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At least 559 records · Page 31Linked to original sources
USGS1987

Progress report on the ground-water, surface-water, and quality-of-water monitoring program, Black Mesa area, northeastern Arizona, 1987

The N aquifer is an important source of water in the 5,400 sq-mi Black Mesa area on the Navajo and Hopi Indian Reservations. The Black Mesa monitoring program is designed to monitor long-term effects on the groundwater resources of the mesa as a result of withdrawals from the aquifer by the strip-mining operation of Peabody Coal Company. Withdrawals from the N aquifer by the mine increased from 95 acre-ft in 1968 to more than 4,480 acre-ft in 1986. Water levels in the confined area of the aquifer declined as much as 90 ft from 1965 to 1987 in some municipal and observation wells within about a 15-mi radius of the mine well field. Part of the drawdown in municipal wells is due to local pumpage. Water levels have not declined in wells tapping the unconfined area of the aquifer. Chemical analyses indicate no significant changes in the quality of water from wells that tap the N aquifer or from springs that discharge from several stratigraphic units, including the N aquifer, since pumping began at the mine. (USGS)

Open-File Report↗
USGS1992

Well-construction, water-level, geophysical, and water-quality data for ground-water monitoring wells for Arnold Air Force Base, Tennessee

Sixty-five wells were installed at 39 sites in the Arnold Air Force Base area in Coffee and Franklin Counties, Tennessee. The wells were installed to provide information on subsurface lithology, aquifer characteristics, ground-water levels, and ground-water quality. Well depths ranged from 11 to 384 feet. Water-quality samples were collected from 60 wells and analyzed for common inorganic ions, trace metals, and volatile organic compounds. The median dissolved-solids concentrations were 60 milligrams per liter in the shallow aquifer, 48 million gallons per liter in the Manchester aquifer, 1,235 milligrams per liter in the Fort Payne aquifer, and 1,712 milligrams per liter in the upper Central Basin aquifer. Caliper, temperature, natural gamma, electric, neutron porosity, gamma-gamma density, and acoustic velocity borehole-geophysical logs were obtained for the six deep wells completed below the Chattanooga Shale. Petrographic and modal analysis were performed on rock samples from each deep well. These six deep wells provide the first information in the study area on hydraulic head and water quality from below the Chattanooga Shale.

Tennessee↗
USGS1999

Specific conductance, water temperature, and water level data, San Francisco Bay, California, water year 1998

Specific conductance and water temperature data are continuously recorded at four sites in San Francisco Bay, California: San Pablo Strait at Point San Pablo, Central San Francisco Bay at Presidio Military Reservation, Pier 24 at Bay Bridge, and South San Francisco Bay at San Mateo Bridge near Foster City (Figure 1). Water level data are recorded only at San Pablo Strait at Point San Pablo. These data were recorded by the Department of Water Resources (DWR) before 1988, by the US Geological Survey (USGS) National Research Program from 1988 to 1989, and by the USGS-DWR cooperative program since 1990. This article presents time-series plots of data from the four sites in San Francisco Bay during water year 1998 (1 October 1997 through 30 September 1998).

California↗
USGS2002

Water level, specific conductance, and water temperature data, San Francisco Bay, California, for Water Year 2000

Time series of water-level, specific-conductance, and watertemperature data were collected at seven sites in San Francisco Bay during water year 2000 (October 1, 1999 through September 30, 2000). Water-level data were recorded only at Point San Pablo. Specific-conductance and water-temperature data were recorded at 15-minute intervals at the following locations (Figure 1): • Carquinez Strait at Carquinez Bridge • Napa River at Mare Island Causeway near Vallejo • San Pablo Bay at Petaluma River Channel Marker 9 • San Pablo Strait at Point San Pablo • Central San Francisco Bay at Presidio Military Reservation • Central San Francisco Bay at Pier 24 • South San Francisco Bay at San Mateo Bridge near Foster City.

California↗
USGS2001

Water Resources Data, North Dakota, Water Year 2000. Volume 2. Ground Water

This edition of the annual hydrologic data report of North Dakota is one of a series of annual reports that document hydrologic data collected from the U.S. Geological Survey's collection networks in each State, Puerto Rico, and the Trust Territories. These records of streamflow, ground-water levels, and quality of water provide the hydrologic information needed by Federal, State, local agencies, and the private sector for developing and managing land and water resources in North Dakota.

Water Data Report↗
USGS2000

Water Resources Data, North Dakota, Water Year 1998. Volume 2. Ground Water

This edition of the annual hydrologic data report of North Dakota is one of a series of annual reports that document hydrologic data collected from the U.S. Geological Survey's collection networks in each State, Puerto Rico, and the Trust Territories. These records of streamflow, ground-water levels, and quality of water provide the hydrologic information needed by Federal, State, local agencies, and the private sector for developing and managing land and water resources in North Dakota.

Water Data Report↗
USGS1995

Water-quality assessment of the Kentucky River Basin, Kentucky; analysis of available surface-water-quality data through 1986, with a section on biological indicators of water quality

As part of the National Water-Quality Assessment program (NAWQA), existing water-quality data and ancillary information from the Kentucky River basin study unit were compiled and evaluated to provide a description of existing conditions and long-term trends in water quality. The data and evaluations were used to develop a conceptual understanding of the observed conditions and trends, including relations to causative factors, both natural and human controlled.

Kentucky↗
USGS1999

Water-quality assessment of south-central Texas — Comparison of water quality in surface-water samples collected manually and by automated samplers

Surface-water sampling protocols of the U.S. Geological Survey National Water-Quality Assessment (NAWQA) Program specify samples for most properties and constituents to be collected manually in equal-width increments across a stream channel and composited for analysis. Single-point sampling with an automated sampler (autosampler) during storms was proposed in the upper part of the South-Central Texas NAWQA study unit, raising the question of whether property and constituent concentrations from automatically collected samples differ significantly from those in samples collected manually. Statistical (Wilcoxon signed-rank test) analyses of 3 to 16 paired concentrations for each of 26 properties and constituents from water samples collected using both methods at eight sites in the upper part of the study unit indicated that there were no significant differences in concentrations for dissolved constituents, other than calcium and organic carbon.

Texas↗
USGS2012

Summary of data from onsite and laboratory analyses of surface water and marsh porewater from South Florida Water Management District Water Conservation Areas, the Everglades, South Florida, March 1995

This report presents results of chemical analysis for samples collected during March, 1995, as part of a study to quantify the interaction of aquatic organic material (referred to here as dissolved organic carbon with dissolved metal ions). The work was done in conjunction with the South Florida Water Management District, the U.S. Environmental Protection Agency, the U.S. Geological Survey South Florida Ecosystems Initiative, and the South Florida National Water Quality Assessment Study Unit. Samples were collected from surface canals and from marsh sites. Results are based on onsite and laboratory measurements for 27 samples collected at 10 locations. The data file contains sample description, dissolved organic carbon concentration and specific ultraviolet absorbance, and additional analytical data for samples collected at several sites in the Water Conservation Areas, the Everglades, south Florida.

Florida↗
USGS1975

Chemical analyses of ground water for saline-water resources studies in Texas Coastal Plain stored in National Water Data Storage and Retrieval System

Chemical analyses of 4,269 water samples from wells in 66 counties in Texas have been processed into the National Water Data Storage and Retrieval System by the Gulf Coast Hydrogeology Project of the U. S. Geological Survey. More than 65,000 chemical analyses of saline waters produced by oil test and production wells have been contributed to the project by major oil companies. The computerized tabulation and the computer-drawn map of the locations of sampling sites are the initial release of oil company, State, and Federal data in Texas Coastal Plain from the data bank.

Open-File Report↗
USGS2010

Water resources of Monroe County, New York, water years 2003-08: Streamflow, constituent loads, and trends in water quality

This report, the sixth in a series published since 1994, presents analyses of hydrologic data in Monroe County for the period October 2002 through September 2008. Streamflows and water quality were monitored at nine sites by the Monroe County Department of Health and the U.S. Geological Survey. Streamflow yields (flow per unit area) were highest in Northrup Creek, which had sustained flows from year-round inflow from the village of Spencerport wastewater-treatment plant and seasonal releases from the New York State Erie (Barge) Canal. Genesee River streamflow yields also were high, at least in part, as a result of higher rainfall and lower evapotranspiration rates in the upper part of the Genesee River Basin than in the other study basins. The lowest streamflow yields were measured in Honeoye Creek, which reflected a decrease in flows due to the withdrawals from Hemlock and Canadice Lakes for the city of Rochester water supply. Water samples collected at nine monitoring sites were analyzed for nutrients, chloride, sulfate, and total suspended solids. The loads of constituents, which were computed from the concentration data and the daily flows recorded at each of the monitoring sites, are estimates of the mass of the constituents that was transported in the streamflow. Annual yields (loads per unit area) also were computed to assess differences in constituent transport among the study basins. All urban sites - Allen Creek and the two downstream sites on Irondequoit Creek - had seasonally high concentrations and annual yields of chloride. Chloride loads are attributed to the application of road-deicing salts to the county's roadways and are related to population and road densities. The less-urbanized sites in the study - Genesee River, Honeoye Creek, and Oatka Creek - had relatively low concentrations and yields of chloride. The highest concentrations and yields of sulfate were measured in Black Creek, Oatka Creek, and Irondequoit Creek at Railroad Mills and are attributable to dissolution of sulfate from gypsum (calcium sulfate) deposits in Silurian shale bedrock that crops out upstream from these monitoring sites. Northrup Creek had the highest concentrations of phosphorus, orthophosphate, and nitrogen, and high yields of nitrate plus nitrite nitrogen and ammonia plus organic nitrogen. These results are attributed to discharges from the Spencerport wastewater-treatment plant (which ceased operation in June 2008), diversions from the New York State Erie (Barge) Canal, and manure and fertilizers applied to agricultural fields. Concentrations and yields of nitrate plus nitrite nitrogen also were high in Oatka Creek and Black Creek; basins with substantial agricultural land uses. Allen Creek had the second highest yield of ammonia plus organic nitrogen. Honeoye Creek, which drains a relatively undeveloped basin, had the lowest yields of nitrogen constituents. The second highest median concentrations and highest sample concentrations of phosphorus and orthophosphate, as well as the highest phosphorus yields, were measured in the Genesee River. A comparison of the yields computed for the two downstream sites on Irondequoit Creek - above Blossom Road and at Empire Boulevard - permitted an assessment of the mitigative effects of the Ellison Park wetland on constituent loads, which would otherwise be transported to Irondequoit Bay. These effects also include those provided by a flow-control structure (installed mid-way through the wetland during February 1997), which was designed to increase the dispersal and short-term detention of stormflows in the wetland. The wetland decreased yields of particulate constituents - phosphorus and ammonia plus organic nitrogen - but had little effect on the yields of dissolved constituents - chloride, sulfate, and nitrate plus nitrite nitrogen. Trends in flow-adjusted concentrations were identified at all sites for most of the nutrient constituents that were evaluated. All of the linear time tren

Scientific Investigations Report↗
USGS2006

Water Budgets and Potential Effects of Land- and Water-Use Changes for Carson Valley, Douglas County, Nevada, and Alpine County, California

To address concerns over continued growth in Carson Valley, the U.S. Geological Survey, in cooperation with Douglas County, Nevada, began a study in February 2003 to update estimates of water-budget components in Carson Valley. Estimates of water-budget components were updated using annual evapotranspiration (ET) rates, rates of streamflow loss to infiltration and gain from ground-water seepage, and rates of recharge from precipitation determined from data collected in 2003 and 2004 for the study and reported in the literature. Overall water budgets were developed for the area of basin-fill deposits in Carson Valley for water years 1941-70 and 1990-2005. Water years 1941-70 represent conditions prior to increased population growth and ground-water pumping, and the importation of effluent. A ground-water budget was developed for the same area for water years 1990-2005. Estimates of total inflow in the overall water budget ranged from 432,000 to 450,000 acre-feet per year (acre-ft/yr) for water years 1941-70 and from 430,000 to 448,000 for water years 1990-2005. Estimates of total inflow for both periods were fairly similar because variations in streamflow and precipitation were offset by increases in imported effluent. Components of inflow included precipitation on basin-fill deposits of 38,000 acre-ft/yr for both periods, streamflow of the Carson River and tributaries to the valley floor of 372,000 acre-ft/yr for water years 1941-70 and 360,000 acre-ft/yr for water years 1990-2005, ground-water inflow ranging from 22,000 to 40,000 acre-ft/yr for both periods, and imported effluent of 9,800 acre-ft/yr for water years 1990-2005 with none imported for water years 1941-70. Estimates of ground-water inflow from the California portion of Carson Valley averaged about 6,000 acre-ft/yr and ranged from 4,000 to 8,000 acre-ft/yr. These estimates compared well with a previous estimate of ground-water inflow across the State line. Estimates of total outflow in the overall water budget were 446,000 acre-ft/yr for water years 1941-70, and 439,000 to 442,000 acre-ft/yr for water years 1990-2005. Variations in ET and outflow of the Carson River were offset by an increase in net ground-water pumping for water years 1990-2005. Components of outflow include ET of 151,000 acre-ft/yr for water years 1941-70 and 146,000 acre-ft/yr for water years 1990-2005, streamflow of the Carson River of 293,000 acre-ft/yr for water years 1941-70 and 278,000 acre-ft/yr for water years 1990-2005, and net ground-water pumping of 2,000 acre-ft/yr for water years 1941-70, and 15,000 to 18,000 acre-ft/yr for water years 1990-2005. The decreased average flows for water years 1990-2005 compared to water years 1940-71 were likely the result of dry conditions from 1987 to 1990. The large volumes of inflow and outflow of the Carson River dominate the overall water budget. Estimates of ground-water recharge for water years 1990-2005 ranged from 35,000 to 56,000 acre-ft/yr, and total sources of ground-water discharge ranged from 41,000 to 44,000 acre-ft/yr. Components of ground-water recharge included ground-water inflow from the Carson Range and Pine Nut Mountains (22,000 to 40,000 acre-ft/yr), ground-water recharge from streamflow (a minimum value of 10,000 acre-ft/yr), and secondary recharge of pumped ground water that returns to the water table (3,000 to 6,000 acre-ft/yr). Components of total ground-water discharge included ground-water ET from native phreatophytes, riparian vegetation, and non-irrigated pasture grasses (11,000 acre-ft/yr); ground-water discharge to streamflow of the Carson River (15,000 acre-ft/yr), and net ground-water pumping (15,000 to 18,000 acre-ft/yr). Changes in land use between water years 1941-70 and 1990-2005 have decreased ET by about 5,000 acre-ft/yr. Increased application of effluent for irrigation between those years has decreased the use of surface water and ground water for irrigation by about 9,500 acre-ft/yr. The total decrease, about 15,000 acre-ft/yr, was approximately equal to the net ground-water pumping of 15,000 to 18,000 acre-ft/yr. The decrease in ET and in the use of streamflow and ground water for irrigation would tend to increase outflow of the Carson River from Carson Valley, offsetting the decrease in outflow caused by ground-water pumping without changes in land use predicted by previous studies of water budgets for Carson Valley.

Scientific Investigations Report↗
USGS2008

Availability, sustainability, and suitability of ground water, Rogers Mesa, Delta County, Colorado: Types of analyses and data for use in subdivision water-supply reports

The population of Delta County, Colorado, like that in much of the Western United States, is forecast to increase substantially in the next few decades. A substantial portion of the increased population likely will reside in rural subdivisions and use residential wells for domestic water supplies. In Colorado, a subdivision developer is required to submit a water-supply plan through the county for approval by the Colorado Division of Water Resources. If the water supply is to be provided by wells, the water-supply plan must include a water-supply report. The water-supply report demonstrates the availability, sustainability, and suitability of the water supply for the proposed subdivision. During 2006, the U.S. Geological Survey, in cooperation with Delta County, Colorado, began a study to develop criteria that the Delta County Land Use Department can use to evaluate water-supply reports for proposed subdivisions. A table was prepared that lists the types of analyses and data that may be needed in a water-supply report for a water-supply plan that proposes the use of ground water. A preliminary analysis of the availability, sustainability, and suitability of the ground-water resources of Rogers Mesa, Delta County, Colorado, was prepared for a hypothetical subdivision to demonstrate hydrologic analyses and data that may be needed for water-supply reports for proposed subdivisions. Rogers Mesa is a 12-square-mile upland mesa located along the north side of the North Fork Gunnison River about 15 miles east of Delta, Colorado. The principal land use on Rogers Mesa is irrigated agriculture, with about 5,651 acres of irrigated cropland, grass pasture, and orchards. The principal source of irrigation water is surface water diverted from the North Fork Gunnison River and Leroux Creek. The estimated area of platted subdivisions on or partially on Rogers Mesa in 2007 was about 4,792 acres of which about 2,756 acres was irrigated land in 2000. The principal aquifer on Rogers Mesa consists of alluvial-fan deposits that overlie shale and, locally, sandstone. Maps of the base of the aquifer, the water table, and the saturated thickness of the aquifer were prepared from data from the well files of the Colorado Division of Water Resources. The base of the aquifer generally is topographically higher than the valleys of the North Fork Gunnison River and Leroux Creek, and direct hydraulic connection of the aquifer to North Fork Gunnison River and Leroux Creek is limited. The aquifer is recharged primarily by infiltration of surface water diverted for irrigation. Ground water discharges to seeps and springs and through slope deposits at the boundaries of the aquifer. Data from the well files also were used to estimate the specific capacity of wells and to estimate the transmissivity and hydraulic conductivity of the aquifer. A water budget was used to estimate recharge to and discharge from the aquifer. Although storage within the aquifer likely varies seasonally and from year to year, it was assumed that there were no long-term changes in ground-water storage. Estimated average annual recharge to and discharge from the aquifer during November 1998 through October 2006 were about 30,767 acre-feet per year. Although sufficient ground water is available on Rogers Mesa for additional domestic water supplies, conversion of irrigated land to residential land use likely would reduce recharge to the aquifer, affecting the sustainability of ground-water supplies on Rogers Mesa. Stream-depletion analyses indicate that the ground water in the aquifer likely would be considered tributary ground water and additional uses of ground water to supply new subdivisions likely would require implementation of augmentation plans. Although sufficient ground water is available on Rogers Mesa for additional domestic water supplies, conversion of irrigated land to residential land use likely would reduce recharge to the aquifer, affecting the sustainability of ground-water supplies on Rogers Mesa. Stream-depletion analyses indicate that the ground water in the aquifer likely would be considered tributary ground water and additional uses of ground water to supply new subdivisions likely would require implementation of augmentation plans. Although the dissolved solids and dissolved sulfate concentrations in ground water from Rogers Mesa aquifer commonly exceeded the U.S. Environmental Protection Agency Secondary Maximum Contaminant Levels for drinking-water supplies, the quality of ground water from the aquifer generally is suitable for residential use. Concentrations of total nitrogen (nitrite plus nitrate, as nitrogen) in ground water ranged from 0.38 to 3.2 milligrams per liter and were less than the State of Colorado maximum contaminant level of 10 milligrams per liter. Concentrations of selenium from seeps and springs at the boundaries of the aquifer commonly exceeded 50 micrograms per liter, the State of Colorado maximum contaminant level for drinking-water supplies. This preliminary evaluation of ground-water supplies on Rogers Mesa could be improved with the collection of additional data including: additional mapping of hydrogeologic features; more accurate locations and altitudes of wells; accurate estimates of water-budget components; measurements of ground-water levels; and collection and analyses of ground-water samples. The use of numerical models of ground-water flow could improve evaluations of the potential effects of changes in land and water use on the water budget, aquifer storage, stream depletion, and well interference.

Colorado↗
USGS1990

National water summary 1987: Hydrologic events and water supply and use

Water use in the United States, as measured by freshwater withdrawals in 1985, averaged 338,000 Mgal/d (million gallons per day), which is enough water to cover the 48 conterminous States to a depth of about 2.4 inches. Only 92,300 Mgal/d, or 27.3 percent of the water withdrawn, was consumptive use and thus lost to immediate further use; the remainder of the withdrawals (72.7 percent) was return flow available for reuse a number of times as the water flowed to the sea. The 1985 freshwater withdrawals were much less than the average 30 inches of precipitation that falls on the conterminous States each year; consumptive use accounted for only 7 percent of the estimated annual runoff of 1,230,000 Mgal/d. Nonetheless, as the State summaries on water supply and use clearly show, water is not always available when and where it is needed. Balancing water demands with available water supplies constitutes one of the major resource allocation issues that will face the United States in the coming decade. Of the 1985 freshwater withdrawals, 78.3 percent (265,000 Mgal/d) came from surface-water sources (streams and lakes), and 21.7 percent (73,300 Mgal/d) came from ground water. Surface water provided drinking water for about 47 percent of the Nation's total population. It was the source of 59.9 percent of the Nation's public-supply systems. For self-supplied withdrawals, surface water accounted for 1.6 percent of the domestic and commercial uses; 64.0 percent of the industrial and mining use; 99.4 percent of the thermoelectric generation withdrawals, mainly for cooling water; and 65.6 percent of the agricultural withdrawals. Eight States accounted for 43 percent of the surface-water use; California, Colorado, and Idaho used surface water primarily for irrigation, and Dlinois, Michigan, Ohio, Pennsylvania, and Texas used surface-water primarily for cooling condensers or reactors in thermoelectric plants. Ground water provided drinking water for 53 percent of the Nation's total population and nearly all the rural population. It was the source of 40.1 percent of the public-supply systems withdrawals. For self-supplied withdrawals, ground water accounted for 11.3 percent of the domestic and commercial use, 17.3 percent of the industrial and mining withdrawals, less than 1 percent of the thermoelectric generation withdrawals, and 34.4 percent of the agricultural withdrawals (irrigation and livestock). Eight States Arizona, Arkansas, California, Florida, Idaho, Kansas, Nebraska, and Texas accounted for 66 percent of the ground water used. In each of those States, as in many other States, irrigation was the major use of ground water. Each offstream-use category described in the State summaries public supply, domestic and commercial, industrial and mining, thermoelectric power, and agriculture (irrigation and livestock) followed its own geographic pattern as described below. Consumptive use of water effectively removes the water from immediate further use downstream of the withdrawal point. Of the total amount of consumptive water use in 1985, agricultural use accounted for about 82.5 percent. More than one-half (53 percent) of irrigation water is consumptively used by evapotranspiration or is incorporated into the crop. This is a good indication of the effect that irrigated agriculture can have in a river basin where irrigation is a major activity. The availability of return flows for reuse depends largely on where the water reenters the system. If the return flows are discharged to a stream, they usually can be reused; if they are discharged to a saltwater estuary, they are effectively lost to further use because of water-quality degradation just as if the water had been consumptively used. Similarly, water that recharges a highly transmissive aquifer can be available for reuse either through pumpage from a well or as discharge to a local stream. Thus, much of the water withdrawn for different uses can and does become available for further use although the quality might degrade with each additional use. The allocation and the management of water resources are the responsibilities of the individual States and water institutions within the States. These institutions are evolving in response to the challenges of water management problems. As the individual State summaries indicate, recent State legislation deals with facilitating water transfers within the States as a means of reducing imbalances between water supplies and use, with emphasizing water conservation in times of drought and at places where groundwater depletion is a problem of long standing, and with reducing threats to public health and the environment from water pollution. Most of the State summaries indicate the expectation that water use will continue to increase in the future and that water contamination will continue to be a major water concern. Both issues will require increasingly intensive water management in the future. Whether the water resources under management are considered to be fully appropriated or over appropriated, as in some Western States, or whether the resource could support additional development, as is the situation in most States, improved water-use information will play a key role in future water management efforts.

Water Supply Paper↗
USGS2003

Simulation of ground-water flow and land subsidence in the Antelope Valley ground-water basin, California

Antelope Valley, California, is a topographically closed basin in the western part of the Mojave Desert, about 50 miles northeast of Los Angeles. The Antelope Valley ground-water basin is about 940 square miles and is separated from the northern part of Antelope Valley by faults and low-lying hills. Prior to 1972, ground water provided more than 90 percent of the total water supply in the valley; since 1972, it has provided between 50 and 90 percent. Most ground-water pumping in the valley occurs in the Antelope Valley ground-water basin, which includes the rapidly growing cities of Lancaster and Palmdale. Ground-water-level declines of more than 200 feet in some parts of the ground-water basin have resulted in an increase in pumping lifts, reduced well efficiency, and land subsidence of more than 6 feet in some areas. Future urban growth and limits on the supply of imported water may continue to increase reliance on ground water. To better understand the ground-water flow system and to develop a tool to aid in effectively managing the water resources, a numerical model of ground-water flow and land subsidence in the Antelope Valley ground-water basin was developed using old and new geohydrologic information. The ground-water flow system consists of three aquifers: the upper, middle, and lower aquifers. The aquifers, which were identified on the basis of the hydrologic properties, age, and depth of the unconsolidated deposits, consist of gravel, sand, silt, and clay alluvial deposits and clay and silty clay lacustrine deposits. Prior to ground-water development in the valley, recharge was primarily the infiltration of runoff from the surrounding mountains. Ground water flowed from the recharge areas to discharge areas around the playas where it discharged either from the aquifer system as evapotranspiration or from springs. Partial barriers to horizontal ground-water flow, such as faults, have been identified in the ground-water basin. Water-level declines owing to ground-water development have eliminated the natural sources of discharge, and pumping for agricultural and urban uses have become the primary source of discharge from the ground-water system. Infiltration of return flows from agricultural irrigation has become an important source of recharge to the aquifer system. The ground-water flow model of the basin was discretized horizontally into a grid of 43 rows and 60 columns of square cells 1 mile on a side, and vertically into three layers representing the upper, middle, and lower aquifers. Faults that were thought to act as horizontal-flow barriers were simulated in the model. The model was calibrated to simulate steady-state conditions, represented by 1915 water levels and transient-state conditions during 1915-95 using water-level and subsidence data. Initial estimates of the aquifer-system properties and stresses were obtained from a previously published numerical model of the Antelope Valley ground-water basin; estimates also were obtained from recently collected hydrologic data and from results of simulations of ground-water flow and land subsidence models of the Edwards Air Force Base area. Some of these initial estimates were modified during model calibration. Ground-water pumpage for agriculture was estimated on the basis of irrigated crop acreage and crop consumptive-use data. Pumpage for public supply, which is metered, was compiled and entered into a database used for this study. Estimated annual pumpage peaked at 395,000 acre-feet (acre-ft) in 1952 and then declined because of declining agricultural production. Recharge from irrigation-return flows was estimated to be 30 percent of agricultural pumpage; the irrigation-return flows were simulated as recharge to the regional water table 10 years following application at land surface. The annual quantity of natural recharge initially was based on estimates from previous studies. During model calibration, natural recharge was reduced from the initial estimate of 40,700 acre-ft per year (acre-ft/yr) to 30,300 acre-ft/yr. Results of the model simulations indicate that ground-water storage declined more than 8.5 million acre-ft from 1915 to 1995. During the period of peak agricultural pumping (1949-53), pumpage averaged 363,000 acre-ft/yr, and 79 percent of the ground water withdrawn came from storage primarily from layer 1 (the upper aquifer). Water released from compaction of the aquitards accounted for about 21,600 acre-ft/yr of the ground water removed from storage. Downward leakage from layer 1 into layer 2 (the middle aquifer) accounted for most (86 percent) of the pumpage from layer 2. For the simulation period 1991-95 (a period representing current conditions when pumpage for public supply exceeded agricultural pumpage), pumpage averaged 81,700 acre-ft/yr, and most of the ground water withdrawn from layer 2 came from downward leakage from layer 1. During this period, ground water removed from storage accounted for 17 percent of the total pumpage and recharge from irrigation return accounted for about 39 percent of the total pumpage. Ground water removed from storage as a result of compaction of aquitards was reduced to about 3,800 acre-ft/yr. The calibrated model was used to simulate the response of the aquifer to future pumping scenarios. Results of the simulation of scenario 1, for which total annual pumpage for 1996-2025 remained at the level specified for 1995, showed that water levels continued to rise (as much as 36 feet) in agricultural areas, continuing the long-term recovery from drawdown caused by historical agricultural pumpage. In the areas where pumping for public supply is concentrated, water levels continued to decline and subsidence continued in the central part of the ground-water basin. Water-level declines were largest (more than 100 feet) in the south-central part of the ground-water basin; most of the public-supply pumpage occurs in this area. As much as 1.9 feet of additional subsidence was simulated in the central part of the ground-water basin from 1996 to 2025. For scenario 2, public-supply pumpage was increased by 3.3 percent annually, and annual agricultural pumpage was increased by 75 percent more than that specified for 1995. Pumpage increases for scenario 2 resulted in significant water-level declines in the southern and northeastern part of the Lancaster subbasin; most pumping for public supply occurs in these areas. Results of this simulation showed that water levels declined more than 150 feet in the south-central part of the ground-water basin and that an additional 5 feet of subsidence was simulated in the central part of the basin.

California↗