrate concentration trend rates are low from 0.01 to 0.12 milligram per liter per year (mg/L/year) with the northern Gooding County HZ having the highest trend rate during the early time period of 0.28 mg/L/year for the step trend and 0.55 mg/L/year for the monotonic trend. All the HZs and both well networks had statistically significant increasing orthophosphate-concentrations trends in groundwater for the early time period except for the Lincoln County HZ and the step-trend for the Minidoka County HZ. Orthophosphate concentration trend rates for the early period were low, ranging from 0.001 to 0.015 mg/L/year. Only two HZs and the well networks had enough orthophosphate concentration data available in the latter time period to do statistical analysis. The two HZs (Minidoka and Southern Gooding/Jerome County) both have decreasing orthophosphate concentration trends, with only the monotonic trend for the Southern Gooding/Jerome County HZ being statistically significant at 90 percent with a rate of −0.001 mg/L/year. Groundwater levels in two well networks in the eastern Snake River Plain aquifer were also evaluated for trends (monotonic and step), with both networks having statistically significant declining groundwater levels for the 1993–2009 trend period. The latter trend period (2010–20) had statistically significant declining groundwater levels for the A&B well network and statistically significant increasing groundwater levels for the Jerome/Gooding well network, which is downgradient from an aquifer recharge area.

Idaho↗

USGS2023

Evaluation of alternative groundwater-withdrawal scenarios on water levels in Kingsbury Pond, upper Charles River Basin, eastern Massachusetts

Kingsbury Pond is a glacial kettle pond in the town of Norfolk, Massachusetts, in the Mill River Basin, which is part of the Upper Charles River Basin in eastern Massachusetts. The pond is hydraulically connected to the surrounding groundwater-flow system, and water levels in the pond fluctuate in response to recharge to the aquifer from precipitation and wastewater return flows through septic systems, to withdrawals from the aquifer at nearby wells, and to precipitation directly on the pond surface. Concerns about the effects of withdrawals on water levels in the pond prompted an investigation to better understand the hydrology of Kingsbury Pond and its response to groundwater withdrawals and to determine if withdrawals from wells in Franklin, Mass., can be modified to simultaneously reduce the effect on water levels in the pond and yet meet the water-supply demands of the Town of Franklin. An existing, transient groundwater-flow model of the Upper Charles River Basin was modified for this study in the area near Kingsbury Pond to improve representation of the hydrologic system near the pond. The mean annual water-level altitude simulated for the pond for nonpumping conditions using the modified model is 136 feet (ft), which falls within the range of likely annual pond-altitude fluctuations of 135 to 140 ft estimated for average hydrologic conditions before the beginning of withdrawals at two nearby wells operated by the Town of Franklin (wells FR–04 and FR–05). The mean annual water-level altitude at the pond decreased by 3.8 ft to 132.2 ft for simulated mean monthly withdrawal rates at all wells within the Upper Charles River Basin from 2010 to 2019 (referred to as the baseline withdrawal condition). A groundwater management model that links the groundwater-flow model with a mathematical optimization method was developed to evaluate the effects of three alternative groundwater-withdrawal scenarios for the Franklin public-water system on water levels in Kingsbury Pond. In the first scenario, monthly withdrawal rates at wells FR–04 and FR–05 were increased from the baseline withdrawal rates to their maximum authorized rates for all months of the year; all other Franklin wells were specified at their baseline withdrawal rates. This scenario resulted in a mean annual water-level altitude at the pond of 129.3 ft, or a mean annual decline of 6.7 ft compared with nonpumping conditions and a decline of 2.9 ft compared with baseline conditions. The results of the second scenario showed that water levels in the pond can be increased relative to 2010–19 conditions while meeting Franklin’s 2010–19 monthly water-supply demands if withdrawals at wells FR–04 and FR–05 were shifted to other Franklin wells. In this scenario, monthly withdrawal rates at wells FR–04 and FR–05 were decreased from their baseline rates to one-third their maximum practical rates for all months of the year; increased withdrawal rates at other Franklin wells were determined by the management model. The decrease in withdrawal rates at wells FR–04 and FR–05 resulted in a mean water-level altitude at the pond of 134.1 ft, which was equivalent to a 51 percent increase (improvement) in the mean annual water level of the pond relative to the baseline condition. A third scenario was done to determine if Franklin’s existing water-supply system has the capacity to meet the mean annual maximum permitted withdrawal rate of the system of 3.45 million gallons per day while maintaining monthly withdrawal rates at wells FR–04 and FR–05 at their 2010–19 rates and water levels in Kingsbury Pond at baseline conditions. The analysis indicated that the capacity of the system cannot meet the increased demand during some months of the year with withdrawal rates at the two wells fixed at their monthly 2010–19 rates; however, the existing system is capable of meeting about 90 percent of the maximum permitted rate (3.10 million gallons per day) by increasing withdrawal rates at other Franklin wells above their 2010–19 rates.

Massachusetts↗

USGS2024

Simulated effects of projected 2014–40 withdrawals on groundwater flow and water levels in the New Jersey Coastal Plain

Abstract Groundwater flow between 2014 through 2040 was simulated in the New Jersey Coastal Plain based on three withdrawal scenarios. Two of the scenarios were based on projected population trends and the assumption of water conservation; the nominal water-loss scenario projected a status quo in the efficiency of water loss in the delivery systems whereas the optimal water-loss scenario projected a better water-loss efficiency resulting in less withdrawals. The third scenario assumes that all wells will withdraw water at their full allocation level which is generally much more than reported withdrawals in 2013 or projected under the other two scenarios. Maps and summaries of heads and drawdowns are presented for nine confined aquifers. All the aquifers have areas with heads below sea level by 2040. Of the three scenarios, the drawdowns are most extreme in the full allocation scenarios; there are large areas of head decline greater than 20 feet in 5 of the 9 confined aquifers. The exceptions are the Vincentown aquifer, despite some areas of large drawdown in the vicinity of wells, and the three Potomac-Raritan-Magothy (PRM) aquifers where withdrawals are regulated by Critical Area restrictions. The nominal and optimal water-loss scenarios have some areas of head declines; most are less than 15 feet. The simulation of these scenarios shows some extensive areas of head recovery as well—especially in the aquifers that are regulated by the Critical Area restrictions. Budgets of inflow and outflow components were calculated for 44 hydrologic budget areas (HBAs). The budget analysis shows that the water movement is complex and varies based on the aquifer geometry and location of pumping wells. Flow components between the unconfined and confined parts of the system were summarized by HUC11 (hydrologic unit code 11) basins.

New Jersey↗

USGS1985

Geochemical controls on dissolved sodium in basalt aquifers of the Columbia Plateau, Washington

Miocene basaltic aquifers of the Columbia Plateau are the principal source of water for agricultural, domestic, and municipal use in Washington State. Irrigation with groundwaters with relatively high sodium concentrations has been cause for concern in recent years, because of the tendency of such waters to reduce soil permeability. Chemical reactions involving groundwater and the basalts are the primary mechanisms responsible for the input of sodium to groundwater in the plateau. This conclusion is supported by the sequence of secondary alteration products found and by progressive changes in groundwater chemistry with depth and position along regional flow paths. Upgradient and shallow groundwaters have low sodium concentrations and sodium-adsorption ratios (SAR's), and are predominantly calcium sodium bicarbonate waters. Groundwaters from deeper and downgradient locations have higher sodium concentrations and SAR 's and are predominantly sodium bicarbonate water. Volcanic glass and cryptocrystalline matrix are the major sources of groundwater sodium, and are dissolved by a combination of silicate hydrolysis and dissolution by carbonic acid. Magnesium, iron, and calcium are removed from solution by the formation of an iron magnesium smectite, calcite , and amorphous iron oxyhydroxide. The addition of sodium, silicon, and potassium by dissolution of basalt exceeds their removal by the precipitation of secondary minerals, and their concentrations increase in the initial stages of this process. In later stages, these continued increases produce a water saturated with clinoptilolite and silica phases, and these begin to precipitate. While the timing of these processes is unclear, the mineralogy of secondary alteration and estimated cooling rates of the basalt flows suggest that observed alteration products formed primarily at low temperature, under conditions similar to those existing at the present time. (USGS)

Water-Resources Investigations Report↗

USGS1986

Three-dimensional steady-state simulation of flow in the sand-and-gravel aquifer, southern Escambia County, Florida

The sand-and-gravel aquifer is the only freshwater aquifer in southern Escambia County, Florida and is the source of public water supply for the area, including the City of Pensacola. The aquifer was simulated by a two-layer, digital model to provide hydrologic information for water resource planning. The lower layer represents the main-producing zone; the upper layer represents all of the aquifer above the main-producing zone including an unconfined zone and discontinuous perched, confined , and confining zones. The model was designed for steady-state simulation and predicts the response of the aquifer (changes in water levels) to groundwater pumping where steady-state conditions have been reached. Input to the model includes matrices representing constant-head nodes, starting head, transmissivity of layer 1, leakance between layers 1 and 2, lateral hydraulic conductivity of layer 2, and altitude of the base layer 2. The sources of water to the model are from recharge by infiltrated precipitation (estimated from base runoff), inflow across boundaries, and induced recharge from river leakance in periods of prolonged groundwater pumping. Model output includes final head and drawdown for each layer and total values for discharge and recharge in the model area. The model was calibrated for 1972 pumping and tested by simulating pumpages during 1939-40, 1958, and 1977. Sensitivity analyses showed water levels in both layers were most sensitive to changes in the recharge matrix and least sensitive to river leakage. Suggestions for further development of the model include subdivision and expansion of the grid, assignment of storage coefficients for transient simulations, more intensive study of the stream-aquifer relations, and consideration of the effects of infiltration basins on recharge. (Author 's abstract)

Water-Resources Investigations Report↗

USGS1988

A digital simulation of the glacial-aquifer system in Sanborn and parts of Beadle, Miner, Hanson, Davison, and Jerauld counties, South Dakota

The drought in South Dakota from 1974-76 and the near drought conditions in 1980-81 have resulted in increased demands on the groundwater resources within many of the irrigated areas of the James River basin in eastern South Dakota. These increases in demand for irrigation water from the glacial aquifer system, and continued requests to the State for additional irrigation well permits, have created a need for a systematic water management program to avoid over-development of this system in the James River basin. An equally spaced grid containing 56 rows and 52 columns used to simulate the glacial aquifer system, was calibrated using water level data collected before significant groundwater development (before 1973). The aquifer was also simulated in 11 annual transient stress periods from 1973 through 1983 and in 12 monthly transient stress periods for 1976. The simulated pre-development potentiometric heads were compared to average water levels from 32 observation wells to check the accuracy of the simulate potentiometric surface. The average arithmetic difference between the simulated and observed water levels was 1.68 ft and the average absolute difference was 4.38 ft. The non-pumping steady-state simulated water budget indicates that recharge from precipitation accounts for 97.1% of the water entering the aquifer and evapotranspiration accounts for 98.2% of the water leaving the aquifer. The sensitivity analysis of the steady-state model indicates that the model is most sensitive to reductions in recharge and least to changes in hydraulic conductivity. The maximum annual recharge varied from 0.10 inch in 1976 to 8.14 inches in 1977. The potential annual evapotranspiration varied from 29.9 inches in 1982 to 48.9 inches in 1976. Withdrawals from the glacial aquifer system increased 2.6 times between 1975 and 1976. The average annual arithmetic difference between the simulated and observed water levels ranged from 3.88 ft in 1974 to 2.23 ft in 1982; the average absolute difference ranged from 4.70 ft in 1973 to 11.70 ft in 1982. In the 1976 monthly transient simulation, the maximum annual recharge rate 0.10 inch was distributed over the months of March, April, and September. The potential monthly evapotranspiration rate ranged from 12.50 inches in August to 0.00 inch during the winter when the ground was frozen. (Author 's abstract)

Water-Resources Investigations Report↗

USGS1993

Development, calibration, and testing of ground-water flow models for the Mississippi River Valley alluvial aquifer in eastern Arkansas using one-square-mile cells

Significant water-level declines in the Mississippi River Valley alluvial aquifer prompted the need to better understand the flow system in the aquifer which, in turn, led to the development of digital groundwater flow models of the alluvial aquifer. Two models were developed in the eastern Arkansas study area with the Arkansas River dividing the study area and functioning as a hydrologic boundary to the models. Both models simulate groundwater flow in one layer with recharge entering the aquifer from head-dependent surface infiltration through the overlying confining unit and from seepage through river beds. Digital models were used to simulate flow in the aquifer during seven stress periods between 1918 and 1987. Pumpage used in the simulations ranged from 83,400,000 to 412,000,000 cu ft/d in the north model and from 12,800,000 to 58,500,000 cu ft/d in the south model. Three different spatial and temporal pumpage scenarios were tested to simulate pumpage stress in the models. The pumpage distribution used in the calibrated model was based on a combination of all three scenarios. Several criteria were used during model development to determine how well the model simulated conditions in the aquifer. Potentiometric maps of model-computed water levels were compared to measured data to check the computed water levels and direction of flow. Hydrographs of observation wells were compared to computed water levels at corresponding model cells to assess the temporal distribution of pumpage. A root-mean-square error analysis was performed during calibration by comparing observation-well and model-computed water levels for 1972. Sensitivity analyses were performed to determine the effects of changes in input parameters on computed heads (water levels). Both models were sensitive to changes in recharge and pumpage but the south model generally was less sensitive than the north model.

Water-Resources Investigations Report↗

USGS1999

Effects on ground-water levels in the Missouri River alluvial aquifer caused by changes in Missouri River stage, Fremont and Monona Counties, Iowa

An analysis of available hydrologic data was conducted to evaluate the effects on groundwater levels in the Missouri River alluvial aquifer caused by changes in Missouri River stage at selected sites in Fremont and Monona Counties in western Iowa. Daily mean ground-water levels and river stage measured during November 1995- September 1996, simulated daily mean river stage for November 1995-December 1996 derived from simulated daily mean discharge for eight alternative water-management plans for the Missouri River, and simulated daily mean ground-water levels for November 1995- December 1996 for selected water-management plans were used in the study. The measured data represent hydrologic conditions for the Current (1998) Water-Control Plan of the U.S. Army Corps of Engineers. Ground-water levels in the alluvial aquifer vary in response to river stage, precipitation, proximity to drainage ditches, evapotranspiration, and pumpage. In Fremont County, measured ground-water levels generally were lower than river stage during the spring, summer, and fall months. In Monona County, measured ground-water levels generally were higher than river stage. Water levels in wells at distances greater than about 8,000 feet from the river in Fremont County and about 6,500 feet in Monona County likely were more affected by precipitation or proximity to drainage ditches than by river stage. Changes in river stage likely affect groundwater levels in Fremont County to a greater degree than in Monona County. In Fremont County, the hydraulic gradient generally is from the river to the aquifer; in Monona County, the gradient generally is from the aquifer to the river. The response of ground- water levels to changes in river stage in Monona County is less apparent than in Fremont County. The higher ground-water levels in Monona County indicate that the effects of other factors, such as differences in recharge from precipitation and aquifer properties, are more dominant than in Fremont County. Generally, the effects of simulated river stage caused higher simulated ground-water levels in Fremont and Monona Counties at distances less than 10,000 feet from the river during the spring months for selected alternatives to the Current Water-Control Plan that target increased benefits to fish and wildlife. Local hydrogeologic conditions will determine how significantly the possible 1- to 4-foot change in ground-water levels affects land use within 10,000 feet of the river. For example, lower river stage and ground-water levels during the mid-summer months could improve drainage in lowland areas during periods of greater-than-normal precipitation. Actual depth to ground water might be controlled by factors other than river stage, such as proximity to drainage ditches and local differences in recharge by precipitation, discharge from evapotranspiration, aquifer properties, and land-surface altitude.

Iowa↗

USGS2022

Determination of recharge areas that supply decades old groundwater to creeks inhabited by the threatened Okaloosa darter

The Okaloosa darter ( Etheostoma okaloosae ) is a diminutive, perch-like, benthic fish that inhabits only six small, clear, and shallow creek systems that flow almost entirely within Eglin Air Force Base in the panhandle of northwest Florida. Listed as Endangered by the U.S. Fish and Wildlife Service (USFWS) in 1973, improvements in erosion control and habitat restoration led to the Okaloosa darter being downlisted from Endangered to Threatened in 2011. However, the long-term management of the species is hampered by the lack of knowledge of the spatial extent of the recharge areas that ultimately support creek flow through groundwater discharge. To address this lack of data, we collected groundwater samples from the sand and gravel aquifer beneath 11 headwater and 11 downgradient sites across six creek basins during February and December 2020. The groundwater samples were collected from 1 to 1.2 m beneath the creek bottom. Concentrations of sulfur hexafluoride (SF 6 ) were analyzed and used to calculate groundwater age (residence time), and indicated that at the 11 headwater sites, recharge occurred between 11 and 28 years ago. Groundwater ages in downgradient parts of the same creeks indicated that recharge occurred between 5 and 25 years ago. When combined with representative values of hydraulic conductivity for the sand and gravel aquifer, the ages reveal that the extent of the maximum recharge distance from the sampling sites ranged from about 222 to 2011 m from the creeks. This new information can be used by natural resource managers as additional evidence to support the USFWS Recovery Plan and proposed delisting of the Okaloosa darter from the Endangered Species List. Moreover, these results may also be useful to fisheries biologists to incorporate groundwater inputs to facilitate fisheries management.

Florida↗

USGS2007

A simulation of groundwater discharge and nitrate delivery to chesapeake bay from the lowermost delmarva peninsula, USA

A groundwater model has been developed for the lowermost Delmarva Peninsula, USA, that simulates saltwater intrusion into local confined aquifers and nitrate delivery to the Chesapeake Bay from the surficial aquifer. A flow path and groundwater-age analysis was performed using the model to estimate the timing of nitrate delivery to the bay over the next several decades. The simulated mean and median residence times of groundwater in the lowermost peninsula are 30 and 15 years, respectively. Current and future nitrate concentrations in coastal groundwater discharge were simulated based on local well data that include nitrate concentrations and groundwater age. A simulated future-trends analysis indicates that nitrate that has been applied to agricultural regions over the last few decades will continue to discharge into the bay for several decades to come. This study highlights the importance of considering the groundwater lag time that affects the mean transport time from diffuse contamination sources.

Conference Paper↗

USGS2023

A Monte-Carlo chemical budget approach to assess ambient groundwater flow in bedrock open boreholes

In low-permeability rocks, ambient groundwater flow in open boreholes may go undetected using conventional borehole-flowmeter tools and alternative approaches may be needed to identify flow. Understanding ambient flow in open boreholes is important for tracking of cross contamination in groundwater. Chlorinated volatile organic compound (CVOC) concentrations from three open boreholes set in a crystalline-rock aquifer (two of three open boreholes) and a siltstone aquifer (one of three open boreholes) were examined using a new approach and associated software program called the AFCE (Aqueous-Flow-Concentration-Estimator). The program allows comparison of coupled chemical datasets through a Monte-Carlo simulation and a chemical-budget approach to assess ambient groundwater flow in open boreholes. The coupled datasets required for the comparison include aqueous CVOC concentrations from groundwater samples from (1) discrete fractures, such as those measured from temporary deployment of straddle-borehole packer assemblies; and (2) the concentration of the open borehole (wellbore) water, as measured by a vertical profile of passive samplers from within the same open borehole. Because results from the passive samplers represent a composite mixture of the results from the discrete samples under ambient groundwater-flow conditions, potentially at unknown proportions, the comparison between coupled datasets affords the ability to discern likely water contributions of CVOC from discrete fractures (or fracture zones), and which fractures may be dominating the water chemistry of the open borehole.

Groundwater Monitoring and Remediation↗