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At least 721 records · Page 40Linked to original sources
USGS2002

Ground-water discharge determined from measurements of evapotranspiration, other available hydrologic components, and shallow water-level changes, Oasis Valley, Nye County, Nevada

Oasis Valley is an area of natural ground-water discharge within the Death Valley regional ground-water flow system of southern Nevada and adjacent California. Ground water discharging at Oasis Valley is replenished from inflow derived from an extensive recharge area that includes the northwestern part of the Nevada Test Site (NTS). Because nuclear testing has introduced radionuclides into the subsurface of the NTS, the U.S. Department of Energy currently is investigating the potential transport of these radionuclides by ground water flow. To better evaluate any potential risk associated with these test-generated contaminants, a number of studies were undertaken to accurately quantify discharge from areas downgradient in the regional ground-water flow system from the NTS. This report refines the estimate of ground-water discharge from Oasis Valley. Ground-water discharge from Oasis Valley was estimated by quantifying evapotranspiration (ET), estimating subsurface outflow, and compiling ground-water withdrawal data. ET was quantified by identifying areas of ongoing ground-water ET, delineating areas of ET defined on the basis of similarities in vegetation and soil-moisture conditions, and computing ET rates for each of the delineated areas. A classification technique using spectral-reflectance characteristics determined from satellite imagery acquired in 1992 identified eight unique areas of ground-water ET. These areas encompass about 3,426 acres of sparsely to densely vegetated grassland, shrubland, wetland, and open water. Annual ET rates in Oasis Valley were computed with energy-budget methods using micrometeorological data collected at five sites. ET rates range from 0.6 foot per year in a sparse, dry saltgrass environment to 3.1 feet per year in dense meadow vegetation. Mean annual ET from Oasis Valley is estimated to be about 7,800 acre-feet. Mean annual ground-water discharge by ET from Oasis Valley, determined by removing the annual local precipitation component of 0.5 foot, is estimated to be about 6,000 acre-feet. Annual subsurface outflow from Oasis Valley into the Amargosa Desert is estimated to be between 30 and 130 acre-feet. Estimates of total annual ground-water withdrawal from Oasis Valley by municipal and non-municipal users in 1996 and 1999 are 440 acre-feet and 210 acre-feet, respectively. Based on these values, natural annual ground-water discharge from Oasis Valley is about 6,100 acre-feet. Total annual discharge was 6,500 acre-ft in 1996 and 6,300 acre-ft in 1999. This quantity of natural ground-water discharge from Oasis Valley exceeds the previous estimate made in 1962 by a factor of about 2.5. Water levels were measured in Oasis Valley to gain additional insight into the ET process. In shallow wells, water levels showed annual fluctuations as large as 7 feet and daily fluctuations as large as 0.2 foot. These fluctuations may be attributed to water loss associated with evapotranspiration. In shallow wells affected by ET, annual minimum depths to water generally occurred in winter or early spring shortly after daily ET reached minimum rates. Annual maximum depths to water generally occurred in late summer or fall shortly after daily ET reached maximum rates. The magnitude of daily water-level fluctuations generally increased as ET increased and decreased as depth to water increased.

Nevada↗
USGS2003

Isotope geochemistry and chronology of offshore ground water beneath Indian River Bay, Delaware

Results of geophysical surveys in Indian River Bay, Delaware, indicate a complex pattern of salinity variation in subestuarine ground water. Fresh ground-water plumes up to about 20 meters thick extending hundreds of meters offshore are interspersed with saline ground water, with varying degrees of mixing along the salinity boundaries. It is possible that these features represent pathways for nutrient transport and interaction with estuarine surface water, but the geophysical data do not indicate rates of movement or nutrient sources and reactions. In the current study, samples of subestuarine ground water from temporary wells with short screens placed 3 to 22 meters below the sediment-water interface were analyzed chemically and isotopically to determine the origins, ages, transport pathways, and nutrient contents of the fresh and saline components. Apparent ground-water ages determined from chlorofluorocarbons (CFCs), sulfur hexafluoride (SF6), tritium (3H), and helium isotopes (3He and 4He) commonly were discordant, but nevertheless indicate that both fresh and saline ground waters ranged from a few years to at least 50 years in age. Tritium-helium (3H-3He) ages, tentatively judged to be most reliable, indicate that stratified offshore freshwater plumes originating in distant recharge areas on land were bounded by relatively young saline water that was recharged locally from the overlying estuary. Undenitrified and partially denitrified nitrate of agricultural or mixed origin was transported laterally beneath the estuary in oxic and suboxic fresh ground water. Ammonium produced by anaerobic degradation of organic matter in estuarine sediments was transported downward in suboxic saline ground water around the freshwater plumes. Many of the chemical and isotopic characteristics of the subestuarine ground waters are consistent with conservative mixing of the fresh (terrestrial) and saline (estuarine) endmember water types. These data indicate that freshwater plumes detected by geophysical surveys beneath Indian River Bay represent lateral continuations of the active surficial nitrate-contaminated freshwater flow systems originating on land, but they do not indicate directly the magnitude of fresh ground-water discharge or nutrient exchange with the estuary. There is evidence that some of the terrestrial ground-water nitrate is reduced before discharging directly beneath the estuary. Local estuarine sediment-derived ammonium in saline pore water may be a substantial benthic source of nitrogen in offshore areas of the estuary.

Delaware↗
USGS2005

Water-quality assessment of part of the Upper Mississippi River Basin, Minnesota and Wisconsin — Ground-water quality along a flow system in the Twin Cities metropolitan area, Minnesota, 1997-98

As part of a national analysis of the effects of land use on ground-water quality for the National Water-Quality Assessment Program, the U.S. Geological Survey sampled wells along a flow system in surficial glacial aquifers in the northwestern part of the Twin Cities metropolitan area during 1997 and 1998. In addition, a reconnaissance steady-state ground-water model was developed to estimate flowpaths and dates of ground-water recharge using a particle-tracking routine. Sediment samples collected during drilling had high horizontal hydraulic conductivities (ranging from about 3.1 to about 190 feet per day, based on sediment-size analysis of well cuttings), small organic carbon concentrations (ranging from less than 0.2 to 160 grams per kilogram), and pH values that were mostly alkaline (ranging from 4.9 to 8.2). Water samples were analyzed for physical properties, major ions, iron, manganese, nutrients, organic carbon, radon, pesticides, volatile organic compounds (VOCs), chlorofluorocarbons, tritium, and isotopes of nitrogen, hydrogen, and oxygen. Most of the water samples had small dissolved-oxygen concentrations (less than 1 milligram per liter). Calcium, magnesium, sodium, bicarbonate, sulfate, and chloride were the primary dissolved constituents in water samples. Nitrite plus nitrate as nitrogen (nitrate) concentrations were less than the U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level of 10 mg/L. Nitrogen isotope ratios indicated that the sources of nitrate primarily were soils, animal waste, or denitrification that increased nitrogen isotope ratios in nitrate. Small concentrations of pesticides were detected in the shallow parts of the aquifer. The herbicide prometon was the most frequently detected pesticide. Herbicides applied to control grasses and weeds in corn (atrazine, simazine, and metolachlor) also were frequently detected in water samples. All pesticide and VOCs detected were below USEPA Maximum Contaminant Levels or Health Advisory Limits. Chlorofluorocarbon compounds and tritium concentrations were used to estimate dates of recharge of ground-water samples. In general, shallower ground-water samples were more recently recharged although most water sampled from the aquifer was recharged after 1955. Although land use had substantial effects on ground-water quality, the distribution of contaminants in the aquifer also is affected by complex combinations of factors and processes that include sources of natural and anthropogenic contaminants, three-dimensional advective flow, physical and hydrologic settings, age and evolution of ground water, and transformation of chemical compounds along the flow system. Compounds such as nitrate and dissolved oxygen were greatest in water samples from the upgradient end of the flow system and near the water table. Specific conductance and dissolved solids increased along the flow system and with depth due to increase in residence time in the flow system and dissolution of aquifer materials.

Minnesota↗
USGS2000

Hydrogeology and simulation of ground-water flow at the Gettysburg Elevator Plant Superfund Site, Adams County, Pennsylvania

Ground water in Triassic-age sedimentary fractured-rock aquifers in the area of Gettysburg, Pa., is used as drinking water and for industrial and commercial supply. In 1983, ground water at the Gettysburg Elevator Plant was found by the Pennsylvania Department of Environmental Resources to be contaminated with trichloroethene, 1,1,1-trichloroethane, and other synthetic organic compounds. As part of the U.S. Environmental Protection Agency?s Comprehensive Environmental Response, Compensation, and Liability Act, 1980 process, a Remedial Investigation was completed in July 1991, a method of site remediation was issued in the Record of Decision dated June 1992, and a Final Design Report was completed in May 1997. In cooperation with the U.S. Environmental Protection Agency in the hydrogeologic assessment of the site remediation, the U.S. Geological Survey began a study in 1997 to determine the effects of the onsite and offsite extraction wells on ground-water flow and contaminant migration from the Gettysburg Elevator Plant. This determination is based on hydrologic and geophysical data collected from 1991 to 1998 and on results of numerical model simulations of the local ground-water flow-system. The Gettysburg Elevator Site is underlain by red, green, gray, and black shales of the Heidlersburg Member of the Gettysburg Formation. Correlation of natural-gamma logs indicates the sedimentary rock strike about N. 23 degrees E. and dip about 23 degrees NW. Depth to bedrock onsite commonly is about 6 feet but offsite may be as deep as 40 feet. The ground-water system consists of two zones?a thin, shallow zone composed of soil, clay, and highly weathered bedrock and a thicker, nonweathered or fractured bedrock zone. The shallow zone overlies the bedrock zone and truncates the dipping beds parallel to land surface. Diabase dikes are barriers to ground-water flow in the bedrock zone. The ground-water system is generally confined or semi-confined, even at shallow depths. Depth to water can range from flowing at land surface to more than 71 feet below land surface. Potentiometric maps based on measured water levels at the Gettysburg Elevator Plant indicate ground water flows from west to east, towards Rock Creek. Multiple-well aquifer tests indicate the system is heterogeneous and flow is primarily in dipping beds that contain discrete secondary openings separated by less permeable beds. Water levels in wells open to the pumped bed, as projected along the dipping stratigraphy, are drawn down more than water levels in wells not open to the pumped bed. Ground-water flow was simulated for steady-state conditions prior to pumping and long-term average pumping conditions. The three-dimensional numerical flow model (MODFLOW) was calibrated by use of a parameter estimation program (MODFLOWP). Steady-state conditions were assumed for the calibration period of 1996. An effective areal recharge rate of 7 inches was used in model calibration. The calibrated flow model was used to evaluate the effectiveness of the current onsite and offsite extraction well system. The simulation results generally indicate that the extraction system effectively captures much of the ground-water recharge at the Gettysburg Elevator Plant and, hence, contaminated ground-water migrating from the site. Some of the extraction wells pump at low rates and have very small contributing areas. Results indicate some areal recharge onsite will move to offsite extraction wells.

Pennsylvania↗
USGS1980

Ground water in the Thousand Oaks area, Ventura County, California

The ground-water basin beneath the city of Thousand Oaks, Calif. , corresponds closely in area with the surface-water drainage basin of Conejo Valley. Before World War II there was little ground-water development. After World War II, urban development put a stress on the ground-water basin; many wells were drilled and water levels in wells were drawn down as much as 300 feet in places. Beginning in 1963, imported water replaced domestic and municipal ground-water systems, and water levels rapidly recovered to predevelopment levels or nearly so. Most of the ground water in the Thousand Oaks area is stored in fractured basalt of the middle Miocene Conejo Volcanics. Depending on the degree of occurrence of open fractures and cavities in the basalt, recoverable ground water in the upper 300 to 500 feet of aquifer is estimated to be between 400,000 and 600,000 acre-feet. The yield of water from wells in the area ranges from 17 to 1,080 gallons per minute. Most of the ground-water in the eastern part of the valley is high insulfate and has a dissolved-solids concentration greater than 1,000 milligrams per liter. In the western part of the valley the ground-water is mostly of a bicarbonate type, and the dissolved-solids concentration is less than 800 milligrams per liter. In most areas of Conejo Valley, ground-water is a viable resource for irrigation of public lands and recreation areas. (USGS)

California↗
USGS1985

Evaluation of the ground-water resources of the lower Susquehanna River basin, Pennsylvania and Maryland

Ground water in the 3,458-square-mile lower Susquehanna River basin occupies secondary openings in bedrock. The distribution of openings is a function of lithology, depth, and topography. Local flow systems account for most of the total ground-water flow. Average annual recharge for the lower basin is 1,857 million gallons per day, most of which discharges to streams. The water table is a subdued replica of land surface; its depth varies with topography, but is generally 20 to 70 feet below land surface. Ground water circulates to depths of 500 to 600 feet below the water table. A digital model of regional, unconfined ground-water flow was developed and used to evaluate the ground-water resources of the lower basin. On the basis of lithologic and hydrologic differences, the area was subdivided into 21 hydrogeologic units, each with different hydrologic characteristics. Each unit was divided into two layers to handle decreasing secondary permeability with depth. A finite-difference grid with square blocks approximately one mile on a side was used. The model was calibrated under steady-state and transient conditions. The model-generated results were compared to estimated water-table altitudes and estimated base flows in the steady-state calibration. In the transient calibration, the model-generated results were compared to observed changes in water-table altitude from November 1, 1980 through April 22, 1981. Hydraulic conductivity increases from hilltops to valley bottoms. The average hydraulic conductivity for carbonate units is about 21 feet per day, which is an order of magnitude greater than the corresponding averages for Paleozoic sedimentary, Triassic sedimentary, and crystalline units. The Cumberland Valley carbonate rocks have the greatest average hydraulic conductivity--about 174 feet per day in valley bottoms. The average gaining-stream leakage coefficient for all carbonate units is about 16 feet per day, which is two orders of magnitude greater than the corresponding averages for the other lithologies. The Cumberland Valley carbonate rocks have the greatest gaining-stream leakage coefficient--about 43 feet per day. The specific yields are 0.035, 0.020, 0.020 and 0.007 for the carbonate, Paleozoic sedimentary, crystalline, and Triassic sedimentary units, respectively. The calibrated model was used to simulate the effects of a ground-water withdrawal of 1 inch per year on water-table altitudes and average annual base flows in the modeled area. The overall effect is least for the carbonate units and greatest for the Triassic sedimentary units. The model also was used to simulate a standardized potential yield for each unit by assuming that the maximum acceptable consequence of a hypothetical withdrawal scheme is an ultimate 50-percent reduction in average annual base flow. Based on this, the potential yield for the modeled area is 891 million gallons per day. The Cumberland Valley carbonate rocks have the greatest potential yield--0.47 million gallons per day per square mile. The carbonate units have the greatest average potential yield, followed by the Paleozoic sedimentary, crystalline, and Triassic sedimentary units. About 90 percent of the eventual decline in water-table altitudes and the eventual reduction in average annual base flows occurs within five years of the implementation of the hypothetical withdrawal scheme. Nearly all of the ground water withdrawn is derived from reduced discharge to streams. The calibrated model can be used to provide estimates of the impacts of ground-water development schemes on regional ground-water levels and base flows of streams. It can not be used to simulate local cones of depression or local base-flow changes. The reliability of the model is a function of its approximation of the physical characteristics of the ground-water flow system, the two calibrations, various simplifying assumptions, and the lack of calibration under ground-water withdrawal conditions. It can be used in steady-state or transient mode to assess the effects of both natural and artificial stresses.

Maryland, Pennsylvania↗
USGS2005

Problems associated with estimating ground water discharge and recharge from stream-discharge records

Ground water discharge and recharge frequently have been estimated with hydrograph-separation techniques, but the critical assumptions of the techniques have not been investigated. The critical assumptions are that the hydraulic characteristics of the contributing aquifer (recession index) can be estimated from stream-discharge records; that periods of exclusively ground water discharge can be reliably identified; and that stream-discharge peaks approximate the magnitude and tinting of recharge events. The first assumption was tested by estimating the recession index from st earn-discharge hydrographs, ground water hydrographs, and hydraulic diffusivity estimates from aquifer tests in basins throughout the eastern United States and Montana. The recession index frequently could not be estimated reliably from stream-discharge records alone because many of the estimates of the recession index were greater than 1000 days. The ratio of stream discharge during baseflow periods was two to 36 times greater than the maximum expected range of ground water discharge at 12 of the 13 field sites. The identification of the ground water component of stream-discharge records was ambiguous because drainage from bank-storage, wetlands, surface water bodies, soils, and snowpacks frequently exceeded ground water discharge and also decreased exponentially during recession periods. The timing and magnitude of recharge events could not be ascertained from stream-discharge records at any of the sites investigated because recharge events were not directly correlated with stream peaks. When used alone, the recession-curve-displacement method and other hydrograph-separation techniques are poor tools for estimating ground water discharge or recharge because the major assumptions of the methods are commonly and grossly violated. Multiple, alternative methods of estimating ground water discharge and recharge should be used because of the uncertainty associated with any one technique.

Ground Water↗
USGS1981

Ground-water resources of the White River basin, Madison County, Indiana

The ground-water resources of the White River basin in and near Madison County, Indiana, were investigated by mapping the aquifers, estimating their hydraulic properties, determining the distribution of potentiometric head in the aquifers, and estimating some of the components of the ground-water budget from data collected in the field. This information was used to con struct and calibrate a five-layer, digital ground-water flow model. The mod el, constructed and calibrated to ground-water-level and seepage data col lected during the study, simulates conditions for the autumn of 1976. The model was used to provide estimates of ground-water potential in terms of yield, drawdown, and depletion in streamflow. Glacial drift covers nearly the entire study area and ranges in thickness from 0 to approximately 300 feet. Beneath the drift lie Ordovician to Devonian limestone, dolomite, and shale. A bedrock valley trends west-northwest through the center of the study area. Relief of the bedrock surface is approximately 325 feet. Four confined sand and gravel aquifers interbedded in the glacial drift, a bedrock aquifer, and an unconfined outwash aquifer are the three most important aquifer systems in the study area. The nearly horizontal, areally discontinuous, confined sand and gravel aquifers have an average thickness of about 15 feet and an average hydraulic conductivity estimated to be 433 feet per day. The bedrock aquifer underlying the entire study area has a thick-ness estimated to be 150 feet and an average transmissivity estimated in previous studies in the basin to be 1,340 square feet per day. The unconfined outwash aquifer, although thin and narrow, is an important aquifer because of its proximity to sources of induced recharge. Water-level fluctuations in observation wells in the basin indicate that the ground-water system is in dynamic equilibrium. Ground-water seepage to streams at 90-percent flow duration on October 1, 1976, was estimated to be between 15.9 and 74.4 cubic feet per second. Ground-water pumpage for 1976 was estimated to be 19.7 million gallons per day (30.5 cubic feet per second). The water budget, as simulated in the model, indicates that the rate of inflow to the ground-water system in the modeled area is 92.4 cubic feet per second. Of this, 78 percent is from effective areal recharge of precipitation, and 22 percent is from ground-water flow across the boundaries into the study area. Thirty-three percent of the ground-water outflow is pumpage, 59 percent is seepage to streams, and the remaining 8 percent is ground-water flow across the boundaries out of the study area. Model simulations of eight pumping plans provide a general assessment of the water-yielding potential of the three major aquifer systems and indicate that as much as 2.5 million gallons per day can be obtained from well fields about half a square mile in area. Model results also indicate that, for as much as 2.5-million gallons per day pumping, flow in the large streams will not be significantly affected, whereas flow in the small streams may be significantly affected. In addition, simulations indicate that adding more wells to the well field northwest of Anderson, Ind., would probably be no more advantageous than increasing the pumpage in the existing wells. How-ever, well interference, well hydraulics, and pumping-level constraints were not considered in the investigation of alternative methods of expanding the well field. In developing the ground-water system in the area, use of many small, scattered well fields that produce less than about 3 million gallons per day may be more favorable hydrologically than a few, heavily pumped well fields.

Indiana↗
USGS2007

An Online Interactive Map Service for Displaying Ground-Water Conditions in Arizona

Monitoring the availability of the nation's ground-water supplies is of critical importance to planners and water managers. The general public also has an interest in understanding the status of ground-water conditions, especially in the semi-arid Southwestern United States where much of the water used by municipalities and agriculture comes from the subsurface. Unlike surface-water indicators such as stage or discharge, ground-water conditions may be more difficult to assess and present. Individual well observations may only represent conditions in a limited area surrounding the well and wells may be screened over single or multiple aquifers, further complicating single-well measurement interpretations. Additionally, changes in ground-water conditions may involve time scales ranging from days to many years, depending on recharge, soil properties and depth to the water table. This lack of an easily identifiable ground-water property indicative of current conditions combined with differing time scales of water-level changes makes the presentation of ground-water conditions a difficult task, particularly on a regional basis. One approach is to spatially present several indicators of ground-water conditions that address different time scales and attributes of the aquifer systems. In this report, we describe a publicly-available online interactive map service that presents several different layers of ground-water-conditions information for the alluvial basins in the Lower Colorado River Basin in Arizona (http://montezuma.wr.usgs.gov/website/azgwconditions/). These data layers include wells experiencing water-level decline, wells experiencing water-level rise, recent trends in ground-water levels, change in water level since predevelopment and change in storage since predevelopment. Recent pumpage totals and projected population numbers are also provided for ground-water basins and counties in the region of the Lower Colorado River in Arizona along with a bibliography of U.S. Geological Survey reports for those seeking further information. The methods used to create these data layers are explained with illustrations of example information available on the Web site.

Open-File Report↗
USGS1999

Deep Aquifer Remediation Tools (DARTs): A new technology for ground-water remediation

P otable ground-water supplies throughout the world are contaminated or threatened by advancing plumes containing radionuclides, metals, and organic compounds. Currently (1999), the most widely used method of ground-water remediation is a combination of extraction, ex-situ treatment, and discharge of the treated water, commonly known as pump and treat. Pump-and-treat methods are costly and often ineffective in meeting long-term protection standards (Travis and Doty, 1990; Gillham and Burris, 1992; National Research Council, 1994). This fact sheet describes a new and potentially cost-effective technology for removal of organic and inorganic contaminants from ground water. The U.S. Geological Survey (USGS) is currently exploring the possibilities of obtaining a U.S. Patent for this technology.

Fact Sheet↗
USGS2003

Arsenic in ground water: Geochemistry and occurrence

Interest in arsenic in ground water has greatly increased in the past decade because of the increased awareness of human health effects and the costs of avoidance or treatment of ground water supplies used for consumption. The goal of this book is to provide a description of the basic processes that affect arsenic occurrence and transport by providing sufficient background information on arsenic geochemistry and descriptions of hi- arsenic ground water, both affected and unaffected by human activity. An understanding of thermodynamics, adsorption, and the speciation of arsenic in solid phases, which are described in first three chapters, is needed to predict the fate of arsenic in ground water systems. Large-scale and deep movement of ground water can and has redistributed arsenic in the near surface environment, as described in the next two chapters. These large-scale systems can affect large volumes of both ground water and surface water, such as in the Yellowstone system, and can produce mineralised zones that subsequently release arsenic to ground water supplies. Regional identification of high-arsenic ground water and its consumption as described in the next three chapters clearly demonstrates a need for increased wat- quality monitoring, particularly in south and southeast Asia. Chapters 9-11 provide examples of high arsenic ground water associated with sulfide mineral oxidation and alkaline conditions. Finally, smaller scale studies of the effects of human activities that have produced high-arsenic ground water and methods for attenuation of ground water are presented

Book↗
USGS2002

Hydrogeology and ground-water-flow simulation of the Cave Springs area, Hixson, Tennessee

The ground-water resource in the Cave Springs area is used by the Hixson Utility District as a water supply and is one of the more heavily stressed in the Valley and Ridge Physiographic Province. In 1999, ground-water withdrawals by the Hixson Utility District averaged about 6.4 million gallons per day (Mgal/d) from two pumping centers. The Hixson Utility District has historically withdrawn about 5.8 Mgal/d from wells at Cave Springs. In 1995 to meet increasing demand, an additional well field was developed at Walkers Corner, located about 3 miles northeast of Cave Springs. From 1995 through 2000, pumping from the first production well at Walkers Corner averaged about 1.8 Mgal/d. A second production well at Walkers Corner was approved for use in 2000. Hixson Utility District alternates the use of the two production wells at Walkers Corner except when drought conditions occur when they are used simultaneously. The second production well increased the capacity of the well field by an additional 2 Mgal/d. The aquifer framework in the study area consists of dense Paleozoic carbonate rocks with secondary permeability that are mantled by thick residual clay-rich regolith in most of the area and by coarse-grained alluvium in the valley of North Chickamauga Creek. Cave Springs, one of the largest springs in Tennessee, derives its flow from conduits in a carbonate rock (karst) aquifer. Production wells at Cave Springs draw water from these conduits. Production wells at Walkers Corner primarily draw water from gravel zones in the regolith near the top of rock. Transmissivities estimated from hydraulic tests conducted across the Cave Springs area span a range from 240 to 900,000 feet squared per day (ft 2 /d) with a median value of 5,200 ft 2 /d. Recharge to the aquifer occurs from direct infiltration of precipitation and from losing streams. Most recharge occurs during the winter and spring months. Computer modeling was used to provide a better understanding of the ground-water-flow system and to simulate the effects of additional ground-water withdrawals. A numerical ground-water-flow model of the ground-water system was constructed and calibrated using MODFLOW 2000. Modeling results indicate that losing streams along the base of the Cumberland Plateau escarpment at the western edge of the study area are an important source of recharge to the ground-water system, supplying about 50 percent of the recharge to the study area. Direct infiltration of precipitation accounts for the remaining recharge to the study area. In 1999, ground-water withdrawals of 6.4 Mgal/d [9.9 cubic feet per second (ft 3 /s)] equaled about 11 percent of the total simulated ground-water recharge. The remaining ground-water recharge discharges to rivers (48 percent, 41.1 ft 3 /s), springs (19 percent, 16.8 ft 3 /s), and Chickamauga Lake (22 percent, 19.0 ft 3 /s). Drawdown at the Walkers Corner well field in 2000 was about 33 feet at the center of a cone of depression that is elongated along strike. If additional pumping at Walkers Corner increases withdrawals by 2 Mgal/d, simulated drawdown at the Walkers Corner well field increases to about 60 feet and simulated ground-water discharges decrease by amounts of 1.0 ft 3 /s to Chickamauga Lake, 0.8 ft 3 /s to North Chickamauga Creek, 0.5 ft 3 /s to Lick Branch-Rogers Spring drainage, 0.5 ft 3 /s to Poe Branch, and 0.2 ft 3 /s to Cave Springs.

Tennessee↗
USGS1996

Age of ground water in basalt aquifers near Spring Creek National Fish Hatchery, Skamania County, Washington

Water samples from four springs and five wells in basalt aquifers near Spring Creek National Fish Hatchery in Skamania County, Washington, were collected and analyzed for selected inorganic ions and stable isotopes. Eight samples were analyzed for carbon-14 (14C), carbon-13 ([3C), and either chlorofluorocarbons (CFCs) or tritium. This work was done to estimate the age (residence time, or time elapsed since recharge) of water issuing from springs at the hatchery. If CFCs are present in ground water, the presence of at least a component of modem (post- 1944) water is indicated. CFC-dating suggests that ground water several hundred feet below land surface in the Underwood Heights area noah of the hatchery, including ground water discharging from the hatchery springs, contains modem water. In contrast, CFC-dating suggests that deeper ground water such as that withdrawn from the Hatchery Well may contain little or no modem water. Concentrations of 14C in water can yield 14C-based ground-water ages, termed 14C-model ages. Unadjusted 14C-model ages (14C-model ages unadjusted for carbon mass transfers) for water discharging from the hatchery springs are on the order of several hundred years. Ground-water samples from three wells in the Underwood Heights area yielded 14C-model ages ranging from modem to several hundred years. Unadjusted 14C-model ages for deep ground water pumped by the Hatchery Well indicate an overall age of several thousand years. However, 14C concentrations may be affected by transfers of carbon into and out of solution. The 13C values of the downgradient ground waters ranged from - 16.4 (per mil) to -18.2 per rail, isotopically heavier than expected for ground water that obtains carbon solely from root respiration in a temperate climate and undergoes no subsequent carbon-isotope fractionation or exchange. Such 13C values in the ground water near the hatchery suggest the possibility of carbon mass transfers during the evolution of these waters. Geochemical mass-transfer modeling suggests that carbon dioxide may degas and calcite may dissolve during the evolution of the modeled ground waters, but that degassing of carbon dioxide is the dominant carbon mass transfer in these ground waters. In addition, modeling results also suggest that, although some calcite may dissolve during the evolution of the water produced by the Hatchery Well, and possibly during the evolution of the shallower ground waters, the amount of calcite dissolution is small. Model testing suggests that the quantity of 14C-dead carbon added from calcite dissolution may not be sufficient to greatly affect the 14C-model ages of these ground waters. In other words, the 14C-model ages that were adjusted for various modeled carbon mass transfers are similar to the unadjusted 14C-model ages. A comparison of CFC data with both adjusted and unadjusted 14C data suggests that water discharging at the hatchery springs contains a mixture of modem and old water, where old water is defined as water recharged prior to 1944. The CFC data support a component of modem water, whereas the 14C data suggest a component of old water. Similar results were obtained from a comparison of CFC data with adjusted and unadjusted 14C data for water collected from Well 3. Well 3 is north of the hatchery springs, on a flow path that appears to be parallel to and similar in length to the flow path leading to the hatchery springs. Water from the Hatchery Well, however, may be devoid of modem water and appears to have an overall age on the order of thousands of years.

Washington↗
USGS2004

Simulation of regional ground-water flow in the Upper Deschutes Basin, Oregon

This report describes a numerical model that simulates regional ground-water flow in the upper Deschutes Basin of central Oregon. Ground water and surface water are intimately connected in the upper Deschutes Basin and most of the flow of the Deschutes River is supplied by ground water. Because of this connection, ground-water pumping and reduction of artificial recharge by lining leaking irrigation canals can reduce the amount of ground water discharging to streams and, consequently, streamflow. The model described in this report is intended to help water-management agencies and the public evaluate how the regional ground-water system and streamflow will respond to ground-water pumping, canal lining, drought, and other stresses. Ground-water flow is simulated in the model by the finite-difference method using MODFLOW and MODFLOWP. The finite-difference grid consists of 8 layers, 127 rows, and 87 columns. All major streams and most principal tributaries in the upper Deschutes Basin are included. Ground-water recharge from precipitation was estimated using a daily water-balance approach. Artificial recharge from leaking irrigation canals and on-farm losses was estimated from diversion and delivery records, seepage studies, and crop data. Ground-water pumpage for irrigation and public water supplies, and evapotranspiration are also included in the model. The model was calibrated to mean annual (1993-95) steady-state conditions using parameter-estimation techniques employing nonlinear regression. Fourteen hydraulic-conductivity parameters and two vertical conductance parameters were determined using nonlinear regression. Final parameter values are all within expected ranges. The general shape and slope of the simulated water-table surface and overall hydraulic-head distribution match the geometry determined from field measurements. The fitted standard deviation for hydraulic head is about 76 feet. The general magnitude and distribution of ground-water discharge to streams is also well simulated throughout the model. Ground-water discharge to streams in the area of the confluence of the Deschutes, Crooked, and Metolius Rivers is closely matched. The model was also calibrated to transient conditions from 1978 to 1997 using traditional trial-and-error methods. Climatic cycles during this period provided an excellent regional hydrologic signal for calibration. Climate-driven water-level fluctuations are simulated with reasonable accuracy over most of the model area. The timing and magnitude of simulated water-level fluctuations caused by annual pulses of recharge from precipitation match those observed reasonably well, given the limitations of the time discretization in the model. Water-level fluctuations caused by annual canal leakage are simulated very well over most of the area where such fluctuations occur. The transient model also simulates the volumetric distribution and temporal variations in ground-water discharge reasonably well. The match between simulated and measured volume of and variations in ground-water discharge is, however, somewhat dependent on geographic scale. The rates of and variations in ground-water discharge are matched best at regional scales. Example simulations were made to demonstrate the utility of the model for evaluating the effects of ground-water pumping or canal lining. Pumping simulations show that pumped water comes largely from aquifer storage when pumping begins, but as the water table stabilizes, the pumping increasingly diminishes the discharge to streams and, hence, streamflow. The time it takes for pumping to affect streamflow varies spatially depending, in general, on the location of pumping relative to the discharge areas. Canal-lining simulations show similar effects.

Water-Resources Investigations Report↗
USGS2005

Influence of fracture anisotropy on ground water ages and chemistry, Valley and Ridge province, Pennsylvania

Model ground water ages based on chlorofluorocarbons (CFCs) and tritium/helium-3 (3H/3He) data were obtained from two arrays of nested piezometers located on the north limb of an anticline in fractured sedimentary rocks in the Valley and Ridge geologic province of Pennsylvania. The fracture geometry of the gently east plunging fold is very regular and consists predominately of south dipping to subhorizontal to north dipping bedding-plane parting and east striking, steeply dipping axial-plane spaced cleavage. In the area of the piezometer arrays, which trend north-south on the north limb of the fold, north dipping bedding-plane parting is a more dominant fracture set than is steeply south dipping axial-plane cleavage. The dating of ground water from the piezometer arrays reveals that ground water traveling along paths parallel to the dip direction of bedding-plane parting has younger 3H/3He and CFC model ages, or a greater component of young water, than does ground water traveling along paths opposite to the dip direction. In predominantly unmixed samples there is a strong positive correlation between age of the young fraction of water and dissolved sodium concentration. The travel times inferred from the model ages are significantly longer than those previously calculated by a ground water flow model, which assumed isotropically fractured layers parallel to topography. A revised model factors in the directional anisotropy to produce longer travel times. Ground water travel times in the watershed therefore appear to be more influenced by anisotropic fracture geometry than previously realized. This could have significant implications for ground water models in other areas underlain by similarly tilted or folded sedimentary rock, such as elsewhere in the Valley and Ridge or the early Mesozoic basins.

Pennsylvania↗
USGS2006

An update of the distribution of selected radiochemical and chemical constituents in perched ground water, Idaho National Laboratory, Idaho, Emphasis 1999-2001

Radiochemical and chemical wastes generated at facilities at the Idaho National Laboratory (INL) were discharged since 1952 to infiltration ponds at the Reactor Technology Complex (RTC) (known as the Test Reactor Area [TRA] until 2005), and the Idaho Nuclear Technology and Engineering Center (INTEC) and buried at the Radioactive Waste Management Complex (RWMC). Disposal of wastewater to infiltration ponds and infiltration of surface water at waste burial sites resulted in formation of perched ground water in basalts and in sedimentary interbeds above the Snake River Plain aquifer. Perched ground water is an integral part of the pathway for waste-constituent migration to the aquifer. The U.S. Geological Survey (USGS), in cooperation with the U.S. Department of Energy, maintains ground-water monitoring networks at the INL to determine hydrologic trends, and to monitor the movement of radiochemical and chemical constituents in wastewater discharged from facilities to both perched ground water and the aquifer. This report presents an analysis of water-quality and water-level data collected from wells completed in perched ground water at the INL during 1999-2001, and summarizes historical disposal data and water-level-and water-quality trends. At the RTC, tritium, strontium-90, cesium-137, dissolved chromium, chloride, sodium, and sulfate were monitored in shallow and deep perched ground water. In shallow perched ground water, no tritium was detected above the reporting level. In deep perched ground water, tritium concentrations generally decreased or varied randomly during 1999-2001. During October 2001, tritium concentrations ranged from less than the reporting level to 39.4?1.4 picocuries per milliliter (pCi/mL). Reportable concentrations of tritium during July-October 2001 were smaller than the reported concentrations measured during July-December 1998. Tritium concentrations in water from wells at the RTC were likely affected by: well's distance from the radioactive-waste infiltration ponds (commonly referred to as the warm-waste ponds); water depth below the ponds; the amount of tritium discharged to radioactive-waste infiltration ponds in the past; discontinued use of radioactive-waste infiltration ponds; radioactive decay; and dilution from disposal of nonradioactive water. During 1999-2001, the strontium-90 concentrations in two wells completed in shallow perched water near the RTC exceeded the reporting level. Strontium-90 concentrations in water from wells completed in deep perched ground water at the RTC varied randomly with time. During October 2001, concentrations in water from five wells exceeded the reporting level and ranged from 2.8?0.7 picocuries per liter (pCi/L) in well USGS 63 to 83.8?2.1 pCi/L in well USGS 54. No reportable concentrations of cesium-137, chromium-51, or cobalt-60 were present in water samples from any of the shallow or deep wells at the RTC during 1999-2001. Dissolved chromium was not detected in shallow perched ground water at the RTC during 1999-2001. Concentrations of dissolved chromium during July-October 2001 in deep perched ground water near the RTC ranged from 10 micrograms per liter (?g/L) in well USGS 61 to 82 ?g/L in well USGS 55. The largest concentrations were in water from wells north and west of the radioactive-waste infiltration ponds. During July-October 2001, dissolved sodium concentrations ranged from 7 milligrams per liter (mg/L) in well USGS 78 to 20 mg/L in all wells except well USGS 68 (413 mg/L). Dissolved chloride concentrations in shallow perched ground water ranged from 10 mg/L in wells CWP 1, 3, and 4 to 53 mg/L in well TRA A 13 during 1999-2001. Dissolved chloride concentrations in deep perched ground water ranged from 5 mg/L in well USGS 78 to 91 mg/L in well USGS 73. The maximum dissolved sulfate concentration in shallow perched ground water was 419 mg/L in well CWP 1 during July 2000. Concentrations of dissolved sulfate in water from wells USGS 54, 60

Scientific Investigations Report↗
USGS1998

Radon-222 in the ground water of Chester County, Pennsylvania

Radon-222 concentrations in ground water in 31 geologic units in Chester County, Pa., were measured in 665 samples collected from 534 wells from 1986 to 1997. Chester County is underlain by schists, gneisses, quartzites, carbonates, sandstones, shales, and other rocks of the Piedmont Physiographic Province. On average, radon concentration was measured in water from one well per 1.4 square miles, throughout the 759 square-mile county, although the distribution of wells was not even areally or among geologic units. The median concentration of radon-222 in ground water from the 534 wells was 1,400 pCi/L (picocuries per liter). About 89 percent of the wells sampled contained radon-222 at concentrations greater than 300 pCi/L, and about 11 percent of the wells sampled contained radon-222 at concentrations greater than 5,000 pCi/L. The highest concentration measured was 53,000 pCi/L. Of the geologic units sampled, the median radon-222 concentration in ground water was greatest (4,400 pCi/L) in the Peters Creek Schist, the second most areally extensive formation in the county. Significant differences in the radon-222 concentrations in ground water among geologic units were observed. Generally, concentrations in ground water in schists, quartzites, and gneisses were greater than in ground water in anorthosite, carbonates, and ultramafic rocks. The distribution of radon-222 in ground water is related to the distribution of uranium in aquifer materials of the various rock types. Temporal variability in radon-222 concentrations in ground water does not appear to be greater than about a factor of two for most (75 percent) of wells sampled more than once but was observed to range up to almost a factor of three in water from one well. In water samples from this well, seasonal variations were observed; the maximum concentrations were measured in the fall and the minimum in the spring.

Pennsylvania↗