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At least 73 records · Page 4Linked to original sources
USGS1998

Review of Aquifer Test Results for the Lansdale Area, Montgomery County, Pennsylvania, 1980–95

Aquifer and aquifer-isolation test results in and around North Penn Area 6 Superfund site, Lansdale, Montgomery County, Pennsylvania are reviewed to provide estimated aquifer properties for use in a numerical model of ground-water flow. This review was made to support of remedial action investigations by U.S. Environmental Protection Agency (USEPA), Region III, Philadelphia. The data reviewed are from files of the U.S. Geological Survey, USEPA, and water companies, and from unpublished consultant reports prepared for USEPA and corporations in the Lansdale area. Tested wells are in fractured sedimentary rocks of the Brunswick Formation, which are Triassic-aged, dipping shales and sandstones. Review procedures include, in some cases, new analyses of drawdown during pumping and recovery by use of analytical models of flow to wells. Estimated aquifer transmissivities (T) range from zero to about 1,300 m 2 /d (meters squared per day); most tests indicate T between 10 and 100 m 2 /d. Aquifer-isolation testing results indicate that most flow enters wells at a few discrete zones, probably fractures or bedding-plane openings. The vertical connection between the zones in a single borehole with multiple producing zones commonly is negligible. This suggests that the formation is vertically anisotropic; the hydraulic conductivity is much larger in the horizontal direction than in the vertical direction. Some evidence of well-field-scale horizontal anisotropy exists, with maximum transmissivity aligned with the regional northeast strike of bedding, but this evidence is weak because of the small number of observation wells, particularly wells screened in isolated depth intervals. Analysis of recovery data after constant-pumping-rate aquifer tests and of drawdown during step tests suggests that a significant fraction, perhaps as much as 85 percent, of the drawdown in some production wells is due to well loss or skin effects in or very near the pumped well and is not caused by resistance to flow in the surrounding formations.

Pennsylvania↗
USGS2008

Update of the Accounting Surface Along the Lower Colorado River

The accounting-surface method was developed in the 1990s by the U.S. Geological Survey, in cooperation with the Bureau of Reclamation, to identify wells outside the flood plain of the lower Colorado River that yield water that will be replaced by water from the river. This method was needed to identify which wells require an entitlement for diversion of water from the Colorado River and need to be included in accounting for consumptive use of Colorado River water as outlined in the Consolidated Decree of the United States Supreme Court in Arizona v. California. The method is based on the concept of a river aquifer and an accounting surface within the river aquifer. The study area includes the valley adjacent to the lower Colorado River and parts of some adjacent valleys in Arizona, California, Nevada, and Utah and extends from the east end of Lake Mead south to the southerly international boundary with Mexico. Contours for the original accounting surface were hand drawn based on the shape of the aquifer, water-surface elevations in the Colorado River and drainage ditches, and hydrologic judgment. This report documents an update of the original accounting surface based on updated water-surface elevations in the Colorado River and drainage ditches and the use of simple, physically based ground-water flow models to calculate the accounting surface in four areas adjacent to the free-flowing river.

Scientific Investigations Report↗
USGS2009

Simulation of Variable-Density Ground-Water Flow and Saltwater Intrusion beneath Manhasset Neck, Nassau County, New York, 1905-2005

The coastal-aquifer system of Manhasset Neck, Nassau County, New York, has been stressed by pumping, which has led to saltwater intrusion and the abandonment of one public-supply well in 1944. Measurements of chloride concentrations and water levels in 2004 from the deep, confined aquifers indicate active saltwater intrusion in response to public-supply pumping. A numerical model capable of simulating three-dimensional variable-density ground-water flow and solute transport in heterogeneous, anisotropic aquifers was developed using the U.S. Geological Survey finite-element, variable-density, solute-transport simulator SUTRA, to investigate the extent of saltwater intrusion beneath Manhasset Neck. The model is composed of eight layers representing the hydrogeologic system beneath Manhasset Neck. Four modifications to the area?s previously described hydrogeologic framework were made in the model (1) the bedrock-surface altitude at well N12191 was corrected from a previously reported value, (2) part of the extent of the Raritan confining unit was shifted, (3) part of the extent of the North Shore confining unit was shifted, and (4) a clay layer in the upper glacial aquifer was added in the central and southern parts of the Manhasset Neck peninsula. Ground-water flow and the location of the freshwater-saltwater interface were simulated for three conditions (time periods) (1) a steady-state (predevelopment) simulation of no pumping prior to about 1905, (2) a 40-year transient simulation based on 1939 pumpage representing the 1905-1944 period of gradual saltwater intrusion, and (3) a 60-year transient simulation based on 1995 pumpage representing the 1945-2005 period of stabilized withdrawals. The 1939 pumpage rate (12.1 million gallons per day (Mgal/d)) applied to the 1905-1944 transient simulation caused modeled average water-level declines of 2 and 4 feet (ft) in the shallow and deep aquifer systems from predevelopment conditions, respectively, a net decrease of 5.2 Mgal/d in freshwater discharge to offshore areas and a net increase of 6.9 Mgal/d of freshwater entering the model from the eastern, western, and southern lateral boundaries. The 1995 pumpage rate (43.3 Mgal/d) applied to the 1945-2005 transient simulation caused modeled average water-level declines of 5 and 8 ft in the shallow and deep aquifer systems from predevelopment conditions, respectively, a net decrease of 13.2 Mgal/d in freshwater discharge to offshore areas and a net increase of 30.1 Mgal/d of freshwater entering the model from the eastern, western, and southern lateral boundaries. The simulated decrease in freshwater discharge to the offshore areas caused saltwater intrusion in two parts of the deep aquifer system under Manhasset Neck. Saline ground water simulated in a third part of the deep aquifer system under Manhasset Neck was due to the absence of the North Shore confining unit near Sands Point. Simulated chloride concentrations greater than 250 milligrams per liter (mg/L) were used to represent the freshwater-saltwater interface, and the movement of this concentration was evaluated for transient simulations. The decrease in the 1905-1944 simulated freshwater discharge to the offshore areas caused the freshwater-saltwater interface in the deep aquifer system to advance landward more than 1,700 ft from its steady-state position in the vicinity of Baxter Estates Village, Long Island, New York. The decrease in the 1945-2005 simulated freshwater discharge to the offshore areas caused a different area of the freshwater-saltwater interface in the deep aquifer system to advance more than 600 ft from its steady-state position approximately 1 mile south of the Baxter Estates Village. However, the 1945-2005 transient simulation underestimates the concentration and extent of saltwater intrusion determined from water-quality samples collected from wells N12508 and N12793, where measured chloride concentrations increased from 625 and 18 mg/L in 1997 t

Scientific Investigations Report↗
USGS2008

Base of principal aquifer for the Elkhorn-Loup model area, North-Central Nebraska

In Nebraska, the water managers in the Natural Resources Districts and the Nebraska Department of Natural Resources are concerned with the effect of ground-water withdrawal on the availability of surface water and the long-term effects of ground-water withdrawal on ground- and surface-water resources. In north-central Nebraska, in the Elkhorn and Loup River Basins, ground water is used for irrigation, domestic supply, and public supply; surface water is used in this area for irrigation, recreation, and hydropower production. In recognition of these sometimes competing ground- and surface-water uses in the Elkhorn and Loup River Basins, the U.S. Geological Survey, the Lewis and Clark Natural Resources District, the Lower Elkhorn Natural Resources District, the Lower Loup Natural Resources District, the Lower Niobrara Natural Resources District, the Lower Platte North Natural Resources District, the Middle Niobrara Natural Resources District, the Upper Elkhorn Natural Resources District, and the Upper Loup Natural Resources District agreed to cooperatively study water resources in the Elkhorn and Loup River Basins. The goals of the overall study were to construct and calibrate a regional ground-water flow model of the area and to use that flow model as a tool to assess current and future effects of ground-water irrigation on stream base flow and to help develop long-term water-resource management strategies for this area, hereafter referred to as the Elkhorn-Loup model area. The Elkhorn-Loup model area covers approximately 30,800 square miles, and extends from the Niobrara River in the north to the Platte River in the south. The western boundary of the Elkhorn-Loup model area coincides with the western boundary of the Middle Niobrara, Twin Platte, and Upper Loup Natural Resources Districts; the eastern boundary coincides with the approximate location of the western extent of glacial till in eastern Nebraska. The principal aquifer in most of the Elkhorn-Loup model area is the High Plains aquifer; the principal aquifer in the remaining part of the Elkhorn-Loup model area is an unnamed alluvial aquifer. The upper surface of the geologic units that directly underlie the aquifer is called the 'base of aquifer' in this report. The geologic unit that forms the base of aquifer in the Elkhorn-Loup model area varies by location. The Tertiary-age Brule Formation generally is the base of aquifer in the west; the Cretaceous-age Pierre Shale generally is the base of aquifer in the east. The purpose of this report is to update the altitude and configuration of the base of the principal aquifer in the Elkhorn-Loup model area and a 2-mile buffer area around the Elkhorn-Loup model area, using base-of-aquifer data from test holes, registered water wells, and oil and gas wells within the Elkhorn-Loup model area and a 20-mile buffer area around the Elkhorn-Loup model area that have become available since the publication of earlier maps of the base of aquifer for this area. The base-of-aquifer map is important for the Elkhorn-Loup ground-water flow model because it defines the model's lower boundary. The accuracy of the Elkhorn-Loup ground-water flow model and the accuracy of the model's predictions about the effects of ground-water irrigation on stream base flow are directly related to the accuracy of the model's lower boundary.

Scientific Investigations Map↗
USGS2003

Simulated ground-water flow in the Hueco Bolson, an alluvial-basin aquifer system near El Paso, Texas

The neighboring cities of El Paso, Texas, and Ciudad Juarez, Chihuahua, Mexico, have historically relied on ground-water withdrawals from the Hueco Bolson, an alluvial-aquifer system, to supply water to their growing populations. By 1996, ground-water drawdown exceeded 60 meters in some areas under Ciudad Juarez and El Paso. A simulation of steady-state and transient ground-water flow in the Hueco Bolson in westernmost Texas, south-central New Mexico, and northern Chihuahua, Mexico, was developed using MODFLOW-96. The model is needed by El Paso Water Utilities to evaluate strategies for obtaining the most beneficial use of the Hueco Bolson aquifer system. The transient simulation represents a period of 100 years beginning in 1903 and ending in 2002. The period 1903 through 1968 was represented with 66 annual stress periods, and the period 1969 through 2002 was represented with 408 monthly stress periods. The ground-water flow model was calibrated using MODFLOWP and UCODE. Parameter values representing aquifer properties and boundary conditions were adjusted through nonlinear regression in a transient-state simulation with 96 annual time steps to produce a model that approximated (1) 4,352 water levels measured in 292 wells from 1912 to 1995, (2) three seepage-loss rates from a reach of the Rio Grande during periods from 1979 to 1981, (3) three seepage-loss rates from a reach of the Franklin Canal during periods from 1990 to 1992, and (4) 24 seepage rates into irrigation drains from 1961 to 1983. Once a calibrated model was obtained with MODFLOWP and UCODE, the optimal parameter set was used to create an equivalent MODFLOW-96 simulation with monthly temporal discretization to improve computations of seepage from the Rio Grande and to define the flow field for a chloride-transport simulation. Model boundary conditions were modified at appropriate times during the simulation to represent changes in well pumpage, drainage of agricultural fields, and channel modifications of the Rio Grande. The model input was generated from geographic information system databases, which facilitated rapid model construction and enabled testing of several conceptualizations of hydrogeologic facies boundaries. Specific yield of unconfined layers and hydraulic conductance of Quaternary faults in the fluvial facies were the most sensitive model parameters, suggesting that ground-water flow is impeded across the fault planes.

Water-Resources Investigations Report↗
USGS1994

Simulations of flow in the Edwards-Trinity aquifer system and contiguous hydraulically connected units, west-central Texas

The Edwards-Trinity aquifer system is currently (1993) being studied as part of the Regional Aquifer-Systems Analysis program. A major goal of the project is to understand and describe the regional ground-water flow system. A finite-element model for simulating two-dimensional steady-state ground-water flow was applied to the major aquifers of the Edwards-Trinity aquifer system and contiguous hydraulically connected units for the winter of 1974-75 and for the predevelopment conditions. The major aquifers are the Edwards-Trinity in the western semiarid part of the study area, the Trinity in the eastern subhumid part of the study area, and the Edwards in the southeastern part of the study area. The Edwards-Trinity, upper part of the Trinity, and part of the Edwards aquifers are shallow and unconfined over most of the study area. Regional ground-water flow is toward the perennial streams for the Edwards-Trinity and Trinity aquifers, as indicated by the observed and simulated potentiometric surfaces. The transmissivity values used in the simulations were within estimated ranges and generally are: 1,000 to 10,000 ft 2 /d (feet squared per day) for the Edwards-Trinity and Trinity aquifers; 100,000 to greater than 1 million ft 2 /d for the Edwards aquifer; and less than 500 to 10,000 ft 2 /d in contiguous hydraulically connected units. Simulated flow through the Edwards-Trinity aquifer system and contiguous hydraulically connected units is about 3 million acre-feet per year. Estimates of areally distributed recharge from the simulations range from 0.1 to 1 inch per year for the Edwards-Trinity aquifer and increase to 4 inches per year for the Trinity aquifer. Recharge to the Edwards aquifer occurs along streambeds that cross outcropped high-permeability rocks of the Edwards Group through joints and faults. Many of the streams are diverted completely underground during periods of no precipitation. The movement of a substantial quantity of water (about 400 cubic feet per second) from the Trinity and Edwards-Trinity aquifers into the Edwards aquifer was simulated. Results of the simulations indicate that anisotropy strongly influences flow in the Edwards aquifer. In the San Antonio and Austin areas, the Edwards aquifer is the most active part of the ground-water flow system with one-third of ground-water discharge occurring in 5 percent of the modeled area for both simulations.

Texas↗
USGS1999

U.S. Geological Survey middle Rio Grande basin study; proceedings of the third annual workshop, Albuquerque, New Mexico, February 24-25, 1999

Approximately 40 percent (about 600,000 people) of the total population of New Mexico lives within the Middle Rio Grande Basin, which includes the City of Albuquerque. Ongoing analyses of the central portion of the Middle Rio Grande Basin by the U.S. Geological Survey (USGS) in cooperation with the City of Albuquerque and other agencies have shown that ground water in the basin is not as readily accessible as earlier studies indicated. A more complete characterization of the ground-water resources of the entire Middle Rio Grande Basin is hampered by a scarcity of data in the northern and southern areas of the basin. The USGS Middle Rio Grande Basin study is a 5-year effort by the USGS and other agencies to improve the understanding of the hydrology, geology, and land-surface characteristics of the Middle Rio Grande Basin. The primary objective of this study is to improve the understanding of the water resources of the basin. Of particular interest is to determine the extent of hydrologic connection between the Rio Grande and the Santa Fe Group aquifer. Additionally, ground-water quality affects the availability of water supplies in the basin. Improving the existing USGS-constructed ground-water flow model of the Middle Rio Grande Basin will integrate all the various tasks that improve our knowledge of the various components of the Middle Rio Grande water budget. Part of this improvement will be accompanied by extended knowledge of the aquifer system beyond the Albuquerque area into the northern and southern reaches of the basin. Other improvements will be based on understanding gained through process-oriented research and improved geologic characterization of the deposits. The USGS and cooperating agencies will study the hydrology, geology, and land-surface characteristics of the basin to provide the scientific information needed for water-resources management and for managers to plan for water supplies needed for a growing population. To facilitate exchange of information among the scientists working on the Middle Rio Grande Basin study, yearly technical meetings have been held for each of the first 3 years of the anticipated 5-year study. These meetings provide an opportunity to present research results and plan new field efforts. This report documents the results of research presented at the third annual technical workshop held in Albuquerque, New Mexico, February 24-25, 1999. The report is organized into this introduction and five chapters that focus on Middle Rio Grande Basin study investigations in progress in the Middle Rio Grande Basin. The first chapter describes geographic data and analysis efforts in the basin. The second chapter details work being done on the hydrogeologic and geologic framework of the basin. The third chapter describes studies on ground-water recharge in the basin. The fourth chapter provides details on the research on the ground-water flow system in the basin, including modeling efforts. The fifth chapter is devoted to an overview of New Mexico District Cooperative Program studies in the basin. The information in this report presents preliminary results of an evolving study. As the study progresses and individual projects publish their results in more detail, the USGS hopes to expand the scientific basis needed for management decisions regarding the Middle Rio Grande Basin.

Open-File Report↗
USGS2006

MODOPTIM: A general optimization program for ground-water flow model calibration and ground-water management with MODFLOW

MODOPTIM is a non-linear ground-water model calibration and management tool that simulates flow with MODFLOW-96 as a subroutine. A weighted sum-of-squares objective function defines optimal solutions for calibration and management problems. Water levels, discharges, water quality, subsidence, and pumping-lift costs are the five direct observation types that can be compared in MODOPTIM. Differences between direct observations of the same type can be compared to fit temporal changes and spatial gradients. Water levels in pumping wells, wellbore storage in the observation wells, and rotational translation of observation wells also can be compared. Negative and positive residuals can be weighted unequally so inequality constraints such as maximum chloride concentrations or minimum water levels can be incorporated in the objective function. Optimization parameters are defined with zones and parameter-weight matrices. Parameter change is estimated iteratively with a quasi-Newton algorithm and is constrained to a user-defined maximum parameter change per iteration. Parameters that are less sensitive than a user-defined threshold are not estimated. MODOPTIM facilitates testing more conceptual models by expediting calibration of each conceptual model. Examples of applying MODOPTIM to aquifer-test analysis, ground-water management, and parameter estimation problems are presented.

Scientific Investigations Report↗
USGS2002

Surface water-ground water interactions along the lower Dungeness River and vertical hydraulic conductivity of streambed sediments, Clallam County, Washington, September 1999-July 2001

The Dungeness River emerges from the Olympic Mountains and flows generally north toward the Strait of Juan De Fuca, crossing the broad, fertile alluvial fan of the Sequim-Dungeness peninsula in northeastern Clallam County, Washington. Increasing competition for the peninsula's ground-water resources, changing water-use patterns, and recent requirements to maintain minimum in-stream flows to enhance endangered salmon and trout populations have severely strained the peninsula's water resources and necessitated a better understanding of the interaction between surface water and groundwater. Three methods were used to characterize the interchange between surface water and groundwater along the lower 11.8 miles of the Dungeness River corridor between September 1999 and July 2001. In-stream mini-piezometers were used to measure vertical hydraulic gradients between the river and the water-table aquifer at 27 points along the river and helped to define the distribution of gaining and losing stream reaches. Seepage runs were used to quantify the net volume of water exchanged between the river and ground water within each of five river reaches, termed 'seepage reaches.' Continuous water-level and water-temperature monitoring at two off-stream well transects provided data on near-river horizontal hydraulic gradients and temporal patterns of water exchange for a representative gaining stream reach and a representative losing stream reach. Vertical hydraulic gradients in the mini-piezometers generally were negative between river miles 11.8 and 3.6, indicating loss of water from the river to ground water. Gradients decreased in the downstream direction from an average of -0.86 at river mile 10.3 to -0.23 at river mile 3.7. Small positive gradients (+0.01 to +0.02) indicating ground-water discharge occurred in three localized reaches below river mile 3.7. Data from the seepage runs and off-stream transect wells supported and were generally consistent with the mini-piezometer findings. An exception occurred between river miles 8.1 and 5.5 where seepage results showed a small gain and the mini-piezometers showed negative gradients. Vertical hydraulic conductivity of riverbed sediments was estimated using hydraulic gradients measured with the mini-piezometers and estimated seepage fluxes. The resulting conductivity values ranged from an average of 1 to 29 feet per day and are similar to values reported for similar river environments elsewhere. The results of this study will be used to calibrate a transient, three-dimensional ground-water flow model of the Sequim-Dungeness peninsula. The model will be used to assess the potential effects on ground-water levels and river flows that result from future water use and land-use changes on the peninsula.

Washington↗
USGS1981

Digital-computer model of the principal ground-water reservoir in Beryl-Enterprise area, Escalante Desert, Utah

The computer model presented in this report was used to simulate the principal ground-water reservoir in the Beryl-Enterprise area, Escalante Desert, Beaver, Iron, and Washington Counties, Utah (Mower, 1981). The details of the formulation of the model, testing of its validity, and the results of predictions are discussed in the cited report. This report was prepared as part of a cooperative program with the Utah Department of Natural Resources, Division of Water Rights, to investigate the water resources of the State. It is an addendum to the principal interpretive report, and it is presented in order to make the model available to anyone desiring to use it for additional predictions. The main program used was the finite-difference model for aquifer simulation in two dimensions documented by Trescott, Pinder, and Larson, (1976). Minor modifications were made to adapt the program to the principal ground-water reservoir in the Beryl-Enterprise area. All the modifications are listed at the top of table 1, and were related to parameter input and output, thus none of the computational subroutines were affected. The parameter arrays (table 1) and map of the area with a grid overlay (pi. 1) are given on following pages. The model simulates an aquifer- under water-table conditions, mostly composed of unconsoliuated basin-fill deposits. The boundaries of the modeled area (pi. 1) generally coincide with the boundaries of the saturated basin fill. However, in the southwest-central part of the model, permeable consolidated rock is included; and that part of the northern boundary between the Black and Wah Wah Mountains is an arbitrary boundary in basin fill between the Beryl-Enterprise area and the Milford area that lies to the northeast. The ignimbrite at Table Butte also was included in the active part of the model. The model includes simulation of discharge by evapotranspiration from phreatophytes. The areal recharge array was used to simulate recharge entering the modeled area at its boundaries and from stream infiltration in the southern corner near Enterprise. In addition, this array included discharge by wells operated during the period simulated as being under steady-state conditions (virtually 1937), and discharging wells simulating flow of water northeast to the Milford area. These wells also were included in the transient-state simulation (1937-77), although any changes in this discharge were modeled using the pumpage array (Group IV, table 1). The wells simulating outflow to the Milford area are shown on plate 1, but the wells pumping in 1937 are not shown unless they also were pumped during 1937-77. The pumpage array was used to simulate: (1) Discharge from wells, (2) discharge after 1977 from a mine in the southwest-central part of the model and recharge resulting form the mine discharge (pi. 1), and (3) changes in discharge in wells operated during the steady-state period. Recharge from irrigation was simulated by reducing pumpage from nodes where irrigation occurs. Discharge from all wells was reduced by 5 percent by multiplying all pumpage by 0.95 in the computer program. North of Newcastle, in T. 35 S., R. 15 W., pumpage was reduced by 35 percent because surface materials are very permeable.

Utah↗
USGS1978

Two-dimensional and three-dimensional digital flow models of the Salinas Valley ground-water basin, California

The Salinas Valley ground-water basin is in central coastal California. The ground-water basin extends from Monterey Bay southeastward along the Salinas River to San Ardo, a distance of about 70 miles, and has a maximum thickness of about 2,000 feet. Annual recharge to the ground-water basin, which is derived mostly from the Salinas River, is about 290,000 acre-feet. Annual discharge, which is mostly from pumpage but also includes the consumptive use of ground water by riparian vegetation along the Salinas River, is about 507,000 acre-feet. About 45 percent of the pumpage, or about 217,000 acre-feet of water annually, returns to the ground-water system. A system of interacting hydrologic models was developed for the Salinas Valley. These models include the small-stream model, river model, two-dimensional ground-water model, and three-dimensional ground-water model. The small-stream model simulates ground-water recharge from small streams that are tributary to the Salinas River. The river model simulates ground-water recharge from the surface-water discharge in the Salinas River. The two-dimensional and three-dimensional ground-water models simulate hydraulic head in the ground-water basin. (Woodard-USGS)

Water-Resources Investigations Report↗
USGS1998

Design, revisions, and considerations for continued use of a ground-water-flow model of the coastal plain aquifer system in Virginia

A digital numerical model of the ground-water-flow system in the Virginia Coastal Plain was constructed as part of the Regional Aquifer System Analysis program of the U.S. Geological Survey and subsequently revised based on reinterpretations of the hydrogeologic framework. The revised model has been incorporated by the Virginia Department of Environmental Quality as a means to evaluate the effects of existing and proposed ground-water withdrawals. The revised model, which is documented here, includes many design features of the original model. Principal modifications are (1) reassignment of some model layers to represent different aquifers, (2) replacement of applied recharge and the uppermost surface-water layer by constant-head cells to represent the unconfined aquifer, and (3) revision of the lateral extents and transmissivity and vertical leakance values assigned to model layers. Ground-water levels simulated by the revised model under steady-state conditions, using withdrawal rates representing the period 1978-80, generally are similar to the water levels simulated by the original model under transient conditions using the same withdrawal rates. Differences probably result largely from changes in transmissivity and vertical leakance values. T he revised model enables the continued evaluation of development of the large and complex aquifer system. Improved information on parts of some aquifers has been incorporated. Only large, regional-scale trends in ground-water water levels, flow directions, and flow rates are simulated, however, and not local trends and (or) short-term changes in water level and flow. Changing hydraulic stresses, knowledge of hydrogeologic conditions, and resource-management needs require periodic revision of the model to maintain its usefulness for the management of ground-water resources in the Virginia Coastal Plain.

Virginia↗
USGS1995

A computerized data base of nitrate concentrations in Indiana ground water

As part of a cooperative study with the Indiana Department of Environmental Management, the U.S. Geological Survey compiled a computerized data base of nitrate concentrations in Indiana ground water. The data included nitrate determinations from more than 29 studies by five Federal and State agencies during June 1973 through August 1991. The National Water Information System software of the U.S. Geological Survey was used to store the data at the U.S. Geological Survey office in Indianapolis, Indiana. Electronic data sets were converted to a standard format of well data, sample data, and analytical data. Data were screened by several error-checking procedures before they were retained in the data base; they were examined for potential duplicates of well location and name. The data base of nitrate concentrations in Indiana ground water contains records of 5,525 samples collected from 4,448 wells in 88 of 92 counties during 1973-91. Those wells included 3,832 drinking-water wells; 536 monitoring wells, 38 livestock-supply wells; and 42 irrigation wells. Nitrate concentrations greater than minimum reporting limits of 0.0 to 0.5 milligrams per liter (mg/L) were determined in 2,453 samples (44 percent of the total). Nitrate in ground water at concentrations greater than 3 mg/L have been considered to be the result of human activities. Nitrate concentrations ranged from 0.005 to 380 mg/L with a median nitrate concentration of 0.3 mg/L. Nitrate concentrations were greater than or equal to 3 mg/L in 704 samples (13 percent of the total). Nitrate concentrations were greater than or equal to the U.S. Environmental Protection Agency Maximum Contaminant Level of 10 mg/L in 188 samples (3.4 percent of the total). Of the 3,832 drinking- water wells in the data base, 147 had at least one sample in which a nitrate concentration was greater than the Maximum Contaminant Level. The percentage of samples with nitrate concentrations greater than or equal to 3 mg/L and greater than or equal to 10 mg/L generally increased during the period 1973 through 1991. The nitrate data base was compiled from numerous data sets that were readily accessible in electronic format. The uses of these data may be limited because they were neither comprehensive nor of a single statistical design. Nonetheless, the nitrate data can be used in several ways: (1) to identify geographic areas with and without nitrate data; (2) to evaluate assumptions, models, and maps of ground-water-contamination potential; and (3) to investigate the relation between environmental factors, land-use types, and the occurrence of nitrate.

Indiana↗