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At least 181 records · Page 10Linked to original sources
USGS1996

Postaudit of head and transmissivity estimates and ground-water flow models of Avra Valley, Arizona

Ground water from regional alluvial-aquifer systems is the main source of water in the alluvial basins of Arizona, such as Avra Valley. Ground-water flow models are used to assess ground-water availability and the effects of development on the regional ground-water resources. A postaudit of regional-head and transmissivity estimates and the ground-water flow models of Avra Valley was used to evaluate potential errors in the distribution of aquifer properties and recharge that can cause predictive errors in ground-water models. Simulations of predevelopment conditions in 1940 and historical development conditions for 1960-79 provided the basis of comparison for assessing predictive errors of historical conditions for two regional ground-water flow models. Potential errors in the estimation of the regional-head and transmissivity and alternate conceptual models were compared with an existing calibrated two-layer flow model for predevelopment (1940) and developed conditions (1940-85). Measured heads can be subdivided into a north-central region and a region south of the basin constriction. A more variable regional-head surface typical of developed aquifer systems was indicated by kriged developed heads (1985) with about 50 percent more uncertainty than predevelopment heads (1940). Incorporating heads from adjacent basins at the ground-water inflow and outflow regions reduced uncertainty in kriged heads for these boundary areas. Universal cokriging of heads with the strongly correlated land-surface altitudes may improve regional-head estimates and model comparisons where head data are sparse. Local transmissivity estimates can be subdivided into northern and south-central regions that are distributed along the valley axis and the Santa Cruz River. Regional geostatistical estimates of transmissivity, which are based solely on local estimates, are low in the northern part of Avra Valley and are high in the south-central part when compared with the head-conditioned model-derived estimates. These differences may be related to a systematic bias between aquifer-test conditions and methods of aquifer-test analysis. Cokriging transmissivities with specific capacity and silt-and-clay content provided the least uncertainty of all kriged estimates. Predictive errors for the Avra Valley model are the result of a different combination of factors that become significant in the simulation of ground-water flow for the periods representing predevelopment, historical development, and future development conditions. Predictive errors for simulation of predevelopment conditions are caused by potential systematic errors in estimates of local transmissivity, uncertainty in long-term mountain-front recharge, and uncertainty in predevelopment heads along the margins of the basin where recharge and transmissivity estimates are constrained by heads during model calibration. Analog-model historical predictions of future development indicate changes to 1985 were as much as 50 to 100 feet different from actual declines that were caused by errors in the spatial distribution and not the total amount of estimated future pumpage. Predictive errors for simulation of historical development (1960-79) appear to be caused to a greater extent by combined errors in estimates of transmissivity and storage properties and to a lesser extent by estimates of net withdrawal and subsidence. Comparison of the two digital models resulted in differences in transmissivity of as much as 30,000 feet squared per day and differences in specific yield of as much as 0.1. In combination with some differences in net withdrawal, these model-parameter differences resulted in local differences in change in storage of as much as 4,000 acre-feet per square mile and are equivalent to historical predictive errors in water levels of as much as 40 feet. Areas with no differences in model parameters yield comparable simulated water-level declines that are similar to measured declines. The pattern of differences in transmissivity and storage parameters are similar to differences between model-derived estimates conditioned on heads and related geostatistical estimates derived from aquifer-test estimates. A postaudit analysis of alternate conceptual models was explored on the basis of well-by-well comparisons of reductions in mean error and variance, and through the use of standardized calibration-error maps for predevelopment heads (1940) and developed heads (1985). Calibration-error maps provide a useful tool for exploring the spatial structure of model errors and the relative adequacy of model fit that is not available from traditional methods of model comparison. Calibration-error maps indicate estimated heads were too high in the southern part of Avra Valley, and estimated heads were too low in the northern part. Increased transmissivity in the southern part of the lower model layer; decreased hydraulic conductivity in the southwestern part of the upper layer; reduced ground-water inflow from Altar Valley; and increased recharge along the Tortolita Mountains, Tucson Mountains, and Brawley Wash yielded a significantly better model for predevelopment but not for developed conditions (1940-85). This may indicate that alternate conceptual models are different for different time periods or require analysis of time-varying model parameters for developed conditions, such as climatically variable recharge. Predictive errors for future simulations (1986-2025) also could potentially include errors of more than 40 ft from omission of subsidence from the simulation of regional ground-water flow in Avra Valley. Further refinement of the changing conceptual model of an aquifer system under continuing development and variable climate, such as Avra Valley, will require a variety of additional geophysical, geochemical, and hydraulic field data.

Arizona↗
USGS2004

Simulation of ground-water flow in glaciofluvial aquifers in the Grand Rapids area, Minnesota

A calibrated steady-state, finite-difference, ground-waterflow model was constructed to simulate ground-water flow in three glaciofluvial aquifers, defined in this report as the upper, middle, and lower aquifers, in an area of about 114 mi2 surrounding the city of Grand Rapids in north-central Minnesota. The calibrated model will be used by Minnesota Department of Health and communities in the Grand Rapids area in the development of wellhead protection plans for their water supplies. The model was calibrated through comparison of simulated ground-water levels to measured static water levels in 351 wells, and comparison of simulated base-flow rates to estimated base-flow rates for reaches of the Mississippi and Prairie Rivers. Model statistics indicate that the model tends to overestimate ground-water levels. The root mean square errors ranged from +12.83 ft in wells completed in the upper aquifer to +19.10 ft in wells completed in the middle aquifer. Mean absolute differences between simulated and measured water levels ranged from +4.43 ft for wells completed in the upper aquifer to +9.25 ft for wells completed in the middle aquifer. Mean algebraic differences ranged from +9.35 ft for wells completed in the upper aquifer to +14.44 ft for wells completed in the middle aquifer, with the positive differences indicating that the simulated water levels were higher than the measured water levels. Percentage errors between simulated and estimated base-flow rates for the three monitored reaches all were less than 10 percent, indicating good agreement. Simulated ground-water levels were most sensitive to changes in general-head boundary conductance, indicating that this characteristic is the predominant model input variable controlling steady-state water-level conditions. Simulated groundwater flow to stream reaches was most sensitive to changes in horizontal hydraulic conductivity, indicating that this characteristic is the predominant model input variable controlling steady-state flow conditions.

Minnesota↗
USGS1999

Review of Phosphorus Control Measures in the United States and Their Effects on Water Quality

Historical information on phosphorus loadings to the environment and the effect on water quality are summarized in this report, which was produced as part of the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Program. Phosphorus is a water-quality constituent of concern because it is often the limiting nutrient responsible for accelerated eutrophication in water bodies. Phosphorus inputs to the environment have increased since 1950 as the use of phosphate fertilizer, manure, and phosphate laundry detergent increased; however, the manufacture of phosphate detergent for household laundry was ended voluntarily in about 1994 after many States had established phosphate detergent bans. Total phosphorus concentrations in raw wastewater effluent contained about 3 milligrams per liter of total phosphorus during the 1940's, increased to about 11 milligrams per liter at the height of phosphate detergent use (1970), and have currently declined to about 5 milligrams per liter. However, in some cases, tertiary wastewater treatment still is needed to effectively improve water quality of streams. Downward trends in phosphorus concentrations since 1970 have been identified in many streams, but median total phosphorus concentrations still exceed the recommended limit of 0.1 milligram per liter across much of the Nation. Data from the NAWQA Program are representative of a variety of phosphorus-control measures, and, therefore, may be used to evaluate the effects of various control strategies. Current areas of concern include: evaluation of the effects of increased manure loadings of phosphorus on soil phosphorus and, subsequently, on ground water and subsurface runoff; determination of point-source and nonpoint-source components of phosphorus loads by geographic modeling and hydrologic separation techniques; and development of methods or indices to evaluate nutrient impairment in streams and rivers to serve as a basis for developing phosphorus criteria or standards.

Water-Resources Investigations Report↗
USGS1978

Methods of estimating recharge to the Floridan aquifer in northeast Florida

Recharge to the principal artesian aquifer in a six-county area in northeast Florida was calculated using closed contour methods , water budgets, and formulas for computing leakage through confining beds. Each estimate was tested in a computer model of groundwater flow to see which estimate was best. Calculations of flow medway between the 24- and 21-meter potentiometric contours show a total flow of 164,000 cubic meters per day, a significant part ofm which must be recharged within the closed 24-meter controur. Flow midway between the 18- and 15-meter contours was calculated to be 342,000 cubic meters per day. A water budget for the same area in water year 1972 indicates a potential recharge rate of 2,570,000 cubic meters per day. Digital modeling of ground-water flow in northeast Florida shows that the recharge rate calculated by closed contour is insufficient to balance the natural discharge of the regional ground-water system, whereas the rate calculated from water budgets is too great. Therefore, the best way to simulate the recharge mechanism of the northeast Florida flow model may be with a constant head boundary, rather than with constant flux. (Woodard-USGS)

Florida↗
USGS2007

Hydrogeologic Setting and Ground-Water Flow in the Leetown Area, West Virginia

The Leetown Science Center is a research facility operated by the U.S. Geological Survey that occupies approximately 455-acres near Kearneysville, Jefferson County, West Virginia. Aquatic and fish research conducted at the Center requires adequate supplies of high-quality, cold ground water. Three large springs and three production wells currently (in 2006) supply water to the Center. The recent construction of a second research facility (National Center for Cool and Cold Water Aquaculture) operated by the U.S. Department of Agriculture and co-located on Center property has placed additional demands on available water resources in the area. A three-dimensional steady-state finite-difference ground-water flow model was developed to simulate ground-water flow in the Leetown area and was used to assess the availability of ground water to sustain current and anticipated future demands. The model also was developed to test a conceptual model of ground-water flow in the complex karst aquifer system in the Leetown area. Due to the complexity of the karst aquifer system, a multidisciplinary research study was required to define the hydrogeologic setting. Geologic mapping, surface- and borehole-geophysical surveys, stream base-flow surveys, and aquifer tests were conducted to provide the hydrogeologic data necessary to develop and calibrate the model. It would not have been possible to develop a numerical model of the study area without the intensive data collection and methods developments components of the larger, more comprehensive hydrogeologic investigation. Results of geologic mapping and surface-geophysical surveys verified the presence of several prominent thrust faults and identified additional faults and other complex geologic structures (including overturned anticlines and synclines) in the area. These geologic structures are known to control ground-water flow in the region. Results of this study indicate that cross-strike faults and fracture zones are major avenues of ground-water flow. Prior to this investigation, the conceptual model of ground-water flow for the region focused primarily on bedding planes and strike-parallel faults and joints as controls on ground-water flow but did not recognize the importance of cross-strike faults and fracture zones that allow ground water to flow downgradient across or through less permeable geologic formations. Results of the ground-water flow simulation indicate that current operations at the Center do not substantially affect either streamflow (less than a 5-percent reduction in annual streamflow) or ground-water levels in the Leetown area under normal climatic conditions but potentially could have greater effects on streamflow during long-term drought (reduction in streamflow of approximately 14 percent). On the basis of simulation results, ground-water withdrawals based on the anticipated need for an additional 150 to 200 gal/min (gallons per minute) of water at the Center also would not seriously affect streamflow (less than 8 to 9 percent reduction in streamflow) or ground-water levels in the area during normal climatic conditions. During drought conditions, however, the effects of current ground-water withdrawals and anticipated additional withdrawals of 150 to 200 gal/min to augment existing supplies result in moderate to substantial declines in water levels of 0.5-1.2 feet (ft) in the vicinity of the Center's springs and production wells. Streamflow was predicted to be reduced locally by approximately 21 percent. Such withdrawals during a drought or prolonged period of below normal ground-water levels would result in substantial declines in the flow of the Center's springs and likely would not be sustainable for more than a few months. The drought simulated in this model was roughly equivalent to the more than 1-year drought that affected the region from November 1998 through February 2000. The potential reduction in streamflow is a result of capture of ground water tha

Scientific Investigations Report↗
USGS2002

Reassessment of ground-water recharge and simulated ground-water availability for the Hawi area of North Kohala, Hawaii

An estimate of ground-water availability in the Hawi area of north Kohala, Hawaii, is needed to determine whether ground-water resources are adequate to meet future demand within the area and other areas to the south. For the Hawi area, estimated average annual recharge from infiltration of rainfall, fog drip, and irrigation is 37.5 million gallons per day from a daily water budget. Low and high annual recharge estimates for the Hawi area that incorporate estimated uncertainty are 19.9 and 55.4 million gallons per day, respectively. The recharge estimates from this study are lower than the recharge of 68.4 million gallons per day previously estimated from a monthly water budget. Three ground-water models, using the low, intermediate, and high recharge estimates (19.9, 37.5, and 55.4 million gallons per day, respectively), were developed for the Hawi area to simulate ground-water levels and discharges for the 1990?s. To assess potential ground-water availability, the numerical ground-water flow models were used to simulate the response of the freshwater-lens system to withdrawals at rates in excess of the average 1990?s withdrawal rates. Because of uncertainty in the recharge estimate, estimates of ground-water availability also are uncertain. Results from numerical simulations indicate that for appropriate well sites, depths, and withdrawal rates (1) for the low recharge estimate (19.9 million gallons per day) it may be possible to develop an additional 10 million gallons per day of fresh ground water from the Hawi area and maintain a freshwater-lens thickness of 160 feet near the withdrawal sites, (2) for the intermediate recharge estimate (37.5 million gallons per day) it may be possible to develop an additional 15 million gallons per day of fresh ground water from the Hawi area and maintain a freshwater-lens thickness of 190 feet near the withdrawal sites, and (3) for the high recharge estimate (55.4 million gallons per day) it may be possible to develop at least an additional 20 million gallons per day of fresh ground water from the Hawi area and maintain a freshwater-lens thickness of 200 feet near the withdrawal sites. Other well-field configurations than the ones considered potentially could be used to develop more fresh ground water than indicated by the scenarios tested in this study. Depth, spacing, and withdrawal rates of individual wells are important considerations in determining ground-water availability. The regional models developed for this study cannot predict whether local saltwater intrusion problems may occur at individual withdrawal sites. Results of this study underscore the importance of collecting new information to better constrain the recharge estimates.

Hawaii↗
USGS2004

Simulation of ground-water flow in the Cedar River alluvial aquifer flow system, Cedar Rapids, Iowa

The Cedar River alluvial aquifer is the primary source of municipal water in the Cedar Rapids, Iowa, area. Since 1992, the U.S. Geological Survey, in cooperation with the City of Cedar Rapids, has investigated the hydrogeology and water quality of the Cedar River alluvial aquifer. This report describes a detailed analysis of the ground-water flow system in the alluvial aquifer, particularly near well field areas. The ground-water flow system in the Cedar Rapids area consists of two main components, the unconsolidated Quaternary deposits and the underlying carbonate bedrock that has a variable fracture density. Quaternary deposits consist of eolian sand, loess, alluvium, and glacial till. Devonian and Silurian bedrock aquifers overlie the Maquoketa Shale (Formation) of Ordovician age, a regional confining unit. Ground-water and surface-water data were collected during the study to better define the hydrogeology of the Cedar River alluvial aquifer and Devonian and Silurian aquifers. Stream stage and discharge, ground-water levels, and estimates of aquifer hydraulic properties were used to develop a conceptual ground-water flow model and to construct and calibrate a model of the flow system. This model was used to quantify the movement of water between the various components of the aluvial aquifer flow system and provide an improved understanding of the hydrology of the alluvial aquifer. Ground-water flow was simulated for the Cedar River alluvial aquifer and the Devonian and Silurian aquifers using the three-dimensional finite-difference ground-water flow model MODFLOW. The model was discretized into 223 rows and 354 columns of cells. Areal cell sizes range from about 50 feet on a side near the Cedar River and the Cedar Rapids municipal wells to 1,500 feet on a side near the model boundaries and farthest away from the Cedar Rapids municipal well fields. The model is separated into five layers to account for the various hydrogeologic units in the model area. Model results indicate that the primary sources of inflow to the modeled area are infiltration from the Cedar River (53.0 percent) and regional flow in the glacial and bedrock materials (34.1 percent). The primary sources of outflow from the modeled area are discharge to the Cedar River (45.4 percent) and pumpage (44.8 percent). Current steady-state pumping rates have increased the flow of water from the Cedar River to the alluvial aquifer by 43.8 cubic feet per second. Steady-state and transient hypothetical pumpage scenarios were used to show the relation between changes in pumpage and changes in infiltration of water from the Cedar River. Results indicate that more than 99 percent of the water discharging from municipal wells infiltrates from the Cedar River, that the time required for induced river recharge to equilibrate with municipal pumpage may be 150 days or more, and that ground-water availability in the Cedar Rapids area will not be significantly affected by doubling current pumpage as long as there is sufficient flow in the Cedar River to provide recharge.

Iowa↗
USGS1998

Hydrogeology, water quality, and geochemistry of the Rush Springs aquifer, western Oklahoma

The Rush Springs aquifer, in western Oklahoma, is equivalent to the Permian-age Rush Springs Formation. It is composed of very fine-grained to fine-grained sandstone that is massive to highly cross-bedded and is underlain by less-permeable Marlow Formation. Reported irrigation well yields exceed 1,000 gallons per minute; yields reported on 89 drillers' logs ranged from 11 to 850 gallons per minute. Transmissivities range from 670 to 1,870 feet squared per day. Specific yields for core samples range from 0.13 to 0.34. Estimates of hydraulic conductivities at one site ranged from 1.05 to 5.62 feet per day. The Rush Springs aquifer is recharged by infiltration of precipitation, ranging from 0.2 to more than 2 inches per year. Discharge is primarily to streams and rivers where the Rush Springs aquifer crops. Estimated total withdrawal was 54.7 million gallons per day in 1990. Over 42 million gallons per day, or 77.8 percent of water withdrawn, was used for irrigation of crops. Thirty-five of the 64 wells sampled produced nitrate concentration that equaled or exceeded drinking water standards. Sulfate concentration also exceeds the drinking water standards in some areas. Two major water types occur in the aquifer, a calcium-magnesium bicarbonate type and a calcium sulfate type. Dissolved solids concentrations in water samples from the aquifer ranged from 52 to 1,840 milligrams per liter. The chemical composition of ground water in the Rush Springs aquifer is the result of chemical reactions between the recharge waters and minerals in the overlying soils and rocks in the Rush Springs and Marlow Formations. Saturation indices of minerals were calculated for 64 water-quality analyses using the geochemical computer model WATEQF. Mass transfer rates were calculated using the mass-balance model NETPATH.

Water-Resources Investigations Report↗
USGS1980

Computer-model analysis of the use of Delaware River water to supplement water from the Potomac-Raritan-Magothy aquifer system in southern New Jersey

A computer model of the Potomac-Raritan-Magothy aquifer system was used to simulate the effects of supplementing ground water with water from the Delaware River. Replacement of ground water pumpage with surface water in a 150-square-mile area near Camden, N.J., was simulated. Artificial recharge of surface water was also simulated in the same area. A series of nine simulations was made. The simulations include the period 1974 to 2000. Two projections for water use were used. Also, in some of the model simulations a line of injection wells was simulated to prevent movement of saline water into pumping centers. The simulations indicate that heads will be as much as 100 feet higher in the year 2000 near the 150-square-mile area than that if ony ground water would be used without supplement of surface water. In the model simulations, heads recover upon application of surface water, but start declining again within 2 years. The rate of head decline after suface-water application is slower than before the application.

New Jersey↗
USGS2001

Hydrogeology, water quality, and simulated effects of ground-water withdrawals from the Floridan aquifer system, Seminole County and vicinity, Florida

The hydrogeology and ground-water quality of Seminole County in east-central Florida was evaluated. A ground-water flow model was developed to simulate the effects of both present day (September 1996 through August 1997) and projected 2020 ground-water withdrawals on the water levels in the surficial aquifer system and the potentiometric surface of the Upper and Lower Floridan aquifers in Seminole County and vicinity. The Floridan aquifer system is the major source of ground water in the study area. In 1965, ground-water withdrawals from the Floridan aquifer system in Seminole County were about 11 million gallons per day. In 1995, withdrawals totaled about 69 million gallons per day. Of the total ground water used in 1995, 74 percent was for public supply, 12 percent for domestic self-supplied, 10 percent for agriculture self-supplied, and 4 percent for recreational irrigation. The principal water-bearing units in Seminole County are the surficial aquifer system and the Floridan aquifer system. The two aquifer systems are separated by the intermediate confining unit, which contains beds of lower permeability sediments that confine the water in the Floridan aquifer system. The Floridan aquifer system has two major water-bearing zones (the Upper Floridan aquifer and the Lower Floridan aquifer), which are separated by a less-permeable semiconfining unit. Upper Floridan aquifer water levels and spring flows have been affected by ground-water development. Long-term hydrographs of four wells tapping the Upper Floridan aquifer show a general downward trend from the early 1950's until 1990. The declines in water levels are caused predominantly by increased pumpage and below average annual rainfall. From 1991 to 1998, water levels rose slightly, a trend that can be explained by an increase in average annual rainfall. Long-term declines in the potentiometric surface varied throughout the area, ranging from about 3 to 12 feet. Decreases in spring discharge also have been observed in a few springs with long-term record. Chloride concentrations in water from the Upper Floridan aquifer in Seminole County range areally from 6.2 to 5,300 milligrams per liter. Chloride concentrations are lowest in the recharge areas of the Floridan aquifer system in the western part of Seminole County and near Geneva. The most highly mineralized water occurs adjacent to the Wekiva River in northwestern Seminole County, around the eastern part of Lake Jesup, and along the St. Johns River in eastern Seminole County. Analysis of limited long-term water-quality data indicates that the chloride concentrations in water for most wells in the Floridan aquifer system in Seminole County have not changed significantly in the 20-year period from 1976 to 1996, and probably not since the mid 1950's. Analysis of water samples collected from some Upper Floridan aquifer springs, however, indicates that the water has become more mineralized during recent years. Increases in specific conductance and concentrations of major cations and anions were observed at several of the springs within the study area where long-term water-quality data were available. Associated with these increases in the mineralization of spring water has been an increase in total nitrate-plus- nitrite as nitrogen concentration. A three-dimensional model was developed to simulate ground-water flow in the surficial and Floridan aquifer systems. The steady-state ground-water flow model was calibrated to water-level data that was averaged over a 1-year period from September 1996 through August 1997. The calibrated flow model generally produced simulated water levels in reasonably close agreement with measured water levels. As a result, the calibrated model was used to simulate the effects of expected increases in ground-water withdrawals on the water levels in the surficial aquifer system and on the potentiometric surface of the Upper and Lower Floridan aquifers in Seminole County. The ca

Water-Resources Investigations Report↗
USGS1973

Ground-water hydrology of northern Napa Valley, California

The alluvium of northern Napa Valley is the principal aquifer of the area and is capable of yielding as much as 3,000 gallons per minute to wells. Generally the larger-yielding wells are along the Napa River where the alluvium is thickest and most permeable. Recharge to the alluvium is chiefly by percolation from streams and infiltration of precipitation. Discharge is chiefly flow to the Napa River, evapotranspiration, and pumpage from wells. Both recharge to, and discharge from, the alluvial aquifer are sensitively influenced by rainfall. About 190,000 acre-feet of water is presently (1972) stored in the alluvium of northern Napa Valley. Future annual water use in the project area will probably vary between 12,000 and 35,000 acre-feet and, for most purposes, can be supplied by the alluvial aquifer even during extended periods of limited rainfall. Generally low transmissivities in the alluvium, however, limit the opportunity for obtaining sustained, large yields from wells in much of the valley and require that large-scale development and operation of wells in much of the area be planned and synchronized. Sustained drought conditions in the Napa Valley accompanied by expected increases in the use of ground water will probably cause significant reductions in the base flow of the Napa River and cause many shallow wells in the area to dry up. Sodium chloride ground water occurs near Calistoga and in the vicinity of Oakville and in some places is not suitable for irrigation. Model studies indicate that limited migration of sodium chloride water into intensively pumped parts of the aquifer probably will not be a serious problem.

California↗
USGS1998

Flow and geochemistry along shallow ground-water flowpaths in an agricultural area in southeastern Wisconsin

Water-quality and geohydrologic data were collected from 19 monitor wells and a stream in an agricultural area in southeastern Wisconsin. These sites were located along a 2,700-ft transect from a local ground-water high to the stream. The transect is approximately parallel to the horizontal direction of ground-water flow at the water table. Most of the wells were installed in unconsolidated deposits at five locations along the transect and include an upgradient well nest, a midgradient well nest, a downgradient well nest, wells in the lowland area near the stream, and wells installed in the stream bottom. The data collected from this study site were used to describe the water quality and geohydrology of the area and to explain and model the variations in water chemistry along selected ground-water flowpaths. Water samples from most wells and the stream were analyzed for major ions, nutrients, pesticides, dissolved organic carbon, aluminum, tritium, CFCs, 15N, 18O, and dissolved gases. Measurements of temperature, pH, specific conductance, and dissolved oxygen were made in the field. Concentrations of all dissolved constituents were below Wisconsin ground-water quality enforcement standards. The concentrations of both nitrate and ammonium in precipitation concentrated by evapotranspiration are roughly equal to the concentrations of either in the shallow ground waters. The nitrogen and oxygen isotope data, however, indicate that soil ammonium, ammonium fertilizer, and animal waste are possible nitrate sources. Concentrated precipitation can also supply dissolved sulfate to the shallow ground waters and may be a principal source of pesticides to the ground water. However, some input of dissolved chloride to the ground water from mineral or anthropogenic sources is necessary. X-ray diffraction analyses of samples from 2 cores show the most abundant mineral to be dolomite, with subordinate quartz, microclme, and plagioclase, and minor amounts of mica, hornblende, and chlorite. Hydraulic conductivities determined from slug tests at selected wells range from 0.006 to 55 feet per day, with most values between 0.4 and 12 feet per day. A cross-sectional ground-water flow model, representing the water-table flow system, was developed for the site and was used to identify possible ground-water flowpaths for geocli^mical modeling. The model was calibrated against measured water levels and was most sensitive to variation in recharge and hydraulic conductivity. The calibrated model shows that downward flow from shallow to deeper wells within a nest may occur at the upgradient and midgradient well nests, but that flow from each well nest travels beneath downgradient nests to the stream. Pathline and travel-time analysis performed on the calibrated flow-model output yielded travel times to well screens that range from 5.8 to 59 years with a recharge of 4 inches per yr. Recharge dates based on tritium and CFC concentrations range from pre-1955 to 1986 and are consistent with flowpaths1 and travel times in the calibrated flow model. Changes in water quality along ground-water flowpaths were evaluated using the geochemical model PHREEQC. Geochemical mole balance models of shallow ground-water formation show that the principal reaction, by an order of magnitude, is dissolution of dolomite with CO2 . Concentration factors in the mole-balance models range from 1 to 11, with most values between 5 and 10, which provides independent support for the concentration factor of 8 based on recharge estimates used in the flow model. Ground water recharging at mid- and downgradient wells is oxic and contains dissolved nitrate, whereas the ground water discharging to the stream is anoxic and contains dissolved ammonium. Redox environments were defined at each well on the basis of relative concentrations of various dissolved redox-active species. Chemically permissible flowpaths inferred from the observed sequence of redox environments at well sites are consistent with flowpaths in the ground-water flow model. The transition from nitrate in recharging ground water to ammonium in ground water discharging to the stream suggests the possibility of nitrate reduction along the flowpath. None of the techniques employed in this study, however, were able to prove the occurrence of this reaction.

Michigan, Wisconsin↗
USGS2007

Precipitation and Runoff Simulations of the Carson Range and Pine Nut Mountains, and Updated Estimates of Ground-Water Inflow and the Ground-Water Budgets for Basin-Fill Aquifers of Carson Valley, Douglas County, Nevada, and Alpine County, California

Recent estimates of ground-water inflow to the basin-fill aquifers of Carson Valley, Nevada, and California, from the adjacent Carson Range and Pine Nut Mountains ranged from 22,000 to 40,000 acre-feet per year using water-yield and chloride-balance methods. In this study, watershed models were developed for watersheds with perennial streams and for watersheds with ephemeral streams in the Carson Range and Pine Nut Mountains to provide an independent estimate of ground-water inflow. This report documents the development and calibration of the watershed models, presents model results, compares the results with recent estimates of ground-water inflow to the basin-fill aquifers of Carson Valley, and presents updated estimates of the ground-water budget for basin-fill aquifers of Carson Valley. The model used for the study was the Precipitation-Runoff Modeling System, a physically based, distributed-parameter model designed to simulate precipitation and snowmelt runoff as well as snowpack accumulation and snowmelt processes. Geographic Information System software was used to manage spatial data, characterize model drainages, and to develop Hydrologic Response Units. Models were developed for * Two watersheds with gaged perennial streams in the Carson Range and two watersheds with gaged perennial streams in the Pine Nut Mountains using measured daily mean runoff, * Ten watersheds with ungaged perennial streams using estimated daily mean runoff, * Ten watershed with ungaged ephemeral streams in the Carson Range, and * A large area of ephemeral runoff near the Pine Nut Mountains. Models developed for the gaged watersheds were used as index models to guide the calibration of models for ungaged watersheds. Model calibration was constrained by daily mean runoff for 4 gaged watersheds and for 10 ungaged watersheds in the Carson Range estimated in a previous study. The models were further constrained by annual precipitation volumes estimated in a previous study to provide estimates of ground-water inflow using similar water input. The calibration periods were water years 1990-2002 for watersheds in the Carson Range, and water years 1981-97 for watersheds in the Pine Nut Mountains. Daily mean values for water years 1990-2002 were then simulated using the calibrated watershed models in the Pine Nut Mountains. The daily mean values of precipitation, runoff, evapotranspiration, and ground-water inflow simulated from the watershed models were summed to provide annual mean rates and volumes for each year of the simulations, and mean annual rates and volumes computed for water years 1990-2002. Mean annual bias for the period of record for models of Daggett Creek and Fredericksburg Canyon watersheds, two gaged perennial watersheds in the Carson Range, was within 4 percent and relative errors were about 6 and 12 percent, respectively. Model fit was not as satisfactory for two gaged perennial watersheds, Pine Nut and Buckeye Creeks, in the Pine Nut Mountains. The Pine Nut Creek watershed model had a large negative mean annual bias and a relative error of -11 percent, underestimated runoff for all years but the wet years in the latter part of the record, but adequately simulated the bulk of the spring runoff most of the years. The Buckeye Creek watershed model overestimated mean annual runoff with a relative error of about -5 percent when water year 1994 was removed from the analysis because it had a poor record. The bias and error of the calibrated models were within generally accepted limits for watershed models, indicating the simulated rates and volumes of runoff and ground-water inflow were reasonable. The total mean annual ground-water inflow to Carson Valley computed using estimates simulated by the watershed models was 38,000 acre-feet, including ground-water inflow from Eagle Valley, recharge from precipitation on eolian sand and gravel deposits, and ground-water recharge from precipitation on the western alluvial fans. The estimate was in close agreement with that obtained from the chloride-balance method, 40,000 acre-feet, but was considerably greater than the estimate obtained from the water-yield method, 22,000 acre-feet. The similar estimates obtained from the watershed models and chloride-balance method, two relatively independent methods, provide more confidence that they represent a reasonably accurate volume of ground-water inflow to Carson Valley. However, the two estimates are not completely independent because they use similar distributions of mean annual precipitation. Annual ground-water recharge of the basin-fill aquifers in Carson Valley ranged from 51,000 to 54,000 acre-feet computed using estimates of ground-water inflow to Carson Valley simulated from the watershed models combined with previous estimates of other ground-water budget components. Estimates of mean annual ground-water discharge range from 44,000 to 47,000 acre-feet. The low range estimate for ground-water recharge, 51,000 acre-feet per year, is most similar to the high range estimate for ground-water discharge, 47,000 acre-feet per year. Thus, an average annual volume of about 50,000 acre-feet is a reasonable estimate for mean annual ground-water recharge to and discharge from the basin-fill aquifers in Carson Valley. The results of watershed models indicate that significant interannual variability in the volumes of ground-water inflow is caused by climate variations. During multi-year drought conditions, the watershed simulations indicate that ground-water recharge could be as much as 80 percent less than the mean annual volume of 50,000 acre-feet.

Scientific Investigations Report↗
USGS2000

Predevelopment water-level map of the Santa Fe Group aquifer system in the middle Rio Grande basin between Cochiti Lake and San Acacia, New Mexico

Because of its increasing population and limited water resources, the Middle Rio Grande Basin between Cochiti Lake and San Acacia, New Mexico, has recently become the subject of intense study. In particular, the U.S. Geological Survey (USGS) in cooperation with the City of Albuquerque has constructed a series of ground-water-flow models of the Tertiary and Quaternary basin-fill deposits of the Santa Fe Group aquifer system (Kernodle and Scott, 1986; Kernodle and others, 1987; Kernodle and others, 1995; Kernodle, 1998; Tiedeman and others, 1998). The ground-water-flow system also has been the focus of hydrochemical studies and other efforts intended largely to help develop an improved flow model. Among the information critical to a thorough understanding of the ground-water-flow system are water-level data that indicate the directions of ground-water flow and the magnitudes of hydraulic gradients in the aquifer prior to perturbation by substantial ground-water withdrawals (under predevelopment conditions).

Water-Resources Investigations Report↗
USGS1997

Chlorofluorocarbon and tritium age determination of ground-water recharge in the Ryan Flat subbasin, Trans-Pecos Texas

A study was conducted to determine the relative influence of mountain-front infiltration in the Ryan Flat subbasin and to determine whether recent recharge (post-1940), which is of importance to water-use planning, has reached the Salt Basin aquifer, Trans-Pecos Texas. The alluvial and volcanic Salt Basin aquifer lies within a bolson, and the average depth to water in most of the subbasin is approximately 250 feet. Concentrations of the chlorofluorocarbons CFC-11, CFC-12, and CFC-113, as well as tritium, were measured in water from 10 wells in the study area. CFC-model recharge dates ranged from pre-1940 to the early 1970's. Ground water in five wells had CFC-model dates of pre-1940 or pre-1945. Ground water in two wells had dates of the mid- to late 1940's. Ground water from one well had a CFC-model recharge age of the early 1950's. Samples from the remaining two wells were most probably contaminated in some manner and are probably unreliable. CFC-model ages were calculated independently for the three chlorofluorocarbons, though the presence of volatile organic compounds affected agreement among them. Tritium activities in the nine wells for which tritium was analyzed indicated pre-1953 recharge and thus agreed approximately with the CFC-model dates. Ground water was analyzed for selected water-quality constituents. Water from all wells met U.S. Environmental Protection Agency national primary and secondary drinking water standards for all tested constituents except fluoride in samples from three wells. Silica concentrations in water from six wells exceeded a range considered typical in natural waters.

Water-Resources Investigations Report↗
USGS2007

Hydrogeologic characteristics of the St. Croix River basin, Minnesota and Wisconsin: Implications for the susceptibility of ground water to potential contamination

Population growth in the St. Croix River Basin in Minnesota and Wisconsin has intensified concerns of county resource managers and the National Park Service, which is charged with protecting the St. Croix National Scenic Riverway, about the potential for ground-water contamination in the basin. This report describes a previously developed method that was adapted to illustrate potential ground-water-contamination susceptibility in the St. Croix River Basin. The report also gives an estimate of ground-water-residence time and surface-water/ground-water interaction as related to natural attenuation and movement of contaminants in five tributary basins. A ground-water-contamination-susceptibility map was adapted from a state-wide map of Wisconsin to the St. Croix River Basin by use of well-driller construction records and regional maps of aquifer properties in Minnesota and Wisconsin. Measures of various subsurface properties were combined to generate a spatial index of susceptibility. The subjective index method developed for the State of Wisconsin by Schmidt (1987) 1 was not derived from analyses of water-quality data or physical processes. Nonetheless, it was adapted for this report to furnish a seamless map across state boundaries that would be familiar to many resource managers. Following this method, areas most susceptible to contamination appear to have coarse-grained sediments (sands or gravels) and shallow water tables or are underlain by carbonate-bedrock aquifers. The least susceptible areas appear to have fine-grained sediments and deep water tables. If an aquifer becomes contaminated, the ground-water-residence time can affect potential natural attenuation along the ground-water-flow path. Mean basin ground-water-residence times were computed for the Apple, Kettle, Kinnickinnic, Snake and Sunrise River Basins, which are tributary basins to the St. Croix Basin, by use of average aquifer properties of saturated thickness, porosity, and recharge rates. The Apple River Basin had the shortest mean ground-water-residence times (20–120 years), owing largely to the moderate saturated thickness and high recharge rate in the basin. The Kinnickinnic and Sunrise River Basins had the longest mean residence times (60–350 and 70–390 years, respectively) chiefly because of the relatively large saturated thickness of the basins. Owing to limitations of the residence-time calculations, actual ground-water-residence times will vary around the mean values within each basin and may range from days or weeks in karst carbonate aquifers to millennia in deep confined sandstone aquifers. Areas of relatively short residence time (less than the median residence time in each basin) were identified by use of ground-water-flow models for each of the five tributary basins. Results of simulations show that these areas, in which contaminants may have relatively less time for natural attenuation along the short flow paths, generally occur near streams and rivers where ground water discharges to the surface. Finally, the ground-water-flow models were used to simulate ground-water/surface-water interaction in the five tributary basins. Results of simulations show that some lakes and reservoirs leak surface water into the ground-water-flow system on their downgradient side, where the surface-water outflow has been restricted by a dam or a naturally constricted outlet. These locations are noteworthy because contaminated surface waters could potentially enter the ground-water-flow system at these locations.

Minnesota, Wisconsin↗
USGS2008

Estimated use of water in the Upper Duck River watershed, central Tennessee, and water-demand projections through 2030

Future municipal water demand was estimated for the Bedford, Coffee, Marshall, and Maury-southern Williamson water-service areas in the upper Duck River watershed in central Tennessee through 2030. The Duck River, a primary source of municipal water, provided a total of 24.3 million gallons per day (Mgal/d) or 92 percent of the total water use in the study area during 2000. Municipal water use increased 46 percent from 1981 to 2000 (from 18.0 to 26.3 Mgal/d). Water demand for municipal use is expected to continue to increase through 2030 because of the recent intensive and anticipated growth in the residential and commercial sectors. Constant-rate models were used to estimate future municipal water demand. Data on residential and nonresidential billing accounts and estimates of public use and losses were used to calibrate the models. Two watershed scenarios for each water-supply system that depends on the Duck River for supply were simulated. Scenario 1 considered monthly water demand during typical weather conditions as represented by monthly per account use during 2003 and a rate of growth in customer accounts from 1999 to 2003. Results showed that total municipal water use could increase about 104 percent to 51 Mgal/d by 2030, residential water use could increase about 140 percent to 24 Mgal/d, nonresidential water use could increase about 110 percent to 17 Mgal/d, and public use and losses could increase about 83 percent to 11 Mgal/d. Scenario 2 considered monthly water demand during drought conditions as represented by monthly per account use during 2000 and recent growth in customer accounts from 1999 to 2003 or, for selected water-supply systems, an increasing rate of growth. Results showed that total municipal water use could increase about 120 percent to 55 Mgal/d, residential water use could increase about 160 percent to 26 Mgal/d, nonresidential water use could increase about 122 percent to 18 Mgal/d, and public use and losses could double and increase to 12 Mgal/d. For both scenarios the model assumed that the Duck River would supply all future surface-water needs in the study area, that ground-water resources would be sufficient to meet growing demands of the water-supply systems that depend on ground water, and that the amount of surface water sold to water-supply systems primarily dependent on ground water would remain the same through 2030.

Tennessee↗
USGS2007

Effects of ground-water withdrawal on Kaunakakai Stream environmental restoration plan, Molokai, Hawaii

The U.S. Army Corps of Engineers, in cooperation with the County of Maui Department of Public Works and Environmental Management, has proposed to construct 2.75 acres of shallow ponds and mudflats near the mouth of Kaunakakai Stream, Moloka`i, Hawai`i to restore habitat for the endangered native Hawaiian Stilt. Kaunakakai Stream is ephemeral upstream from the habitat-restoration site. Where the pond and wetland bottoms are below the water table, the ponds and wetland will be sustained by ground-water discharge during dry-weather conditions. Because ground water is the main source of water for the proposed ponds and wetland, a reduction of ground-water levels and discharge near the mouth of Kaunakakai Stream will have an effect on the availability of habitat. In response to concerns about the possible effects of ground-water withdrawal on the habitat restoration project near the mouth of Kaunakakai Stream, the U.S. Geological Survey undertook the present investigation to estimate, using an existing numerical ground-water-flow model, the changes in ground-water level and coastal discharge caused by redistributed and additional ground-water withdrawals. Steady-state water-level and coastal-discharge changes, relative to recent base-case conditions, were estimated for each of six withdrawal scenarios. Redistributed and additional ground-water withdrawals in the six scenarios were simulated from selected sites in the area between Kualapu`u and `Ualapu`e. For the scenarios tested, model results indicate that withdrawals from existing and proposed wells cause a water-level decline of about 0.1 ft in the vicinity of the Kaunakakai habitat-restoration site. In addition, model results indicate a reduction of ground-water discharge, ranging from 98,000 to 170,000 gal/d, to the model element containing the habitat-restoration site, although the existing spatial discretization in the model is too coarse to reliably estimate the reduction of ground-water discharge to the stream. Reduction in discharge to the habitat-restoration site is likely less than the total indicated by the model element because the site covers a small fraction (about 5 percent) of the area of a model element. Ground-water-level declines near the habitat-restoration site will reduce (1) the available wetted habitat area by an amount that is dependent on the bottom slope of the ponds near their edges, (2) the maximum water depth of the ponds by about 0.1 ft, and (3) the average water depth by an amount that is dependent on the bottom shape of the ponds. The salinity of ground-water discharging into the wetland area likely will increase by an unknown amount in response to increased withdrawals upgradient from the site. A numerical model capable of simulating density-dependent flow and transport is needed to evaluate the effects of withdrawal on salinity in the area.

Hawai'i↗