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At least 811 records · Page 45Linked to original sources
USGS2009

Development and Implementation of a Transport Method for the Transport and Reaction Simulation Engine (TaRSE) based on the Godunov-Mixed Finite Element Method

A model to simulate transport of materials in surface water and ground water has been developed to numerically approximate solutions to the advection-dispersion equation. This model, known as the Transport and Reaction Simulation Engine (TaRSE), uses an algorithm that incorporates a time-splitting technique where the advective part of the equation is solved separately from the dispersive part. An explicit finite-volume Godunov method is used to approximate the advective part, while a mixed-finite element technique is used to approximate the dispersive part. The dispersive part uses an implicit discretization, which allows it to run stably with a larger time step than the explicit advective step. The potential exists to develop algorithms that run several advective steps, and then one dispersive step that encompasses the time interval of the advective steps. Because the dispersive step is computationally most expensive, schemes can be implemented that are more computationally efficient than non-time-split algorithms. This technique enables scientists to solve problems with high grid Peclet numbers, such as transport problems with sharp solute fronts, without spurious oscillations in the numerical approximation to the solution and with virtually no artificial diffusion.

Scientific Investigations Report↗
USGS2004

A method for simulating transient ground-water recharge in deep water-table settings in central Florida by using a simple water-balance/transfer-function model

A relatively simple method is needed that provides estimates of transient ground-water recharge in deep water-table settings that can be incorporated into other hydrologic models. Deep water-table settings are areas where the water table is below the reach of plant roots and virtually all water that is not lost to surface runoff, evaporation at land surface, or evapotranspiration in the root zone eventually becomes ground-water recharge. Areas in central Florida with a deep water table generally are high recharge areas; consequently, simulation of recharge in these areas is of particular interest to water-resource managers. Yet the complexities of meteorological variations and unsaturated flow processes make it difficult to estimate short-term recharge rates, thereby confounding calibration and predictive use of transient hydrologic models. A simple water-balance/transfer-function (WBTF) model was developed for simulating transient ground-water recharge in deep water-table settings. The WBTF model represents a one-dimensional column from the top of the vegetative canopy to the water table and consists of two components: (1) a water-balance module that simulates the water storage capacity of the vegetative canopy and root zone; and (2) a transfer-function module that simulates the traveltime of water as it percolates from the bottom of the root zone to the water table. Data requirements include two time series for the period of interest?precipitation (or precipitation minus surface runoff, if surface runoff is not negligible) and evapotranspiration?and values for five parameters that represent water storage capacity or soil-drainage characteristics. A limiting assumption of the WBTF model is that the percolation of water below the root zone is a linear process. That is, percolating water is assumed to have the same traveltime characteristics, experiencing the same delay and attenuation, as it moves through the unsaturated zone. This assumption is more accurate if the moisture content, and consequently the unsaturated hydraulic conductivity, below the root zone does not vary substantially with time. Results of the WBTF model were compared to those of the U.S. Geological Survey variably saturated flow model, VS2DT, and to field-based estimates of recharge to demonstrate the applicability of the WBTF model for a range of conditions relevant to deep water-table settings in central Florida. The WBTF model reproduced independently obtained estimates of recharge reasonably well for different soil types and water-table depths.

Scientific Investigations Report↗
USGS1995

Analysis of ground-water flow in the Catahoula aquifer system in the vicinity of Laurel and Hattiesburg, Mississippi

The upper, middle, and lower Catahoula aquifers in the vicinity of the cites of Laurel and Hattiesburg in southern Mississippi are made up of irregular, discontinuous sand zones in the Catahoula Formation of Miocene age. In places thee three aquifers may be hydraulically well connected, and are referred to as the Catahoula aquifer system. Withdrawal from the Catahoula aquifers increased from 28 million gallons per day (Mgal/d) to 41 Mgal/d during 1970 to 1985, and decreased to 38 Mgal/d during 1990. Most withdrawal in the Laurel area is from the lower and middle Catahoula, and most withdrawal in the Hattiesburg area is from the middle and upper Catahoula aquifers. In the Laurel area, water levels in selected wells in the lower Catahoula aquifer declined at rates ranging from about 1 to 3.6 feet/ year until the late 1980's in response to the increase in pumping. A three-dimensional model was developed to represent ground-water flow in the Catahoula aquifers. Simulated water levels in the lower Catahoula aquifer, the layer most affected by pumping, were lowered from predevelopment levels as much as 130 feet in the Laurel area and 100 feet in the Hattiesburg area, according to the model analysis of 1992 conditions. Three scenarios of increased pumpage, for the period 1992-2020, were simulated. Under the low-growth scenario, water- level declines would be 20 feet or less below 1992 water levels in the middle and upper Catahoula aquifer in the Hattiesburg area, and about 60 feet in the lower Catahoula aquifer in the Laurel area. Under the moderate-growth scenario, water-level declines would be 40 feet or less below 1992 water levels in the middle Catahoula aquifer in the Hattiesburg area. Water-level declines would be about 110 feet in the lower Catahoula aquifer in the Laurel area, and water levels would approach the top of the aquifer. Under the high-growth scenario, water-level declines would be 40 feet or less in the upper Catahoula aquifer and about 80 feet in the middle Catahoula, with the largest declines occurring in the Hattiesburg area. Water levels would decline about 130 feet and would be drawn down below the top of the lower Catahoula aquifer in the Laurel area under the high-growth scenario.

Water-Resources Investigations Report↗
USGS2006

Deep resistivity structure of Yucca Flat, Nevada Test Site, Nevada

The Department of Energy (DOE) and the National Nuclear Security Administration (NNSA) at their Nevada Site Office are addressing groundwater contamination resulting from historical underground nuclear testing through the Environmental Management program and, in particular, the Underground Test Area project. One issue of concern is the nature of the somewhat poorly constrained pre Tertiary geology and its effects on ground-water flow in the area adjacent to a nuclear test. Ground water modelers would like to know more about the hydrostratigraphy and geologic structure to support a hydrostratigraphic framework model that is under development for the Yucca Flat Corrective Action Unit (CAU). During 2003, the U.S. Geological Survey, supported by the DOE and NNSA-NSO, collected and processed data from 51 magnetotelluric (MT) and audio-magnetotelluric (AMT) stations at the Nevada Test Site in and near Yucca Flat to assist in characterizing the pre-Tertiary geology in that area. The primary purpose was to refine the character, thickness, and lateral extent of pre Tertiary confining units. In particular, a major goal has been to define the upper clastic confining unit (late Devonian - Mississippian-age siliciclastic rocks assigned to the Eleana Formation and Chainman Shale) in the Yucca Flat area. The MT and AMT data have been released in separate USGS Open File Reports. The Nevada Test Site magnetotelluric data interpretation presented in this report includes the results of detailed two-dimensional (2 D) resistivity modeling for each profile (including alternative interpretations) and gross inferences on the three dimensional (3 D) character of the geology beneath each station. The character, thickness, and lateral extent of the Chainman Shale and Eleana Formation that comprise the Upper Clastic Confining Unit are generally well determined in the upper 5 km. Inferences can be made regarding the presence of the Lower Clastic Confining Unit at depths below 5 km. Large fault structures such as the CP Thrust fault, the Carpetbag fault, and the Yucca fault that cross Yucca Flat are also discernable as are other smaller faults. The subsurface electrical resistivity distribution and inferred geologic structures determined by this investigation should help constrain the hydrostratigraphic framework model that is under development.

Open-File Report↗
USGS2005

Simulation of ground-water flow to assess geohydrologic factors and their effect on source-water areas for bedrock wells in Connecticut

Generic ground-water-flow simulation models show that geohydrologic factors?fracture types, fracture geometry, and surficial materials?affect the size, shape, and location of source-water areas for bedrock wells. In this study, conducted by the U.S. Geological Survey in cooperation with the Connecticut Department of Public Health, ground-water flow was simulated to bedrock wells in three settings?on hilltops and hillsides with no surficial aquifer, in a narrow valley with a surficial aquifer, and in a broad valley with a surficial aquifer?to show how different combinations of geohydrologic factors in different topographic settings affect the dimensions and locations of source-water areas in Connecticut. Three principal types of fractures are present in bedrock in Connecticut?(1) Layer-parallel fractures, which developed as partings along bedding in sedimentary rock and compositional layering or foliation in metamorphic rock (dips of these fractures can be gentle or steep); (2) unroofing joints, which developed as strain-release fractures parallel to the land surface as overlying rock was removed by erosion through geologic time; and (3) cross fractures and joints, which developed as a result of tectonically generated stresses that produced typically near-vertical or steeply dipping fractures. Fracture geometry is defined primarily by the presence or absence of layering in the rock unit, and, if layered, by the angle of dip in the layering. Where layered rocks dip steeply, layer-parallel fracturing generally is dominant; unroofing joints also are typically well developed. Where layered rocks dip gently, layer-parallel fracturing also is dominant, and connections among these fractures are provided only by the cross fractures. In gently dipping rocks, unroofing joints generally do not form as a separate fracture set; instead, strain release from unroofing has occurred along gently dipping layer-parallel fractures, enhancing their aperture. In nonlayered and variably layered rocks, layer-parallel fracturing is absent or poorly developed; fracturing is dominated by well-developed subhorizontal unroofing joints and steeply dipping, tectonically generated fractures and (or) cooling joints. Cross fractures (or cooling joints) in nonlayered and variably layered rocks have more random orientations than in layered rocks. Overall, nonlayered or variably layered rocks do not have a strongly developed fracture direction. Generic ground-water-flow simulation models showed that fracture geometry and other geohydrologic factors affect the dimensions and locations of source-water areas for bedrock wells. In general, source-water areas to wells reflect the direction of ground-water flow, which mimics the land-surface topography. Source-water areas to wells in a hilltop setting were not affected greatly by simulated fracture zones, except for an extensive vertical fracture zone. Source-water areas to wells in a hillside setting were not affected greatly by simulated fracture zones, except for the combination of a subhorizontal fracture zone and low bedrock vertical hydraulic conductivity, as might be the case where an extensive subhorizontal fracture zone is not connected or is poorly connected to the surface through vertical fractures. Source-water areas to wells in a narrow valley setting reflect complex ground-water-flow paths. The typical flow path originates in the uplands and passes through either till or bedrock into the surficial aquifer, although only a small area of the surficial aquifer actually contributes water to the well. Source-water areas in uplands can include substantial areas on both sides of a river. Source-water areas for wells in this setting are affected mainly by the rate of ground-water recharge and by the degree of anisotropy. Source-water areas to wells in a broad valley setting (bedrock with a low angle of dip) are affected greatly by fracture properties. The effect of a given fracture is to channel the

Scientific Investigations Report↗
USGS1993

Hydrogeology of, and simulated ground-water flow in, the valley-fill aquifers of the upper Rockaway River basin, Morris County, New Jersey

Public water supply in the Rockaway River valley depends almost entirely on ground water from wells in the valley-fill deposits. Ground-water withdrawals from these deposits in 1986 were about 9.1 million gallons per day. A steady-state ground-water flow model was developed to quantify the effects of ground-water withdrawals on water levels in the valley-fill aquifers and on ground-water discharge to the Rockaway River. The ground-water-flow model, which represents an aquifer system consisting of an unconfined and a confined aquifer separated by a discontinuous confining unit, was implemented to examine aquifer resonse to current and predicted ground-water withdrawals in areas of proposed well sites and the effect of increased ground-water withdrawals on ground-water discharge to the river. Ground-water flow to wells in the valley-fill aquifers is sustained by increased vertical flow between the two aquifers, the diversion of ground water that had discharged to the Rockaway River, and induced seepage resulting from pumping near the Rickaway River. If the rate of ground-water recharge decreases or if the rate of ground-water withdrawals from the valley-fill aquifers increases, ground-water discharge to the Rockaway River above the Boonton Reservoir will decrease by an equivalent amount. The average annual base flow of the Rockaway River above the Boonton Reservoir will meet the minimum passing flow requirement of 7 million gallons per day under conditions of average annual ground-water recharge, and increased ground-water withdrawals anticipated by the years 2000 and 2040. For anticipated increases in withdrawals to 11.5 million gallons per day by the the year 2000, and 14.6 million gallons per day by the year 2040, base flow to the Rockaway River above the Boonton Reservoir may not be sufficient to meet the minimum required reservoir outflow during extended periods of decreased recharge, such as drought, as much as 5 and 11 percent of the time, respectively.

Water-Resources Investigations Report↗
USGS1984

Projected water-level declines in the Ogallala aquifer in Lea County, New Mexico

A two-dimensional digital ground-water flow model was constructed of the Ogallala aquifer in Lea County, New Mexico. Simulations of predevelopment steady-state and historical pumping conditions were used to fit the model. Projections of water-level declines were made based on the condition of no additional development and the condition of a 0.44 percent annual increase in irrigation withdrawals to 1990 and a 0.88 percent annual increase in nonirrigation withdrawals to 2020. Based on no additional development, projected maximum water-level declines from 1980 were 31 feet in 2000 and 59 feet in 2020. The amount of recoverable water remaining in the aquifer as simulated by the model was 24.6 million acre-feet in 2000 and 22.4 million acre-feet in 2020, compared to an estimated 28 million acre-feet in 1980, with increased withdrawals, projected maximum declines were 33 feet in 2000 and 67 feet in 2020. As simulated by the model, 24.4 million acre-feet of recoverable water remained in 2000 and 21.8 million acre-feet remained in 2020. The sensitivity of the model to variation in hydraulic conductivity, specific yield, and recharge was tested over their range of uncertainty. Projected water-level declines were most sensitive to specific yield. In response to changes in specific yield, declines varied by as much as 6.2 feet from the standard simulation to 2020 with no additional development. (USGS)

Water-Resources Investigations Report↗
USGS2000

Estimating the probability of elevated nitrate (NO2+NO3-N) concentrations in ground water in the Columbia Basin Ground Water Management Area, Washington

Logistic regression was used to relate anthropogenic (man-made) and natural factors to the occurrence of elevated concentrations of nitrite plus nitrate as nitrogen in ground water in the Columbia Basin Ground Water Management Area, eastern Washington. Variables that were analyzed included well depth, depth of well casing, ground-water recharge rates, presence of canals, fertilizer application amounts, soils, surficial geology, and land-use types. The variables that best explain the occurrence of nitrate concentrations above 3 milligrams per liter in wells were the amount of fertilizer applied annually within a 2-kilometer radius of a well and the depth of the well casing; the variables that best explain the occurrence of nitrate above 10 milligrams per liter included the amount of fertilizer applied annually within a 3-kilometer radius of a well, the depth of the well casing, and the mean soil hydrologic group, which is a measure of soil infiltration rate. Based on the relations between these variables and elevated nitrate concentrations, models were developed using logistic regression that predict the probability that ground water will exceed a nitrate concentration of either 3 milligrams per liter or 10 milligrams per liter. Maps were produced that illustrate the predicted probability that ground-water nitrate concentrations will exceed 3 milligrams per liter or 10 milligrams per liter for wells cased to 78 feet below land surface (median casing depth) and the predicted depth to which wells would need to be cased in order to have an 80-percent probability of drawing water with a nitrate concentration below either 3 milligrams per liter or 10 milligrams per liter. Maps showing the predicted probability for the occurrence of elevated nitrate concentrations indicate that the irrigated agricultural regions are most at risk. The predicted depths to which wells need to be cased in order to have an 80-percent chance of obtaining low nitrate ground water exceed 600 feet in the irrigated agricultural regions, whereas wells in dryland agricultural areas generally need a casing in excess of 400 feet. The predicted depth to which wells need to be cased to have at least an 80-percent chance to draw water with a nitrate concentration less than 10 milligrams per liter generally did not exceed 800 feet, with a 200-foot casing depth typical of the majority of the area.

Washington↗
USGS2005

Ground-water vulnerability to nitrate contamination at multiple thresholds in the mid-Atlantic region using spatial probability models

The U.S. Geological Survey, in cooperation with the U.S. Environmental Protection Agency?s Regional Vulnerability Assessment Program, has developed a set of statistical tools to support regional-scale, ground-water quality and vulnerability assessments. The Regional Vulnerability Assessment Program?s goals are to develop and demonstrate approaches to comprehensive, regional-scale assessments that effectively inform managers and decision-makers as to the magnitude, extent, distribution, and uncertainty of current and anticipated environmental risks. The U.S. Geological Survey is developing and exploring the use of statistical probability models to characterize the relation between ground-water quality and geographic factors in the Mid-Atlantic Region. Available water-quality data obtained from U.S. Geological Survey National Water-Quality Assessment Program studies conducted in the Mid-Atlantic Region were used in association with geographic data (land cover, geology, soils, and others) to develop logistic-regression equations that use explanatory variables to predict the presence of a selected water-quality parameter exceeding a specified management concentration threshold. The resulting logistic-regression equations were transformed to determine the probability, P(X), of a water-quality parameter exceeding a specified management threshold. Additional statistical procedures modified by the U.S. Geological Survey were used to compare the observed values to model-predicted values at each sample point. In addition, procedures to evaluate the confidence of the model predictions and estimate the uncertainty of the probability value were developed and applied. The resulting logistic-regression models were applied to the Mid-Atlantic Region to predict the spatial probability of nitrate concentrations exceeding specified management thresholds. These thresholds are usually set or established by regulators or managers at National or local levels. At management thresholds of 1 milligram per liter and 3 milligrams per liter as nitrogen, the probability of nitrate concentrations exceeding these levels is greater than 50 percent (0.50) throughout much of the Mid-Atlantic Region. This includes extensive areas throughout central Maryland, southeastern Pennsylvania, northwestern Pennsylvania, and the Delmarva Peninsula. In addition, extensive areas in North Carolina and Virginia also have high probabilities of nitrate concentrations in ground water exceeding management thresholds of 1 milligram per liter and 3 milligrams per liter. The mapped areas showing a high predicted probability of nitrate concentrations in ground water exceeding 1 milligram per liter and 3 milligrams per liter correspond to areas that are mapped as cultivated land cover and/or overlying carbonate rocks. At a management threshold of 10 milligrams per liter (corresponding to the U.S. Environmental Protection Agency standard for nitrate in drinking water of 10 milligrams per liter), the predicted probability of nitrate concentrations in ground water exceeding this level is low for most of the Mid-Atlantic Region, except for the Delmarva Peninsula, southeastern Pennsylvania, and areas mapped as carbonate rocks in Virginia, Maryland, and Pennsylvania.

Scientific Investigations Report↗
USGS2003

Analytical and numerical simulation of the steady-state hydrologic effects of mining aggregate in hypothetical sand-and-gravel and fractured crystalline-rock aquifers

Analytical solutions and numerical models were used to predict the extent of steady-state drawdown caused by mining of aggregate below the water table in hypothetical sand-and-gravel and fractured crystalline-rock aquifers representative of hydrogeologic settings in the Front Range area of Colorado. Analytical solutions were used to predict the extent of drawdown under a wide range of hydrologic and mining conditions that assume aquifer homogeneity, isotropy, and infinite extent. Numerical ground-water flow models were used to estimate the extent of drawdown under conditions that consider heterogeneity, anisotropy, and hydrologic boundaries and to simulate complex or unusual conditions not readily simulated using analytical solutions. Analytical simulations indicated that the drawdown radius (or distance) of influence increased as horizontal hydraulic conductivity of the aquifer, mine penetration of the water table, and mine radius increased; radius of influence decreased as aquifer recharge increased. Sensitivity analysis of analytical simulations under intermediate conditions in sand-and-gravel and fractured crystalline-rock aquifers indicated that the drawdown radius of influence was most sensitive to mine penetration of the water table and least sensitive to mine radius. Radius of influence was equally sensitive to changes in horizontal hydraulic conductivity and recharge. Numerical simulations of pits in sand-and- gravel aquifers indicated that the area of influence in a vertically anisotropic sand-and-gravel aquifer of medium size was nearly identical to that in an isotropic aquifer of the same size. Simulated area of influence increased as aquifer size increased and aquifer boundaries were farther away from the pit, and simulated drawdown was greater near the pit when aquifer boundaries were close to the pit. Pits simulated as lined with slurry walls caused mounding to occur upgradient from the pits and drawdown to occur downgradient from the pits. Pits simulated as refilled with water and undergoing evaporative losses had little hydro- logic effect on the aquifer. Numerical sensitivity analyses for simulations of pits in sand-and-gravel aquifers indicated that simulated head was most sensitive to horizontal hydraulic conductivity and the hydraulic conductance of general-head boundaries in the models. Simulated head was less sensitive to riverbed conductance and recharge and relatively insensitive to vertical hydraulic conductivity. Numerical simulations of quarries in fractured crystalline-rock aquifers indicated that the area of influence in a horizontally anisotropic aquifer was elongated in the direction of higher horizontal hydraulic conductivity and shortened in the direction of lower horizontal hydraulic conductivity compared to area of influence in a homogeneous, isotropic aquifer. Area of influence was larger in an aquifer with ground-water flow in deep, low-permeability fractures than in a homogeneous, isotropic aquifer. Area of influence was larger for a quarry intersected by a hydraulically conductive fault zone and smaller for a quarry intersected by a low-conductivity fault zone. Numerical sensitivity analyses for simulations of quarries in fractured crystalline-rock aquifers indicated simulated head was most sensitive to variations in recharge and horizontal hydraulic conductivity, had little sensitivity to vertical hydraulic conductivity and drain cells used to simulate valleys, and was relatively insensitive to drain cells used to simulate the quarry.

Water-Resources Investigations Report↗
USGS1984

Shallow ground-water flow and drainage characteristics of the Brown ditch basin near the East Unit, Indiana Dunes National Lakeshore, Indiana, 1982

Brown ditch drains wetlands between three parallel ridges of sand dunes near the East Unit of Indiana Dunes National Lakeshore in Poter County, Indiana. Dune and lacustrine sands form a surficial aquifer that is the source of base flow to the ditch. Profiles established in July and August 1982 show that the average streambed slope of the ditch in the Lakeshore (0.19 percent) is six times that of the upstream east reach in the adjacent town, The Pines (0.03 percent). Although the ditch contains debris and vegetation, the reach in the Lakeshore seems to convey all the base flow it receives. In contrast, the upstream east arm contains several ponded sections where flow is sluggish. Digital model simulations show that dredging to form a uniformly graded streambed and eliminate ponding in the upstream east arm of the ditch south of The Pines could lower the water table 0.2 to 2.0 feet in The Pines. Lowering the ditch stage in the Lakeshore by 0.5 to 1.0 foot would cause additional water-table decline of less than 0.5 foot in The Pines. Lowering the ditch stage both in the Lakeshore and in The Pines could lower the water-table in the Lakeshore by nearly 1.0 foot. (USGS)

Water-Resources Investigations Report↗
USGS1992

Water resources and aquifer yields in the Charles River basin, Massachusetts

In 1984, about 66 Mgal/d (million gallons per day) of municipally supplied water was used by towns in the middle and upper Charles River basin and by the city of Cambridge in the lower basin. The Division of Water Resources of the Massachusetts Department of Environmental Management estimates that by the year 2020 an additional 11 Mgal/d of water will be needed to meet demands. The largest sources of un used water in the basin are 15 stratified-drift aquifers along the Charles River that are hydraulically con nected to the river and to its major tributaries. These aquifers, which consist mainly of medium to coarse sand and gravel, are generally unconfined, narrow, and discontinuous. They are as much as 100 feet thick, and transmissivity ranges from about 1,000 to 25,000 cubic feet per day per square foot times foot of aquifer thickness. The depth to the water table from the land surface generally is less than 15 feet and the water table fluctuates about 3 to 5 feet annually. Recharge to the aquifers is mostly derived from precipita tion, and the aquifers continuously discharge ground water to streams, ponds, lakes, and wetlands. Aquifer yields from the 15 aquifers under normal climatic conditions were related to the estimated duration of flow of the streams that drain the aquifers. Long-term (180 days or more) aquifer yields that are derived from interception of ground-water discharge range from less than 1 to more than 5 Mgal/d at the streamflow that is exceeded 95 percent of the time. However, because withdrawal may reduce streamflow in the Charles River to unacceptable levels, these yield estimates were adjusted to meet commonly cited minimum-streamflow criteria. For example, if minimum streamflows in the Charles River are maintained at no less than the annual 7-day, 10-year, mean low flow for 95 percent of the time, only three of the 15 aquifers could sustain yields of more than 1 Mgal/d from intercepted ground-water discharge. However, none of the aquifers could yield an appreciable amount of water from this source if streamflows are maintained at no less than this minimum for 99 percent of the time. Yields also are available to wells front water in duced by pumping to infiltrate through a streambed. Yields front induced infiltration from the Charles River were estimated for four aquifers and range from about 4 to 11 Mgal/d at the 95-percent flow duration. If minimum-streamflow criteria are to be maintained, these yield estimates would be substantially lower. A streamflow-accounting model was used to demonstrate the possible streamflow reductions that could result from further development of the water resources of the middle and upper Charles River basin. Results indicate that pumpage of an additional 11 Mgal/d could reduce flow in the river at Waltham by about 6 ft 3 /s (cubic feet per second) (4 Mgal/d) throughout the year--alt amount equal to about 24 percent of the flow that is exceeded 95 percent of the time. Also, model results indicate that increases in punzpage and the export of was-tewater from the basin of 23 Mgal/d would reduce streamflow at Waltham by about 14 ft 3 / s (9 Mgal/d) throughout the year.

Massachusetts↗
USGS2002

Hydrologic investigation of Powell Marsh and its relation to Dead Pike Lake, Vilas County, Wisconsin

An analytic element ground-water-flow model was constructed to help understand the ground- and surface-water hydrology in the vicinity of Dead Pike Lake and Powell Marsh, Vilas County, Wisconsin. The model was used to simulate the effect of removing Powell Marsh control structures (ditches and Vista Pond) on the hydrology of Dead Pike Lake. Measurements and model simulation results show that ground water in the vicinity of Powell Marsh moves to the northwest and west. If Powell Marsh structures are removed from the simulation, it does not affect the general direction of ground-water flow nor the total flow to Dead Pike Lake. Without the simulated structures, slightly more ground-water flow enters Dead Pike Lake and slightly less surface-water flows at the Dead Pike Lake inlet than with the simulated structures. Ground-water levels measured in piezometers installed along a flow path indicated that ground-water flow primarily is horizontal in the marsh and moves upward in the vicinity of a ditch where it discharges. Flow from Vista Pond is downward to the ground-water system but eventually also discharges upward to the ditches. Based on analyses of water samples from piezometers, the ditch, and Vista Pond, it was shown that dissolved iron is transported in the ground water. When ground water is discharged, iron and manganese react with dissolved oxygen, then precipitates, and forms the oxyhydroxide floc present in the Powell Marsh ditches. The processes involved in the transport and floc formation are not unique to the ditches, but are an expected outcome where discharging ground water and oxygenated surface water meet. Therefore, although floc formed in the ditches would no longer be available for transport if ditches were removed, it is likely that the floc formation would be redirected to the near-shore areas of Dead Pike Lake where increased groundwater discharge is expected.

Wisconsin↗
USGS1996

Simulated effects of alternative withdrawal strategies on ground-water-flow patterns, New Jersey Pinelands

A steady-state, three-dimensional ground-water- flow model of the unconfined part of the Kirkwood- Cohasey aquifer system beneath the upper parts of the Rancocas Creek and Wading River Basins in the New Jersey Pinelands was developed to (1) define ground-water-flow patterns and residence times in an aquifer system typical of the New Jersey Coastal Plain and (2) demonstrate the effects of alternative withdrawal strategies of ground- water-flow patterns and streams. Ground-water flow near the McDonald's-Middle Branch area was analyzed by using a particle tracker to demonstrate the effects of three hypothetical withdrawal scenarios on the configurations of source areas of ground-water flow to withdrawal wells, streams, and other discharge outlets in the Kirkwood-Cohansey aquifer system. Under natural conditions, most ground-water discharge to streams and wetlands. Ground-water residence times ranged from slightly greater than zero to about 200 years. Much of the ground water remained in the system for less than 20 years because it discharged to streams. Residence times of ground water were reduced significantly by persistent withdrawals. The configurations of source areas of flow to local stream systems and to the Piney Point aquifer are affected by the location of a withdrawal well. Results of withdrawal simulations indicate that well-location strategies applied in the Kirkwood-Cohansey aquifer system can alleviate the adverse effects of withdrawals on streams and that large-scale regional withdrawals in confined aquifers can adversely effect streams although the effects are dispersed over numerous streams.

Water-Resources Investigations Report↗
USGS1977

Water resources of the Maunabo Valley, Puerto Rico

The Maunabo Valley, in southestern Puerto Rico, consists of a 3.5-square-mile alluvial plain surrounded by hills of metavolcanic and igneous intrusive rocks. The principal source of ground water in the basin is a shallow unconfined aquifer in the valley alluvium. Continuous pumping of the shallow aquifer has induced the flow of saltwater and has caused the chloride concentration to increase from about 30 to 540 milligrams per liter in the lower part of the valley. A study from January 1971 to December 1974 indicated that the hydraulic conductivities of the aquifer materials, in the basin, are less than 1 foot per day for the metavolcanic and igneous rocks. Estimated conductivities range from 10 to 100 feet per day for the alluvium. The average transmissivity of the alluvial aquifer is estimated to be 4,000 feet squared per day and the average specific capacity is 20 gallons per minute per foot of drawdown. In December 1974, the alluvial aquifer contained an estimated 10 ,000 acre-feet of water in recoverable storage. The data suggest that water supplies to meet future needs could be supplemented by the construction of surface-water-control structures and additional wells. Analysis of a digital simulation model suggests that wells be located in the upper reaches of the alluvial aquifer where a sustained yield of 3,000 gallons per minute could be obtained. (Woodard-USGS)

Maunabo Valley, Puerto Rico↗
USGS1995

Numerical Simulation of Regional Changes in Ground-Water Levels and in the Freshwater-Saltwater Interface Induced by Increased Pumpage at Barbers Point Shaft, Oahu, Hawaii

The effect on the regional ground-water system of southern Oahu from increased pumpage at Barbers Point shaft was estimated by a numerical ground-water model developed for the Oahu Regional Aquifer Systems Analysis (RASA) study. The RASA model was updated by revising pumping and ground-water recharge data. Pumpage data used in the new simulations were based on the allocated pumping rates for 1995 as set by the State Commission on Water Resource Management. On the basis of numerical simulation, Barbers Point shaft can sustain a withdrawal rate of 4.34 million gallons per day without adversely affecting wells in the Waianae aquifer. From results of numerical simulations, it is estimated that, as a result of increasing pumpage in Barbers Point shaft by 2 million gallons per day above the 1995-allocated rate of 2.337 million gallons per day, regional declines in ground-water levels will be about 0.4 to 0.7 feet throughout the Waianae aquifer and about 0.8 ft at the shaft. The corresponding rise of the freshwater-saltwater interface, as a result of declines in ground-water levels, is estimated to be about 20 to 30 feet. Numerical simulation also indicates that changes in ground-water levels greater than about 0.1 feet do not extend across either the Waianae-Koolau unconformity or the south Schofield barrier. The model-estimated position of the freshwater-saltwater interface, as a result of additional pumpage, ranges from 500 to 860 feet below sea level in the southern and northern parts of the aquifer, respectively, and about 540 feet below sea level at the shaft. On the basis of an estimate of the thickness of the transition-zone, the freshwater lens would remain about 240 feet thick below the shaft. In addition, the estimated declines in ground-water levels throughout the aquifer are small compared with the thickness of the freshwater lens and these declines would not be expected to affect the yields of other wells in terms of quantity. Chloride concentrations in the water pumped at Barbers Point shaft were about 240 milligrams per liter in 1992. The estimated background chloride concentration is 200 to 220 milligrams per liter because of low rainfall and the contamination of recharge water from natural salt accumulation in the soil. A reduction in irrigation through 1995 is expected to reduce recharge to the aquifer from irrigation-return water and chloride concentrations associated with the irrigation water throughout the Waianae aquifer. As a result of these combined effects, chloride concentrations of water pumped from the Barbers Point shaft will likely decrease, although the length of time required for this lowering is unknown.

Water-Resources Investigations Report↗
USGS2005

Snowmelt discharge characteristics Sierra Nevada, California

Alpine snow is an important water resource in California and the western U.S. Three major features of alpine snowmelt are the spring pulse (the first surge in snowmelt-driven river discharge in spring), maximum snowmelt discharge, and base flow (low river discharge supported by groundwater in fall). A long term data set of hydrologic measurements at 24 gage locations in 20 watersheds in the Sierra Nevada was investigated to relate patterns of snowmelt with stream discharge In wet years, the daily variations in snowmelt discharge at all the gage locations in the Sierra Nevada correlate strongly with the centrally located Merced River at Happy Isles, Yosemite National Park (i.e., in 1983, the mean of the 23 correlations was R= 0.93 + 0.09) ; in dry years, however, this correlation breaks down (i.e., in year 1977, R=0.72 + 0.24). A general trend towards earlier snowmelt was found and modeled using correlations with the timing of the spring pulse and the river discharge center of mass. For the 24 river and creek gage locations in this study, the spring pulse appeared to be a more sensitive measure of early snowmelt than the center of mass. The amplitude of maximum daily snowmelt discharge correlates strongly with initial snow water equivalent. Geologic factors, base rock permeability and soil-to-bedrock ratio, influence snowmelt flow pathways. Although both surface and ground water flows and water levels increase in wet years compared to dry years, the increase was greater for surface water in a watershed with relatively impermeable base rock than for surface water in a watershed with highly permeable base rock The relation was the opposite for base flow (ground water). The increase was greater for groundwater in a watershed with permeable rock compared to ground water in a watershed with impermeable rock. A similar, but weaker, surface/groundwater partitioning was observed in relatively impermeable granitic watersheds with differing soil-to-bedrock ratios. The increase in surface flow was greater in a watershed with a low, compared to a high, soil-to-bedrock ratio; whereas the increase in ground water flow was greater in a watershed with a high, compared to a low, soil-to-bedrock ratio. Transects that include long-term observations of shallow well-water depth and chemistry would complement traditional hydroclimate data and provide a more complete understanding of hydrologic controls of snowmelt.

California↗
USGS1984

Simulated ground-water flow in the Potomac aquifers, New Castle County, Delaware

Flow in three aquifers of the Potomac Formation in New Castle County, Delaware, was simulated. Results of the calibrated flow model show the maximum transmissivity of the lower aquifer is between 1,000 and 1,500 square feet per day; the middle aquifer, between 3,000 and 3,500 square feet per day; and the upper aquifer, between 5,000 and 6,000 square feet per day. Vertical leakance values for the three confining beds are similar and range 1 x 10 to the (1-8) power per day in downdip areas and .01 per day in subcrop areas. A storage coefficient of 0.00056 was used for each aquifer. A specific storage value of 0.000006 per foot was used for each confining bed. Estimated storage coefficients were not changed during calibration. Changes in water levels resulting from five scenarios of future pumpage were evaluated. Results of these simulations show: (1) the reduction in pumpage at Amoco causing a 120 foot local recovery of heads; (2) water levels generally will change less than 25 feet if there is no change in pumpage; (3) proposed pumpage in western Delaware may cause drawdowns below the tops of the aquifers; and (4) expected increases in pumpage would cause the greatest head decline, about 40 feet, in the lower aquifer at Getty. (USGS)

Water-Resources Investigations Report↗