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At least 1,639 records · Page 91Linked to original sources
USGS1997

Hydraulic properties and ground-water flow in the St Peter-Prairie du Chien-Jordan aquifer, Rochester area, southeastern Minnesota

The hydraulic properties were updated and their effects on ground-water flow in the St. Peter-Prairie du Chien-Jordan aquifer in the Rochester area in southeastern Minnesota were evaluated, using new information compiled since a study by Delin (1990). Since 1988, new information on the hydrogeology of the ground-water system in the Rochester area has become available from well-drilling and construction activity associated with Rochester's rapid growth. The St. Peter-Prairie du Chien-Jordan aquifer consists of the St. Peter Sandstone, the Prairie du Chien Group (limestones and dolomites), and the Jordan Sandstone. Horizontal hydraulic conductivity and transmissivity were determined from 15 aquifer tests and specific-capacity information compiled for 310 wells. A 140-square-mile area of the aquifer bounded on the west, south, and east by a ground-water divide contributes water to the Rochester, Minnesota, municipal wells. Transmissivities for the St. Peter-Prairie du Chien-Jordan aquifer in the study area range from less than 5,000 square feet per day (ft 2 /d) to greater than 20,000 ft 2 /d. Transmissivities greater than 20,000 ft 2 /d occur in the west-central, northwestern, and east-central parts of the study area. Transmissivities of less than 5,000 ft 2 /d occur in the northern, northeastern, central, and southern parts of the study area. The areas of greatest potential well yield coincide with areas of greatest transmissivity. Delin (1990) developed a ground-water-flow model to simulate flow of ground water in the St. Peter-Prairie du Chien-Jordan aquifer in the Rochester area. The 1988 Rochester model was rerun using revised horizontal hydraulic conductivity arrays in the model, based on the transmissivity distribution determined for this study. The results of the simulations using horizontal hydraulic conductivities based on the transmissivity distribution determined for this study may indicate that transmissivity values derived from specific-capacity information generally are too high. The transmissivity distribution determined for this study, however, is valid as an indicator of the spatial variability of the relative magnitude of transmissivity and potential well yield for the St. Peter-Prairie du Chien-Jordan aquifer in the study area. Water-level changes in wells from January through February 1988 to February through March 1995 ranged from -6.8 to +15.3 feet. Water-level changes in 12 Rochester municipal wells for the same period ranged from -7.4 to +8.0 feet. Water levels in wells generally rose in the northern and eastern parts of the study area and generally declined in the southwestern and western parts. Near Rochester, water levels in wells generally declined near the city boundaries and showed little change or rose in the central part of the city. Water-level changes from 1988 to 1995 near the ground-water divide generally were less than 2 feet, resulting in no appreciable changes in the location of the divide.

Minnesota↗
USGS1994

Application of a Geographic Information System for regridding a ground-water flow model of the Columbia Plateau Regional Aquifer System, Walla Walla River basin, Oregon-Washington

Computerized Geographic Information Systems (GIS) have become viable and valuable tools for managing,analyzing, creating, and displaying data for three-dimensional finite-difference ground-water flow models. Three GIS applications demonstrated in this study are: (1) regridding of data arrays from an existing large-area, low resolution ground-water model to a smaller, high resolution grid; (2) use of GIS techniques for assembly of data-input arrays for a ground-water model; and (3) use of GIS for rapid display of data for verification, for checking of ground-water model output, and for the cre.ation of customized maps for use in reports. The Walla Walla River Basin was selected as the location for the demonstration because (1) data from a low resolution ground-water model (Columbia Plateau Regional Aquifer System Analysis [RASA]) were available and (2) concern for long-term use of water resources for irrigation in the basin. The principal advantage of regridding is that it may provide the ability to more precisely calibrate a model, assuming chat a more detailed coverage of data is available, and to evaluate the numerical errors associated with a particular grid design.Regridding gave about an 8-fold increase in grid-node density.Several FORTRAN programs were developed to load the regridded ground-water data into a finite-difference modular model as model-compatible input files for use in a steady-state model run.To facilitate the checking and validating of the GIS regridding process, maps and tabular reports were produced for each of eight ground-water parameters by model layer. Also, an automated subroutine that was developed to view the model-calculated water levels in cross-section will aid in the synthesis and interpretation of model results.

Water-Resources Investigations Report↗
USGS1984

Ground-water resources of the Mattapoisett River aquifer, Plymouth County, Massachusetts: Summary for water-resource managers

Proposed increases in municipal pumpage in the Mattapoisett River valley will triple ground-water withdrawals in the next two decades. Because of State and local concern about the long-term effects of these withdrawals on ground-water levels and streamflow, a digital ground-water-flow model was developed to assist water-resource management. Ten development scenarios representing existing and proposed withdrawals were simulated using drought conditions. Under conditions simulating 1965, the driest year of record, predicted water levels in the aquifer are as much as 9 feet lower than average. Under severely dry conditions, simulating only enough recharge to keep the river flowing with no pumping, the predicted water levels are as much as 19 feet lower than average. During the greatest pumping demands, the predicted drawdown in five wells could cause well failure. Simulated pumping demands in six scenarios use all available ground-water discharge. Under severely dry conditions, it is predicted that the downstream half of the river will stop flowing under most pumping plans. (USGS)

Massachusetts↗
USGS2005

Trace metal concentrations in shallow ground water

Trace metal clean sampling and analysis techniques were used to examine the temporal patterns of Hg, Cu, and Zn concentrations in shallow ground water, and the relationships between metal concentrations in ground water and in a hydrologically connected river. Hg, Cu, and Zn concentrations in ground water ranged from 0.07 to 4.6 ng L −1 , 0.07 to 3.10 μg L −1 , and 0.17 to 2.18 μg L −1 , respectively. There was no apparent seasonal pattern in any of the metal concentrations. Filtrable Hg, Cu, and Zn concentrations in the North Branch of the Milwaukee River ranged from below the detection limit to 2.65 ng Hg L −1 ,0.51 to 4.30 μg Cu L −1 , and 0.34 to 2.33 μg Zn L −1 . Thus, metal concentrations in ground water were sufficiently high to account for a substantial fraction of the filtrable trace metal concentration in the river. Metal concentrations in the soil ranged from 8 to 86 ng Hg g −1 , 10 to 39 μg Cu g −1 , and 15 to 84 μg Zn g −1 . Distribution coefficients, K D , in the aquifer were 7900,22,000, and 23,000 L kg −1 for Hg, Cu, and Zn, respectively. These values were three to 40 times smaller than K D values observed in the Milwaukee River for suspended particulate matter.

Ground Water↗
USGS1954

Ground-water data collected in the Missouri River basin units in Kansas during 1953

Ground-water studies in the Missouri River basin were begun by the United States Geological Survey during the fall of 1945 as a part of a program for the development of the resources of the basin by the United States Bureau of Reclamation and other Federal Agencies. The studies of the ground-water resources in the part of Kansas that lies within the Missouri River basin have been coordinated with the cooperative program of ground water studies which were already being made in Kansas by the U. S. Geological Survey, the State Geological Survey of Kansas, the Division of Sanitation of the Kansas State Board of Health, and the Division of Water Resources of the Kansas State Board of Agriculture. Areas in which ground-water data have been and are being collected are the following: Almena unit in Norton and Phillips Counties; Bostwick unit in Jewell, Republic, and Cloud Counties; Cedar Bluff unit in Ellis, Rush, and Trego Counties; Glen Elder unit in Mitchell County; Kanopolis unit in Ellsworth, McPherson, and Saline Counties; Kirwin unit in Phillips, Smiths and Osborne Counties; St. Francis unit in Cheyenne County; Webster unit in Osborne County; and Wilson unit in Lincoln County. (See fig. 1.) Data relating to the Ladder Creek project in Greeley, Gove, Lane, Logan, Scott, Wallace, and Wichita Counties will be published later in a separate report.

Kansas↗
USGS1957

Ground-water geology of the Bruneau–Grand View area, Owyhee County, Idaho

The Bruneau-Grand View area is part of an artesian basin in northern Owyhee County, Idaho. The area described in this report comprises about 600 square miles, largely of undeveloped public domain, much of which is open, or may be opened, for desert-entry filing. Many irrigation-entry applications to the Federal Government are pending, and information about ground-water geology is needed by local citizens and well drillers, by Federal agencies that have custody of the land, and by local and State agencies that administer water rights. The areal geology and ground-water conditions in the Bruneau-Grand View area seemingly typify several basins in southwestern Idaho, and this study is a step toward definition and analysis of regional problems in ground-water geology and the occurrence and availability of ground water for irrigation or other large-scale uses. Owyhee County is subdivided physiographically into a plateau area, the Owyhee uplift, and the Snake River valley. The Bruneau-Grand View area is largely within the Snake River valley. The climate is arid and irrigation is essential for stable agricultural development. Nearly all usable indigenous surface water in the area is appropriated, including freshet flow in the Bruneau River, which is used for power generation at the C. J. Strike Dam. However, with storage facilities additional land could be irrigated, and some land may be irrigated with Snake River water if suitable reclamation projects are constructed. Sedimentary and igneous rocks exposed in the area range in age from Miocene to Recent. The igneous rocks include silicic and basic intrusive and extrusive bodies, and the sedimentary rocks are compacted stream and lake sediments. The rocks contain economically important artesian aquifers; the principal ones are volcanic rocks in which ground water is imperfectly confined beneath sediments of the Idaho formation, thus forming a leaky artesian system. The altitude of the piezometric surface of the artesian water does not exceed about 2,700 feet above mean sea level. In some areas, where the land surface is below that altitude, the artesian system discharges water through springs and seeps and locally causes waterlogging and development of alkali soil. In chemical quality much of the water is unsuitable for irrigation and domestic use. The water contains a relatively moderate amount of dissolved solids, but the percent sodium and the concentration of fluoride are excessive for some uses. The quality of the water for irrigation ranges from excellent in the southern part of the artesian system to unsuitable in the northern part. All the artesian ground water that was sampled contained excessive amounts of fluoride. There is a substantial supply of undeveloped artesian water in the area, but sustained use of the water for irrigation may not be feasible unless provisions can be made for adequate soil drainage and soil amendment, because of the high percentage of sodium in the water. Detailed hydrologic and geologic study of the area should precede development.

Idaho↗
USGS1951

Ground-water situation in Oregon

The water that occurs beneath the land surface follows definite and well-known rules of hydraulics, the same as water on the surface. However, ground water must be studied by methods, some of which are unique to that type of water occurrence, in order to evaluate the part it plays in the over-all water scheme. Water that falls on the land surface as rain or snow and water that rests upon the surface may in places pass laterally or downward through the pores of the earth materials. There it may take one or more of a variety of paths before again flowing out on the surface or being expelled to the atmosphere by evaporation and by the transpiration of plants. Water so diverted underground is delayed or diverted from its course toward the sea and that digression results in many services of prime importance to mankind. Underground, the water generally exceeds in total quantity the water present on the land surface at any one time. The discussion of ground water can be clarified somewhat by a description of the major parts or phases of the normal path of water underground.

Oregon↗
USGS1999

Simulation of ground-water flow and movement of the freshwater-saltwater interface in the New Jersey coastal plain

The confined aquifers of the New Jersey Coastal Plain are sands that range in thickness from 50 to 600 feet and are separated by confining units. The confining units are composed of silts and clays that range in thickness from 500 to 1,000 feet. The aquifers are recharged by precipitation on their outcrop areas. This water then flows laterally downdip and vertically to the deeper confined aquifers. The confined aquifers ultimately discharge to the Raritan and Delaware Bays and to the Atlantic Ocean. In 1988, ground-water withdrawals from confined and unconfined New Jersey Coastal Plain aquifers were approximately 345 million gallons per day, more than 75 percent of which was pumped from the confined aquifers. These withdrawals have created large cones of depression in several Coastal Plain aquifers near populated areas, particularly in Camden and Monmouth Counties. The continued decline of water levels in confined aquifers can cause saltwater intrusion, reduce stream discharge near the outcrop areas, and threaten the quality of the ground-water supply. SHARP, a quasi-three-dimensional finite-difference computer model that can simulate freshwater and saltwater flow, was used to simulate the ground-water flow system in the New Jersey Coastal Plain, including the location and movement of the freshwater-saltwater interface in nine aquifers and eight intervening confining units. The freshwater-saltwater interface is defined as the hypothetical line seaward of which the chloride concentration is equal to or greater than 10,000 milligrams per liter. Model simulations were used to estimate the location and movement of the freshwater-saltwater interface resulting from (1) eustatic sea-level changes over the past 84,000 years, (2) ground-water withdrawals from 1896 through 1988, (3) and future ground-water withdrawals from 1988 to 2040 from Coastal Plain aquifers. Simultion results showed that the location and movement of the freshwater-saltwater interface are more dependent on the historical sea level than on the stresses imposed on the flow system by ground-water withdrawals from the Coastal Plain aquifers from 1896 to 1988. Results of a predictive simulation in which pumpage from existing wells was increased by 30 percent indicate that additional withdrawals from each of the eight confined aquifers in the Coastal Plain would broaden and deepen the existing cones of depression and result in significant drawdowns from the 1988 potentiometric surfaces. Drawdowns of 30 feet were simulated at the center of the cone of depression in the Upper, Middle, and Lower Potomac-Raritan-Magothy aquifers in Camden and Ocean Counties. Simulated drawdowns exceeded 80 feet at the center of the cone of depression in the Wenonah-Mount Laurel and Englishtown aquifers in Monmouth County. Drawdowns of 30 feet were simulated in the lower Kirkwood-Cohansey and confined Kirkwood aquifers in Cape May County. Simulation results showed that the increase in ground-water withdrawals would result in only minimal movement of the freshwater-saltwater interface by 2040, despite large drawdowns.

New Jersey↗
USGS2006

Vulnerability of recently recharged ground water in the High Plains aquifer to nitrate contamination

Nitrate concentrations greater than background levels have been detected in ground water of the High Plains aquifer. Empirically based models and corresponding maps were developed that predict the vulnerability of the aquifer to nonpoint-source nitrate contamination. The models predict the probability of detecting nitrate concentrations larger than 4 milligrams per liter in ground water of the High Plains aquifer that was recharged during the last 50 years. The models were calibrated by correlating concentrations of nitrate in ground water from 336 wells that intercept recently recharged (less than 50 years) water with anthropogenic and hydrogeologic explanatory variables. Particle-tracking simulations delineated well-contributing areas and determined well-screen depths that intercept recently recharged ground water. The models were developed using multivariate logistic regression, and a map was generated from these models using a geographic information system. Two multivariate logistic regression models of vulnerability were found to have the most statistical significance and the best model fit and predictive ability. The two models represent the Northern High Plains and the combined Central and Southern High Plains, and they indicate that ground-water vulnerability of the entire High Plains aquifer is best explained by the spatial distribution of nonirrigated and irrigated agricultural lands, organic matter of the soil, depth to the regional water table, and clay content of the unsaturated zone. Vulnerability of the Northern High Plains is greater in areas that have more nonirrigated and irrigated agricultural lands and less organic matter in the soil. The vulnerability of the Central and Southern High Plains also is greater in areas that have more nonirrigated and irrigated agricultural lands and also in areas with shallow depths to water table and less clay in the unsaturated zone. The majority (53.3 percent) of the High Plains aquifer has less than a 40-percent predicted probability of nitrate concentrations larger than 4 milligrams per liter. Approximately 21.1 percent of the High Plains aquifer has a relatively high (greater than 60 percent) predicted probability of nitrate concentrations greater than or equal to 4 milligrams per liter. Areas with relatively high predicted probability are located in the southwestern, southern, and eastern areas of the Northern High Plains, in the eastern arm of the Central High Plains, and in southern areas of the Southern High Plains. Areas of the aquifer with relatively low (less than 40 percent) predicted vulnerability to nitrate concentrations greater than or equal to 4 milligrams per liter are located in the northwestern and north-central areas of the Northern High Plains, the central and southern areas of the Central High Plains, and a band across the north-central part of the Southern High Plains. Uncertainty of these vulnerability predictions was estimated by Latin hypercube sampling to address propagation of model and data errors inherently associated with estimates of model coefficients and explanatory variables. Results of the Latin hypercube sampling simulations are presented as uncertainty maps of the lower 5th and upper 95th percentile of the output probability distribution, which represents the 90-percent prediction interval that contains the true probability of detecting nitrate greater than or equal to 4 milligrams per liter. Generally, these uncertainty maps show greater prediction uncertainty in areas with relatively higher predicted vulnerability and lower uncertainty in areas with relatively lower predicted vulnerability.

Scientific Investigations Report↗
USGS2007

Distinguishing sources of ground water recharge by using δ2H and δ18O

Stable isotope values of hydrogen and oxygen from precipitation and ground water samples were compared by using a volumetrically based mixing equation and stable isotope gradient to estimate the season and location of recharge in four basins. Stable isotopes were sampled at 11 precipitation sites of differing elevation during a 2-year period to quantify seasonal stable isotope contributions as a function of elevation. Supplemental stable isotope data collected by the International Atomic Energy Association during a 14-year period were used to reduce annual variability of the mean seasonal stable isotope data. The stable isotope elevation relationships and local precipitation elevation relationships were combined by using a digital elevation model to calculate the total volumetric contribution of water and stable isotope values as a function of elevation within the basins. The results of these precipitation calculations were compared to measured ground water stable isotope values at the major discharge points near the terminus of the basins. Volumetric precipitation contributions to recharge were adjusted to isolate contributing elevations. This procedure provides an improved representation of recharge contributions within the basins over conventional stable isotope methods. Stable isotope values from wells and springs at the terminus of each basin were used to infer the elevations of precipitation important for recharge of the regional ground water flow system. Ancillary climatic, geologic, and stable isotope values were used to further constrain the location where precipitation is entering the ground water flow system.

Arizona↗
USGS1993

Preliminary evaluation of hydrogeology and ground-water quality in valley sediments in the vicinity of Killarney Lake, Kootenai County, Idaho

Ground water near Killarney Lake in the Coeur d'Alene River Valley, Idaho, contains arsenic, cadmium, lead, and zinc in concentrations that would make it unsuitable for a potable (drinking water) supply. Dissolved arsenic in one well was more than six times higher than the U.S. Environmental Protection Agency' s maximum contaminant level. However, ground water discharging to the Coeur d'Alene River in the area probably has minimal effects on river water quality because of poor transmissive characteristics of the fine-grained valley sediments. Hydraulic conduc- tivity values are between 1.0 x 100 and 6.3 x 100 feet per day, calculated from slug-test data from three of the six monitoring wells installed for this study; the ground-water-flow gradient is 0.0015 or less, determined from the water-level contour map; and the valley sediments are about 400 feet thick. Although the sediments near Killarney Lake do not transmit large quantifies of water, coarser grained sediments upstream from Killarney Lake. Because sediments along the Coeur d'Alene River are contaminated downstream from the South Fork Coeur d'Alene River near Enaville, where mining wastes were discharged for more than 100 years, it is possible that ground water near Cataldo could contribute significant quantities of contaminants to the Coeur d'Alene River.

Water-Resources Investigations Report↗
USGS1985

Land subsidence caused by ground water withdrawal in urban areas

At least eight urban areas in the world have encountered significant economic impact from land subsidence caused by pumping of ground water from unconsolidated sediment. The areas, most of which are coastal, include Bangkok, Houston, Mexico City, Osaka, San Jose, Shanghai, Tokyo, and Venice. Flooding related to decreased ground elevation is the principal adverse effect of the subsidence. Lesser effects include regional tilting, well-casing failures, "rising" buildings, and ground failure or rupture. Subsidence of most of these urban areas began before the phenomenon was discovered and understood. Thus, the subsidence problems were unanticipated. Methods to arrest subsidence typically have included control of ground water pumping and development of surface water to offset the reductions of ground water pumping. Ground water recharge has also been practiced. Areas threatened by flooding have been protected by extensive networks of dikes and sea walls, locks, and pumping stations to remove storm runoff. ?? 1985 D. Reidel Publishing Company.

GeoJournal↗
USGS2005

Role of ground water in geomorphology, geology, and paleoclimate of the southern High Plains, USA

Study of ground water in the Southern High Plains is central to an understanding of the geomorphology, deposition of economic minerals, and climate change record in the area. Ground water has controlled the course of the Canadian and Pecos rivers that isolated the Southern High Plains from the Great Plains and has contributed significantly to the continuing retreat of the westward escarpment. Evaporative and dissolution processes are responsible for current plateau topography and the development of the signature 20,000 small playa basins and 40 to 50 large saline lake basins in the area. In conjunction with eolian processes, ground water transport controls the mineralogy of commercially valuable mineral deposits and sets up the distribution of fine efflorescent salts that adversely affect water quality. As the water table rises and retreats, lunette and tufa formation provides valuable paleoclimate data for the Southern High Plains. In all these cases, an understanding of ground water processes contributes valuable information to a broad range of geological topics, well beyond traditional interest in water supply and environmental issues.

Colorado, Kansas, New Mexico, Texas↗
USGS1995

Relation of land use to nitrogen concentration in ground water in the Patuxent River basin, Maryland

A decrease in nitrogen inputs could improve water quality in the Chesapeake Bay. In order to provide information about nitrogen transport to the bay, the U.S. Geological Survey examined historical land use associated with nonpoint sources of nitrogen and nitrogen concentrations in ground water in the Patuxent River Basin, Maryland; The Patuxent River is a major tributary to the bay. Most nitrogen in ground water was present as nitrate. In the Piedmont Physiographic Province part of the Patuxent River basin, nitrate concentrations in ground water were higher in agricultural areas than in forested and urban areas. Nitrate concentrations were related to land use at well sites because wells yielded water that infiltrated within the same contributing area where the wells are located. Agricultural activities possibly were a source for the transport of large amounts of nitrogen to ground water and probably also to base flow in nearby streams. The high nitrate concentrations were not attributed to sampling bias. In the Coastal Plain Physiographic Province, most nitrate concentrations were low and were unrelated to land use at the well site because the wells were screened in deep, regional aquifers. Ground water in a few shallow wells had high nitrate concentrations, possibly related to nearby agricultural and urban land use. Increased nitrate concentrations in shallow ground water could increase concentrations in stream base flow and eventually could increase concentrations in regional aquifers and ground water that discharges directly to the Chesapeake Bay.

Water-Resources Investigations Report↗
USGS1998

In-situ alteration of minerals by acidic ground water resulting from mining activities: Preliminary evaluation of method

The chemical composition of the Cu-mining-related acidic ground water (pH ∼ 3.5 to near neutral) in Pinal Creek Basin, Arizona has been monitored since 1980. In-situ experiments are planned using alluvial sediments placed in the ground-water flow path to measure changes in mineral and chemical composition and changes in dissolution rates of subsurface alluvial sediments. The test results should help refine developed models of predicted chemical changes in ground-water composition and models of streamflow. For the preliminary test, sediment from the depth of the well screen of a newly drilled well was installed in three wells, the source well (pH 4.96) and two up-gradient wells (pHs 4.27 and 4.00). The sediment was placed in woven macrofilters, fastened in series to polyvinyl chloride (PVC) pipes, and hung at the screened level of each well. After interacting with the slowly moving ground water for 48 days, the test sediments were removed for analysis. There was no evidence that any of the materials used were biologically or chemically degraded or that the porosity of the filters was diminished by ferric hydroxide precipitation. These materials included 21-μm-pore (21PEMF) and 67-μm-pore polyester and the 174-μm-pore fluorocarbon Spectra/mesh macrofilters containing the in-situ sediment, the polypropylene (PP) macrofilter support structures, and the Nylon (NY) monofilament line used to attach the samples to the PVC pipe. Based on chemical and mineral composition and on particle-size distribution of the sediment before and after ground-water exposure, the 21PEMF macrofilter was chosen as the most suitable macrofilter for the long-term in-situ experiment. Tests also showed that the PP support structures and the NY monofilament line were sufficiently durable for this experiment.

Arizona↗
USGS2005

Plants as indicators of focused ground water discharge to a northern Minnesota lake

Determining the discharge of ground water to Shingobee Lake (66 ha), north-central Minnesota, is complicated by the presence of numerous springs situated adjacent to the lake and in the shallow portion of the lakebed. Springs first had to be located before these areas of more rapid discharge could be quantified. Two methods that rely on the distribution of aquatic plants are useful for locating springs. One method identifies areas of the near-shore lakebed where floating-leaf and emergent aquatic vegetation are absent. The second method uses the distribution of marsh marigold (Caltha palustris L.) to locate springs that discharge on land near the shoreline of the lake. Marsh marigold produces large (2 to 4 cm diameter) yellow flowers that provide a ready marker for locating ground water springs. Twice as many springs (38) were identified using this method as were identified using the lack of near-shore vegetation. A portable weir was used to measure discharge from onshore springs, and seepage meters were used to measure discharge from near-shore springs. Of the total 56.7 L s −1 that enters the lake from ground water, approximately 30% comes from onshore and near-shore springs.

Minnesota↗
USGS1996

Status of ground-water resources at U.S. Navy Support Facility, Diego Garcia; summary of hydrologic and climatic data, January 1994 through September 1996

This report describes the status of ground-water resources at U.S. Navy Support Facility, Diego Garcia. Data presented are from January 1994 through September 1996, with a focus on data from July through September 1996 (third quarter of 1996). A complete database of ground-water withdrawals and chloride-concentration records since 1985 is maintained by the U.S. Geological Survey. Total rainfall for the period July through September 1996 was 8.94 inches, which is 60 percent less than the mean rainfall of 22.23 inches for the period July through September. July and August are part of the annual dry season, while September is the start of the annual wet season. Ground-water withdrawal during July through September 1996 averaged 1,038,300 gallons per day. Withdrawal for the same 3 months in 1995 averaged 888,500 gallons per day. Ground-water withdrawals have steadily increased since about April 1995. At the end of September 1996, the chloride concentration of water from the elevated tanks at Cantonment and Air Operations were 68 and 150 milligrams per liter, respectively. The chloride concentration from all five production areas increased throughout the third quarter of 1996, and started the upward trend in about April 1995. Chloride concentration of ground water in monitoring wells at Cantonment and Air Operations also increased throughout the third quarter of 1996, with the largest increases from water in the deepest monitoring wells. Chloride concentrations have not been at this level since the dry season of 1994. A fuel-pipeline leak at Air Operations in May 1991 decreased total islandwide withdrawals by 15 percent. This lost pumping capacity is being offset by increased pumpage at Cantonment. Six wells do not contribute to the water supply because they are being used to hydraulically divert fuel migration away from water-supply wells by a program of ground-water withdrawal and injection.

Open-File Report↗
USGS1995

Ground-water flow, geochemistry, and effects of agricultural practices on nitrogen transport at study sites in the Piedmont and Coastal Plain physiographic provinces, Patuxent River Basin, Maryland

The effects of agricultural practices on nitrogen transport were assessed at two 10-acre study sites in the Patuxent River Basin, Maryland, during 1986- 92. Nitrogen load was larger in ground water than in surface runoff at both sites. Denitrification and (or) long traveltimes of ground water at the study site in the Piedmont Province resulted in lower concentrations of nitrate than at the site in the Coastal Plain Province. The study period was brief compared to traveltimes of nitrogen in ground water of several decades. Therefore, the effects of agricultural practices were observed only in parts of both sites. At the Piedmont site, nitrate concentration in two springs was 7 mg/L (milligrams per liter) two years after corn was grown under no-till cultivation, and decreased to 3.5 mg/L during 4 years while cultivation practices and crops included no-till soybeans, continuous alfalfa, and contoured strips alternated among corn, alfalfa, and soybeans. Nitrogen load in ground water decreased from 12 to 6 (lb/acre)/yr (pounds per acre per year). At the Coastal Plain site, the concentration of nitrate in ground water decreased from 10 mg/L after soybeans were grown under no-till cultivation for 2 years, to 9 mg/L after soybeans were grown under conventional till cultivation for 3 years. No-till cultivation in 1988 resulted in a greater nitrogen load in ground water (12.55 (lbs/acre)/yr), as well as greater ground-water recharge and discharge, than conventional till cultivation in 1991 (11.51 (lbs/ acre)/yr), even though the amount and timing of precipitation for both years were similar.

Maryl↗