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At least 1,657 records · Page 92Linked to original sources

USGS2017

Simulated effects of Lower Floridan aquifer pumping on the Upper Floridan aquifer at Barbour Pointe, Chatham County, Georgia

Steady-state simulations using a revised regional groundwater-flow model based on MODFLOW were run to assess the potential long-term effects on the Upper Floridan aquifer (UFA) of pumping the Lower Floridan aquifer (LFA) at well 36Q398, located at Barbour Pointe in coastal Georgia near Savannah. Simulated pumping of well 36Q398 at a rate of 750 gallons per minute (gal/min; or 1.08 million gallons per day [Mgal/d]) indicated a maximum drawdown of about 2.19 feet (ft) in the UFA directly above the pumped well and at least 1 ft of drawdown within a nearly 190-square-mile area (scenario A). Induced vertical leakage from the UFA provided about 98 percent of the water to the pumped well. Simulated pumping of well 36Q398 caused increased downward leakage in all layers above the LFA, decreased upward leakage in all layers above the LFA, increased inflow to and decreased outflow from lateral specified-head boundaries in the UFA and LFA, and an increase in the volume of induced inflow from the general-head boundary representing outcrop units. Water budgets for scenario A indicated that changes in inflows and outflows through general-head boundaries would compose about 45 percent of the simulated pumpage from well 36Q398, with the remaining 55 percent of the pumped water derived from flow across lateral specified-head boundaries. Additional steady-state simulations were run to evaluate a pumping rate in the UFA of 240 gal/min (0.346 Mgal/d), which would produce an equivalent maximum drawdown in the UFA as pumping from well 36Q398 in the LFA at a rate of 750 gal/min (called the “drawdown offset”; scenario B). Simulated pumping in the UFA for the drawdown offset produced about 2.18 ft of drawdown, comparable to 2.19 ft of drawdown in the UFA simulated in scenario A. Water budgets for scenario B also provided favorable comparisons with scenario A, indicating that 42 percent of the drawdown-offset pumpage (0.346 Mgal/d) in the UFA originates as increased inflow and decreased outflow across general-head boundaries from overlying units in the surficial and Brunswick aquifer systems and that the remaining simulated pumpage originates as flow across general- and specified-head boundaries within the UFA and LFA. The revised model was evaluated for sensitivity by first altering horizontal and vertical hydraulic conductivity in the Lower Floridan semiconfining unit and then adjusting horizontal and vertical hydraulic conductivity in the LFA to match the 35.6 ft of drawdown at pumping well 36Q398. These adjustments also affected the maximum simulated drawdown in the UFA and the equivalent offset pumping in the UFA that would produce the same amount of drawdown. The maximum drawdown in the UFA ranged from 1.82 to 2.57 ft and the equivalent offset pumping in the UFA ranged from 199 to 278 gal/min. The revised model reasonably depicts changes in groundwater levels resulting from pumping the LFA at Barbour Pointe at a rate of 750 gal/min. Results are limited, however, by the same model assumptions and design as the original model, and placement of boundaries and type of boundary used exert the greatest control on overall groundwater flow and interaquifer leakage in the system. Simulation results have improved regional characterization of the Floridan aquifer system, which could be used by State officials in evaluating requests for groundwater withdrawal from the LFA.

Georgia↗

USGS2016

Groundwater contaminant science activities of the U.S. Geological Survey in New England

Aquifers in New England provide water for human needs and natural ecosystems. In some areas, however, aquifers have been degraded by contaminants from geologic and human sources. In recent decades, the U.S. Geological Survey has been a leader in describing contaminant occurrence in the bedrock and surficial aquifers of New England. In cooperation with Federal, State, and local agencies, the U.S. Geological Survey has also studied the vulnerability of groundwater to contaminants, the factors affecting the geographic distribution of contaminants, and the geochemical processes controlling contaminant transport and fate. This fact sheet describes some of the major science needs in the region related to groundwater contaminants and highlights recent U.S. Geological Survey studies that provide a foundation for future investigations.

New England↗

USGS2016

Assessing groundwater depletion and dynamics using GRACE and InSAR: Potential and limitations

In the last decade, remote sensing of the temporal variation of ground level and gravity has improved our understanding of groundwater dynamics and storage. Mass changes are measured by GRACE (Gravity Recovery and Climate Experiment) satellites, whereas ground deformation is measured by processing synthetic aperture radar satellites data using the InSAR (Interferometry of Synthetic Aperture Radar) techniques. Both methods are complementary and offer different sensitivities to aquifer system processes. GRACE is sensitive to mass changes over large spatial scales (more than 100,000 km 2 ). As such, it fails in providing groundwater storage change estimates at local or regional scales relevant to most aquifer systems, and at which most groundwater management schemes are applied. However, InSAR measures ground displacement due to aquifer response to fluid-pressure changes. InSAR applications to groundwater depletion assessments are limited to aquifer systems susceptible to measurable deformation. Furthermore, the inversion of InSAR-derived displacement maps into volume of depleted groundwater storage (both reversible and largely irreversible) is confounded by vertical and horizontal variability of sediment compressibility. During the last decade, both techniques have shown increasing interest in the scientific community to complement available in situ observations where they are insufficient. In this review, we present the theoretical and conceptual bases of each method, and present idealized scenarios to highlight the potential benefits and challenges of combining these techniques to remotely assess groundwater storage changes and other aspects of the dynamics of aquifer systems.

Groundwater↗

USGS2017

New insights into nitrate dynamics in a karst groundwater system gained from in situ high-frequency optical sensor measurements

Understanding nitrate dynamics in groundwater systems as a function of climatic conditions, especially during contrasting patterns of drought and wet cycles, is limited by a lack of temporal and spatial data. Nitrate sensors have the capability for making accurate, high-frequency measurements of nitrate in situ, but have not yet been evaluated for long-term use in groundwater wells. We measured in situ nitrate continuously in two groundwater monitoring wells —one rural and one urban—located in the recharge zone of a productive karst aquifer in central Texas in order to resolve changes that occur over both short-term (hourly to daily) and long-term (monthly to yearly) periods. Nitrate concentrations, measured as nitrate-nitrogen in milligrams per liter (mg/L), during drought conditions showed little or no temporal change as groundwater levels declined. During aquifer recharge, extremely rapid changes in concentration occurred at both wells as documented by hourly data. At both sites, nitrate concentrations were affected by recharging surface water as evidenced by nitrate concentrations in groundwater recharge (0.8–1.3 mg/L) that were similar to previously reported values for regional recharging streams. Groundwater nitrate concentrations responded differently at urban and rural sites during groundwater recharge. Concentrations at the rural well (approximately 1.0 mg/L) increased as a result of higher nitrate concentrations in groundwater recharge relative to ambient nitrate concentrations in groundwater, whereas concentrations at the urban well (approximately 2.7 mg/L) decreased as a result of the dilution of higher ambient nitrate concentrations relative to those in groundwater recharge. Notably, nitrate concentrations decreased to as low as 0.8 mg/L at the urban site during recharge but postrecharge concentrations exceeded 3.0 mg/L. A return to higher nitrate concentrations postrecharge indicates mobilization of a localized source of elevated nitrate within the urbanized area of the aquifer. Changes in specific conductance were observed at both sites during groundwater recharge, and a significant correlation between specific conductance and nitrate (correlation coefficient [R] = 0.455) was evident at the urban site where large (3-fold) changes in nitrate occurred. Nitrate concentrations and specific conductance measured during a depth profile indicated that the water column was generally homogeneous as expected for this karst environment, but changes were observed in the most productive zone of the aquifer that might indicate some heterogeneity within the complex network of flow paths. Resolving the timing and magnitude of changes and characterizing fine-scale vertical differences would not be possible using conventional sampling techniques. The patterns observed in situ provided new insight into the dynamic nature of nitrate in a karst groundwater system.

Texas↗

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↗

USGS2016

Evaluating geothermal and hydrogeologic controls on regional groundwater temperature distribution

A one-dimensional (1-D) analytic solution is developed for heat transport through an aquifer system where the vertical temperature profile in the aquifer is nearly uniform. The general anisotropic form of the viscous heat generation term is developed for use in groundwater flow simulations. The 1-D solution is extended to more complex geometries by solving the equation for piece-wise linear or uniform properties and boundary conditions. A moderately complex example, the Eastern Snake River Plain (ESRP), is analyzed to demonstrate the use of the analytic solution for identifying important physical processes. For example, it is shown that viscous heating is variably important and that heat conduction to the land surface is a primary control on the distribution of aquifer and spring temperatures. Use of published values for all aquifer and thermal properties results in a reasonable match between simulated and measured groundwater temperatures over most of the 300 km length of the ESRP, except for geothermal heat flow into the base of the aquifer within 20 km of the Yellowstone hotspot. Previous basal heat flow measurements (&sim;110 mW/m 2 ) made beneath the ESRP aquifer were collected at distances of >50 km from the Yellowstone Plateau, but a higher basal heat flow of 150 mW/m 2 is required to match groundwater temperatures near the Plateau. The ESRP example demonstrates how the new tool can be used during preliminary analysis of a groundwater system, allowing efficient identification of the important physical processes that must be represented during more-complex 2-D and 3-D simulations of combined groundwater and heat flow.

Idaho↗

USGS2021

Assessment of water availability in the Osage Nation using an integrated hydrologic-flow model

The Osage Nation of northeastern Oklahoma, conterminous with Osage County, covers about 2,900 square miles. The area is primarily rural with 62 percent of the land being native prairie grass, and much of the area is used for cattle ranching and extraction of petroleum and natural gas. Protection of water rights are important to the Osage Nation because of its reliance on cattle ranching and the potential for impairment of water quality by petroleum extraction. Additionally, the potential for future population increases, demands for water from neighboring areas such as the Tulsa metropolitan area, and expansion of petroleum and natural-gas extraction on water resources of this area further the need for the Osage Nation to better understand its water availability. Therefore, the U.S. Geological Survey, in cooperation with the Osage Nation, completed a hydrologic investigation to assess the status and availability of surface-water and groundwater resources in the Osage Nation. A transient integrated hydrologic-flow model was constructed using the U.S. Geological Survey fully integrated hydrologic-flow model called the MODFLOW One-Water Hydrologic Model. The integrated hydrologic-flow model, called the Osage Nation Integrated Hydrologic Model (ONIHM), was constructed and uses an orthogonal grid of 276 rows and 289 columns, and each grid cell measures 1,312.34 feet (ft; 400 meters) per side, with eight variably thick vertical layers that represented the alluvial and bedrock aquifers within the study area, including the alluvial aquifer, the Vamoosa-Ada aquifer, and the minor Pennsylvanian bedrock aquifers, and the confining units. Landscape and groundwater-flow processes were simulated for two periods: (1) the 1950–2014 period from January 1950 through September 2014 and (2) the forecast period from October 2014 through December 2099. The 1950–2014 period ONIHM simulated past conditions using measured or estimated inputs, and the forecast-period ONIHM simulated three separate potential forecast conditions under constant dry, average, or wet climate conditions using calibrated input values from the 1950–2014 period ONIHM. The 1950–2014 period ONIHM was calibrated by linking the Parameter Estimation software (PEST) with the MODFLOW One-Water Hydrologic Model. PEST uses statistical parameter estimation techniques to identify the best set of parameter values to minimize the difference between measured or estimated calibration targets and their simulated equivalent values (residuals). Tikhonov regularization and singular-value decomposition-assist features of PEST were used during the calibration process. The 1950–2014 period ONIHM was calibrated to 713 measured groundwater levels at 195 wells; 95,636 estimated monthly mean groundwater levels at 124 wells; 5,307 measured streamflows at 13 streamgages; and 8,679 simulated mean monthly streamflows at 10 streamgages extracted from a surface-water model by adjusting 231 parameters. The estimated groundwater-level observations and streamflows were included as observations to improve the spatial and temporal density of observation targets during calibration. The best set of parameter values obtained during the calibration process of the 1950–2014 model was then used as the input parameter values for the forecast model simulations. A comparison of the calibration targets to their corresponding simulated values indicated that the model adequately reproduced streamflows and groundwater levels for some streamgages and wells and underestimated streamflows and groundwater levels at other locations. Measured and simulated streamflows correlated adequately with a coefficient of determination of 0.938, as did water levels with a coefficient of determination of 0.795. The 1950–2014 period ONIHM underestimated certain groundwater levels and streamflows, but generally measured or estimated calibration targets correlated well with simulated equivalents, which indicated that the model can adequately simulate the response of the hydrologic system to stresses in the 1950–2014 and forecast periods. In the 1950–2014 period ONIHM, the calibrated mean horizontal hydraulic conductivity for layer 1 alluvial aquifer was 30.7 feet per day, and the seven lower layers had a calibrated mean horizontal hydraulic conductivity of less than 3.3 feet per day. The mean calibrated groundwater-level residual was 16.6 ft, and the mean calibrated streamflow residual of the Arkansas River at Ralston, Oklahoma, streamgage (U.S. Geological Survey station 07152500) was within 6 percent (373 cubic feet per second) of mean measured streamflow for the 1950–2014 period ONIHM. The ONIHM simulated landscape fluxes of precipitation; groundwater applied by irrigation wells; evapotranspiration from precipitation, groundwater, and irrigation; runoff from precipitation; and deep percolation from precipitation. The largest loss of water from the landscape was evapotranspiration from precipitation with a calibrated mean annual outflow of 32 inches (in.): mean annual precipitation was about 36 in. Calibrated mean annual runoff and deep percolation (recharge to the water table) rates were 4.7 inches per year (in/yr) and 0.70 in/yr, respectively, for the 1950–2014 period ONIHM. The calibrated 1950–2014 period ONIHM groundwater fluxes included net farm net recharge (calculated as the difference between the inflow of recharge to the water table and the outflow of evapotranspiration from the water table such that negative values indicate that evapotranspiration from the water table was greater than deep percolation [recharge to the water table] and vice versa). Net farm net recharge was the largest flux from the groundwater system with a mean annual net outflow of 153.4 cubic feet per second. Stream leakage was the largest flux to the groundwater system with a mean annual net inflow of 152.5 cubic feet per second, indicating that, on average, the groundwater/surface-water interaction was a “losing” system where stream water leaked into the subsurface and recharged the water table. Simulated monthly trends demonstrated that net stream leakage was the largest inflow to the groundwater-flow system for 10 of the 12 months; for the other 2 months (January and March), farm net recharge (January) and net storage (March) were the largest inflow to the groundwater-flow system. A saline groundwater interface map was created for the study and compared to the water levels from the final stress period of the 1950–2014 model to identify the presence of fresh/marginal groundwater throughout the study area. Fresh/marginal groundwater was characterized as groundwater with less than 1,500 milligrams per liter of total dissolved solids. Fresh/marginal groundwater thickness ranged from 0 to 438.2 ft within the study area. The thickest regions of fresh/marginal groundwater were in the eastern part of the study area near Sand Creek, Bird Creek, and Hominy Creek and in the Arkansas River alluvial aquifer in the region downstream from the Arkansas River at Ralston, Okla. Like the 1950–2014 model, forecast model results for the landscape indicated that transpiration from precipitation was the largest flux out of the landscape for all three forecasts, constituting 77, 73, and 58 percent of precipitation for the dry, average, and wet forecasts, respectively. The dry and average forecast landscape fluxes demonstrated similar trends and magnitudes, whereas the wet forecast landscape fluxes indicated the largest changes compared to the average forecast fluxes. Most notably, runoff increased from a mean of 1.1 and 1.6 in/yr for the dry and average forecasts, respectively, to 10 in/yr for the wet forecast. Similar changes occurred for the other wet forecast landscape fluxes. The calibrated 1950–2014 period ONIHM simulated three forecasts to assess the effects of potential climatic changes on the hydrologic system from October 2014 to December 2099. The three forecasts simulated theoretical dry, average, and wet conditions using precipitation and potential evapotranspiration datasets from selected years in the calibrated 1950–2014 period ONIHM. Annual precipitation amounts were 26.89, 35.47, and 50.73 in. for the dry, average, and wet forecasts, respectively. Groundwater-flow component forecast results indicated that stream leakage is always a net inflow to the groundwater-flow system for dry, average, and wet conditions, meaning the study area stream network is always predominantly a “losing” regime where stream water infiltrates into the underlying aquifer. Storage was only a net outflow from the groundwater-flow system and indicated a replenishment to groundwater storage that resulted in an increase in groundwater levels only during the wet forecast. Further, these gains in groundwater storage for the wet forecast occurred only during February through June. Mean fresh/marginal groundwater saturated thicknesses were 125 and 126 ft for the dry and average forecast conditions, respectively, and wet forecast average thickness was 145 ft and ranged from 0 to 443 ft. The spatial extents of fresh/marginal groundwater at the end of the dry, average, and wet forecast model periods (December 2099) did not change substantially from the end of the 1950–2014 model period (September 2014).

Kansas, Oklahoma↗

USGS1990

Purgeable organic compounds in ground water at the Idaho National Engineering Laboratory, Idaho; 1988 and 1989

Groundwater samples from 38 wells at the Idaho National Engineering Laboratory were analyzed for 36 purgeable organic compounds in 1988-89. Thirty-six of the wells obtain water from the Snake River Plain aquifer and were equipped with dedicated or portable pumps. Water samples from one well that obtains water from the aquifer and one that obtains water from a perched groundwater zone were collected using a thief sampler. Analyses of water from 22 wells indicated the aquifer locally contained detectable concentrations of at least 1 of 19 purgeable organic compounds, mainly carbon tetrachloride, 1,1,1-trichloroethane, and trichloroethylene. Except for five wells, the maximum concentration of a specific compound in groundwater was 6.4 microgram/L or less; concentrations of most compounds were less than 0.2 microgram/L. Water from four wells at and near the Test Area North contained from 44 to 29, 000 micrograms/L of trichloroethylene. Water from a well that obtains water from a discontinuous perched groundwater zone at the Radioactive Waste Management Complex contained 1,400 micrograms/L of carbon tetrachloride, 940 micrograms/L of chloroform, 250 micrograms/L of 1,1,1- trichloroethane, and 1,100 micrograms/L trichloroethylene. Selected purgeable organic compounds, such as total xylene and methylene chloride, were detected in some groundwater samples and some blank samples consisting of boiled deionized water. Their presence in the blank samples suggest the compounds could have been inadvertently introduced into the groundwater sampled during or subsequent to collection. (USGS)

Open-File Report↗

USGS2012

Aquifer test at well SMW-1 near Moenkopi, Arizona

The Hopi villages of Lower Moencopi and Upper Moenkopi are on the Hopi Indian Reservation south of Tuba City in northern Arizona. These adjacent Hopi villages, located west and north of the confluence of Pasture Canyon Wash and Moenkopi Wash, are dependent on groundwater withdrawals from three wells that penetrate the N aquifer and from two springs that discharge from the N aquifer. The N aquifer is the principal aquifer in this region of northern Arizona and is composed of thick beds of sandstone between less permeable layers of siltstone and mudstone. The fine-grained character of the N aquifer inhibits rapid movement of water and large yields to wells; however, the aquifer is moderately productive at yields generally less than 25 gallons per minute in the study area. In recent years, the water level has declined in the three public-supply wells and the flow from the springs has decreased, causing concern that the current water supply will not be able to accommodate peak demand and allow for residential and economic growth. In addition to the challenge imposed by declining groundwater levels, the water-supply wells and springs are located about 2 miles downgradient from the Tuba City Landfill site where studies are ongoing to determine if uranium and other metals in groundwater beneath the landfill are higher than regional concentrations in the N aquifer. In August 2008, the U.S. Geological Survey, in cooperation with the Hopi Tribe, conducted an aquifer test on well SMW-1, designed to help the Hopi Tribe determine the potential yield and water quality of the N aquifer south of Moenkopi Wash as a possible source of additional water supply. Well SMW-1 was drilled south of Moenkopi Wash to a depth of 760 feet below land surface before being backfilled and cased to about 300 feet. The well penetrates, in descending order, the Navajo Sandstone and the Kayenta Formation, both units of the N aquifer. The pre-test water level in the well was 99.15 feet below land surface. A 9.25-hour step-drawdown test and a 72-hour constant-rate test followed by recovery tests were used to investigate the performance of the test well and to estimate the transmissivity and potential yield of the N aquifer south of Moenkopi Wash. The test data were analyzed using the Cooper-Jacob method adjusted for confined conditions, the Papadopulos-Cooper method that accounts for wellbore storage, and the Theis method on the recovery data. Results of the tests indicate that in the vicinity of the well, the N aquifer has a transmissivity of about 50 feet squared per day. The test well, as completed, should yield about 15 gallons per minute with about 75 feet of drawdown (less than half of the available saturated thickness of the aquifer at the well).

Arizona↗

USGS2003

Conjunctive-management models for sustained yield of stream-aquifer systems

Conjunctive-management models that couple numerical simulation with linear optimization were developed to evaluate trade-offs between groundwater withdrawals and streamflow depletions for alluvial-valley stream-aquifer systems representative of those of the northeastern United States. A conjunctive-management model developed for a hypothetical stream-aquifer system was used to assess the effect of interannual hydrologic variability on minimum monthly streamflow requirements. The conjunctive-management model was applied to the Hunt-Annaquatucket-Pettaquamscutt stream-aquifer system of central Rhode Island. Results show that it is possible to increase the amount of current withdrawal from the aquifer by as much as 50% by modifying current withdrawal schedules, modifying the number and configuration of wells in the supply-well network, or allowing increased streamflow depletion in the Annaquatucket and Pettaquamscutt rivers. Alternatively, it is possible to reduce current rates of streamflow depletion in the Hunt River by as much as 35% during the summer, but such reductions would result increases in groundwater withdrawals.

Journal of Water Resources Planning and Management↗

USGS2020

Pesticides and their degradates in groundwater reflect past use and current management strategies, Long Island, New York, USA

Long Island, New York, has a mix of urban/suburban to agricultural/horticultural land use and nearly 3 million residents that rely on a sole-source aquifer for drinking water. The analysis of shallow groundwater (<40 m below land surface) collected from 54 monitoring wells across Long Island detected 53 pesticides or pesticide degradates. Maximum concentrations for individual pesticides or pesticide degradates ranged from 3 to 368,000 ng/L. The highest concentrations and most frequent pesticide detections occurred in samples collected from the pesticide management (PM) network, set in an agricultural/horticultural area in eastern Long Island with coordinated pesticide management by state and local agencies. The other two networks (Suffolk and Nassau/Queens) were set in suburban and urban areas, respectively, and had less frequent detections and lower pesticide concentrations than the PM network. Pesticide detections and concentration patterns (herbicide, insecticide, or fungicide) differed among the three networks revealing broad differences in land use. The predominance of fungicides metalaxyl, 1 H -1,2,4-triazole (propiconazole/myclobutanil degradate), and 4-hydroxychlorothalonil (HCTL, chlorothalonil degradate) in samples from the PM network reflects their intensive use in agricultural settings. Total fungicide concentrations in the PM network ranged from <10 to >300,000 ng/L. The widespread detection of imidacloprid and triazine herbicides, simazine and atrazine, reveal a mixture of current and past use pesticides across the Long Island region. Low concentrations (<200 ng/L) of the triazines in the Suffolk and Nassau/Queens networks may reflect a change in land use and application. Acetanilide herbicides and aldicarb have been discontinued for 20 and 40 years, respectively, yet the concentrations of their degradates were among the highest observed in this study. Acetanilide (total concentrations up to 10,000 ng/L) and aldicarb degradates (up to 270 ng/L) are present in the PM network at much lower concentrations than previous Long Island studies and reflect changes in agricultural practices and pesticide management.

New York↗

USGS2009

Estimated land-surface subsidence in Harris County, Texas, 1915-17 to 2001

Land-surface subsidence, or land subsidence, in Harris County, Texas, which encompasses much of the Houston area, has been occurring for decades. Land subsidence has increased the frequency and extent of flooding, damaged buildings and transportation infrastructure, and caused adverse environmental effects. The primary cause of land subsidence in the Houston area is withdrawal of groundwater, although extraction of oil and gas also has contributed. Throughout most of the 20th century, groundwater was the primary source of municipal, agricultural, and industrial water supply for Harris County. Currently (2009) a transition to surface water as the primary source of supply, guided by a groundwater regulatory plan developed by the Harris-Galveston Subsidence District (2001), is in effect. The aquifers in Harris County contain an abundant amount of potable groundwater, but they also contain layers of clay. Groundwater withdrawals caused compaction of the clay layers, which in turn resulted in the widespread, substantial land-surface subsidence that has occurred in the Houston area.

Scientific Investigations Map↗

USGS2022

Estimated effects of pumping on groundwater storage and Walker River stream efficiencies in Smith and Mason Valleys, west-central Nevada

The Walker River originates in the Sierra Nevada Mountains and flows nearly 160 miles to its terminus at Walker Lake in west-central Nevada. The river provides a source of irrigation water for tens of thousands of acres of agricultural lands in California and Nevada and is the principal source of inflow to Walker Lake. Extraction of groundwater for agricultural use became prevalent in the late 1950s and early 1960s to supplement irrigation demands not met by surface-water diversions during times of drought. There is growing concern that continued groundwater withdrawals within the Walker River Basin are likely contributing to depleted streamflow of the Walker River and the long-term depletion of groundwater storage in the basin. This report documents changes in groundwater storage-volume and trends in Walker River stream efficiency, a measure of change in flow due to gaining or losing conditions, in the two largest agricultural valleys in the Walker River Basin, Smith and Mason Valleys, for a multi-decade period. Groundwater-level maps from previous studies were used for the beginning (1970) and middle (2006) points of this study. Groundwater levels measured from 1991–95 and 2016–20 were used to construct median groundwater-level maps that represented conditions in 1995 and 2020. Valley wide groundwater-level change was calculated by comparing groundwater-level maps for the periods 1970–95, 1996–2006, and 2007–20 and by observing the overall change from 1970 to 2020. Groundwater storage-volume change was calculated using groundwater-level change and previously defined specific yield values. Between 1970 and 2020, groundwater storage-volume declined 287,600 acre-feet in Smith Valley and 269,000 acre-feet in Mason Valley. Using groundwater storage-volume decline and annual groundwater pumpage rates, a maximum groundwater pumpage rate can be computed to support management of water resources. In Smith Valley, groundwater pumping in excess of 22,300 acre-feet per year would likely result in groundwater storage decline. In Mason Valley, groundwater pumping in excess of 75,200 acre-feet per year would likely result in groundwater storage decline. Stream efficiency was calculated using continuous streamflow data and monthly diversion volumes on two reaches: (1) the West Walker River in Smith Valley, from 1948 to 2020 and (2) the Walker River in Mason Valley, from 1958 to 2020. Stream efficiency during non-irrigation season in Smith and Mason Valleys declined at a statistically significant rate of 1.1 and 0.6 percent per year, respectively. Trends in stream efficiency corresponded to occurrence of prolonged drought, deviation from average annual streamflows, and total groundwater pumpage. Long-term declines in groundwater storage-volume and stream efficiency demonstrate that the alluvial aquifer system is becoming increasingly depleted, such that the river can no longer replenish groundwater storage while simultaneously balancing groundwater and surface-water withdrawals. The introduction of supplemental groundwater pumpage was intended to offset surface-water deficits during dry years; however, pumpage occurs even in years when average or above average streamflows meet surface-water demands. Reliance on supplemental groundwater pumpage has resulted in widespread groundwater storage-volume decline and decreased stream efficiency. With each successive drought cycle, the ability of Walker River to sustain streamflows and convey water downstream has diminished. Above average wet periods have a marginal and short-lived effect on rebounding the groundwater levels outside of the river corridor. Moreover, if the trend continues, each future drought cycle may further reduce groundwater supplies and that may further decrease streamflow reliability.

Nevada↗

USGS2016

Hydrogeologic investigations of the Miocene Nogales Formation in the Nogales Area, Upper Santa Cruz Basin, Arizona

Hydrogeologic investigations were conducted to evaluate the groundwater resource potential for the Miocene Nogales Formation in the Nogales area, southern Arizona. Results indicate that parts of the formation may provide new, deeper sources of groundwater for the area. Geologic mapping determined the hydrogeologic framework of the formation by defining lithologic, mineralogic, and stratigraphic characteristics; identifying potential aquifers and confining units; and mapping faults and fractures which likely influence groundwater flow. Geophysical modeling was used to determine the basin geometry and thickness of the Nogales Formation and younger alluvial aquifers and to identify target areas (deep subbasins) which may prove to be productive aquifers. Volcaniclastic sandstone samples from the formation were analyzed for porosity, bulk density, saturated hydraulic conductivity, and fabric. Effective porosity ranges from 16 to 42 percent, bulk density from 1.6 to 2.47 grams per cubic centimeter, and saturated hydraulic conductivity (SHC) from 4 to 57 centimeters per day (4.9×10 -5 to 6.7×10 -4 centimeters per second). Thin sections show that sandstone framework grains consist of quartz, feldspar, biotite, hornblende, pumice, volcanic glass, and opaque minerals. The matrix in most samples consists of pumice fragments, and some contain predominantly silt and clay. Samples with a mostly silt and clay matrix have lower porosity and SHC compared to samples with mostly pumice, which have higher and wider ranges of porosity and SHC. Pore space in the Nogales Formation sediments includes moldic, intercrystalline, and fracture porosity. Some intercrystalline pore space is partially filled with calcite cement. About one third of the samples contain fractures, which correspond to fractures noted in outcrops in all members of the formation. Scanning electron microscope (SEM) and x-ray diffraction (XRD) analyses indicate that most of the samples contained the zeolite clinoptilolite and mixed-layer clay. X-ray diffraction analyses verified clinoptilolite as the only zeolite in Nogales Formation samples; they also verified the presence of smectite and illite clay and some kaolinite. Samples which contain greater amounts of clinoptilolite and lesser amounts of smectite have high porosity and SHC in narrow ranges. However, samples with abundant smectite and lesser amounts of clinoptilolite span the entire ranges of porosity and SHC for the formation. All members of the Nogales Formation are fractured and faulted as a result of Tertiary Basin and Range extensional deformation, which was broadly contemporaneous with deposition of the formation. These structures may have significant influence on groundwater flow in the upper Santa Cruz basin because, although many of the sediments in the formation have characteristics indicating they may be productive aquifers based only on porous-media flow, fracturing in these sediments may further enhance permeability and groundwater flow in these basin-fill aquifers by orders of magnitude.

Arizona↗

USGS1996

Geohydrology of the Weldon Spring ordnance works, St. Charles County, Missouri

Bedrock units at the Weldon Spring ordnance works in St. Charles County, Missouri, dip to the northeast at about 60 feet per mile, as measured by the top of the Chouteau Group. The top of the bedrock forms a generally east-west trending ridge through the Weldon Spring training area and the Weldon Spring chemical plant. This surface contains a large, broad bedrock low centered about the unnamed tributary to Dardenne Creek that contains Burgermeister spring. The low has been interpreted to be a paleodrainage that existed before deposition of glacial drift. This feature consists of smaller, more elongate paleovalleys at and west of the chemical plant where more dense drillhole data provide better definition. The uppermost bedrock unit throughout most of the ordnance works is the BurlingtonKeokuk Limestone of Mississippian age. It is subdivided based on weathering characteristics into a lower, unweathered unit; an upper, weathered unit; and a strongly weathered subunit of the weathered unit. The unweathered unit is a light to medium gray, coarse to less commonly fine crystalline, thin to massive bedded, fossiliferous, cherty limestone. The unweathered unit can be silty or argillaceous, or can locally be dolostone or siltstone. The weathered unit is characterized by an increase in mostly horizontal fractures and partings, increased porosity, vugs, voids, breccia, and discoloration by iron oxides. A strongly weathered subunit of the weathered unit is identified in some monitoring wells where these features are particularly abundant or intense. The overburden units are, in ascending order: residuum, basal till, glacial till, including a glacial outwash subunit, the Ferrelview Formation, loess, alluvium, and fill. Some of the thickest overburden occurs in the northern part of the training area and north of the training area and may be caused by a larger thickness of glacial drift. The paleodrainage centered about the unnamed tributary to Dardenne Creek that contains Burgermeister spring appears to have been partially filled by glacial drift, and a surface-water divide now exists southeast of the tributary. The upper, more permeable part of the shallow aquifer consists of the residuum, basal till, glacial outwash (where there is no glacial till below it), and the weathered unit of the Burlington-Keokuk Limestone. The lower, less permeable part of the shallow aquifer consists of the unweathered unit of the Burlington-Keokuk Limestone and the Fern Glen Formation. Generally, the upper part of the shallow aquifer thins to the north, reflecting the thin to absent weathered unit north of the training area and chemical plant. A glacial drift confining unit consists of parts of the glacial till and the Ferrelview Formation. Ground water as recharge and discharge probably moves in fractures through this unit. It confines ground water where the potentiometric surface of the shallow aquifer is above its base. There are stream reaches where the streams have cut through the glacial drift confining unit to expose the underlying shallow aquifer. A potentiometric surface map of the shallow aquifer shows a large ground-water mound in the south-central part of the training area. This mound is part of a generally east-west trending ground-water ridge through the training area and the chemical plant that defines a ground-water divide. Precipitation that percolates downward through fractures in the glacial drift confining unit recharges the shallow aquifer. Where the glacial drift confining unit is not present, precipitation can be expected to recharge the shallow aquifer more readily. There is the potential for groundwater flow in permeable overburden units where the potentiometric surface is above the top of bedrock. Generally, the residuum and locally other overburden units of the shallow aquifer potentially become more important as mediums of ground-water flow north and downgradient of the ground-water ridge. This is probably limited where clay-rich zones in the residuum confine ground water below in the bedrock. Because the thickness of the weathered unit generally decreases to the north, it generally becomes a less important medium of ground-water flow downgradient to the north. Also to the north, the potentiometric surface of the shallow aquifer is above the base of the glacial drift confining unit over a large area, indicating that the aquifer is confined. Upward ground-water gradients measured in monitoring well pairs, Burgermeister and other springs, the gaining unnamed tributary to Dardenne Creek upstream of Burgermeister spring, and Dardenne Creek indicate ground-water discharge in the northern part of the ordnance works.

Missouri↗

USGS2020

Geophysical surveys, hydrogeologic characterization, and groundwater flow model for the Truxton basin and Hualapai Plateau, northwestern Arizona

This is the third and final report in a series that describe the groundwater resources of the Hualapai Indian Reservation. These reports document the findings of a comprehensive groundwater study conducted on the reservation and adjacent areas from 2015 through 2018 by the U.S. Geological Survey in cooperation with the Bureau of Reclamation. The first report described the hydrologic framework and characterization of the Truxton aquifer on the Hualapai Indian Reservation (Bills and Macy, 2016). The second report described the hydrogeologic characterization of the Hualapai Plateau part of the reservation (Mason, Macy, and others, 2020). This report includes five chapters. Chapter A (Mason, Knight, and others, 2020) is a summary of this multichapter volume and briefly describes the study area. Chapter B (Mason, Bills, and Macy, 2020) describes the geology and hydrology of the Truxton basin and Hualapai Plateau. Chapter C (Kennedy, 2020) describes the results of a gravity geophysical survey of the Truxton basin. Chapter D (Ball, 2020) describes the findings of an airborne electromagnetic survey of the Truxton aquifer and Hualapai Plateau. Chapter E (Knight, 2020) describes the results of a transient groundwater model created for the entire Truxton aquifer both on and off the reservation. The groundwater-flow model is used to estimate projected groundwater levels based on future groundwater withdrawal scenarios.

Arizona↗

USGS2012

Well network installation and hydrogeologic data collection, Assateague Island National Seashore, Worcester County, Maryland, 2010

The U.S. Geological Survey, as part of its Climate and Land Use Change Research and Development Program, is conducting a multi-year investigation to assess potential impacts on the natural resources of Assateague Island National Seashore, Maryland that may result from changes in the hydrologic system in response to projected sea-level rise. As part of this effort, 26 monitoring wells were installed in pairs along five east-west trending transects. Each of the five transects has between two and four pairs of wells, consisting of a shallow well and a deeper well. The shallow well typically was installed several feet below the water table—usually in freshwater about 10 feet below land surface (ft bls)—to measure water-level changes in the shallow groundwater system. The deeper well was installed below the anticipated depth to the freshwater-saltwater interface—usually in saltwater about 45 to 55 ft bls—for the purpose of borehole geophysical logging to characterize local differences in lithology and salinity and to monitor tidal influences on groundwater. Four of the 13 shallow wells and 5 of the 13 deeper wells were instrumented with water-level recorders that collected water-level data at 15-minute intervals from August 12 through September 28, 2010. Data collected from these instrumented wells were compared with tide data collected north of Assateague Island at the Ocean City Inlet tide gage, and precipitation data collected by National Park Service staff on Assateague Island. These data indicate that precipitation events coupled with changes in ambient sea level had the largest effect on groundwater levels in all monitoring wells near the Atlantic Ocean and Chincoteague and Sinepuxent Bays, whereas precipitation events alone had the greatest impact on shallow groundwater levels near the center of the island. Daily and bi-monthly tidal cycles appeared to have minimal influence on groundwater levels throughout the island and the water-level changes that were observed appeared to vary among well sites, indicating that changes in lithology and salinity also may affect the response of water levels in the shallow and deeper groundwater systems throughout the island. Borehole geophysical logs were collected at each of the 13 deeper wells along the 5 transects. Electromagnetic induction logs were collected to identify changes in lithology; determine the approximate location of the freshwater-saltwater interface; and characterize the distribution of fresh and brackish water in the shallow aquifer, and the geometry of the fresh groundwater lens beneath the island. Natural gamma logs were collected to provide information on the geologic framework of the island including the presence and thickness of finer-grained deposits found in the subsurface throughout the island during previous investigations. Results of this investigation show the need for collection of continuous water-level data in both the shallow and deeper parts of the flow system and electromagnetic induction and natural gamma geophysical logging data to better understand the response of this groundwater system to changes in precipitation and tidal forcing. Hydrologic data collected as part of this investigation will serve as the foundation for the development of numerical flow models to assess the potential effects of climate change on the coastal groundwater system of Assateague Island.

Maryl↗

USGS2014

Simulation of groundwater flow and saltwater movement in the Onslow County area, North Carolina: predevelopment-2010

Onslow County, North Carolina, is located within the designated Central Coastal Plain Capacity Use Area (CCPCUA). The CCPCUA was designated by law as a result of groundwater level declines of as much as 200 feet during the past four decades within aquifers in rocks of Cretaceous age in the central Coastal Plain of North Carolina and a depletion of water in storage from increased groundwater withdrawals in the area. The declines and depletion of water in storage within the Cretaceous aquifers increase the potential for saltwater migration—both lateral encroachment and upward leakage of brackish water. Within the CCPCUA, a reduction in groundwater withdrawals over a period of 16 years from 2003 to 2018 is mandated. Under the CCPCUA rules, withdrawals in excess of 100,000 gallons per day from any of the Cretaceous aquifer well systems are subject to water-use reductions of as much as 75 percent. To assess the effects of the CCPCUA rules and to assist with groundwater-management decisions, a numerical model was developed to simulate the groundwater flow and chloride concentrations in the surficial Castle Hayne, Beaufort, Peedee, and Black Creek aquifers in the Onslow County area. The model was used to (1) simulate groundwater flow from 1900 to 2010; (2) assess chloride movement throughout the aquifer system; and (3) create hypothetical scenarios of future groundwater development. After calibration of a groundwater flow model and conversion to a variable-density model, five scenarios were created to simulate future groundwater conditions in the Onslow County area: (1) full implementation of the CCPCUA rules with three phases of withdrawal reductions simulated through 2028; (2) implementation of only phase 1 withdrawal reductions of the CCPCUA rules and simulated through 2028; (3) implementation of only phases 1 and 2 withdrawal reductions of the CCPCUA rules and simulated through 2028; (4) full implementation of the CCPCUA rules with the addition of withdrawals from the Castle Hayne aquifer in Onslow County at the fully permitted amount in the final stress period and simulated through 2028; and (5) full implementation of the CCPCUA rules as in scenario 1 except simulated through 2100. Results from the scenarios give an indication of the water-level recovery in the Black Creek aquifer throughout each phase of the CCPCUA rules in Onslow County. Furthermore, as development of the Castle Hayne aquifers was increased in the scenarios, cones of depression were created around pumping centers. Additionally, the scenarios indicated little to no change in chloride concentrations for the time periods simulated.

North Carolina↗