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USGS2007

Halite brine in the Onondaga Trough near Syracuse, New York: Characterization and simulation of variable-density flow

Halite brine (saturation ranging from 45 to 80 percent) lies within glacial-drift deposits that fill the Onondaga Trough, a 40-km long bedrock valley deepened by Pleistocene ice near Syracuse, N.Y. The most concentrated brine occupies the northern end of the trough, more than 15 kilometers (km) beyond the northern limit of halite beds in the Silurian Salina Group, the assumed source of salt. The chemical composition of the brine and its radiocarbon age estimated from geochemical modeling with NETPATH suggest that the brine formed through dissolution of halite by glacial melt water, and later mixed with saline bedrock water about 16,500 years ago. Transient variable-density flow simulations were conducted with SEAWAT to assess current (2005) ground-water flow conditions within the glacial drift. A transient three-dimensional (3D) model using a grid spacing of 100 meters (m) and maximum layer spacing of 30 m was used to simulate a 215-year period from 1790 to 2005. The model was calibrated to observations of water levels, chloride concentrations, and discharges of water and chloride. The model produced an acceptable match to the measured data and provided a reasonable representation of the density distribution within the brine pool. The simulated mass of chloride in storage declined steadily during the 215-year period; however, the decline was mainly due to dispersion, which is probably overestimated because of the large layer spacing. Model results suggest that saline water from waste-disposal operations associated with a chemical plant has migrated beneath the western shore of Onondaga Lake. Two-dimensional (2D) cross-sectional models of the aquifer system within the Onondaga Trough were prepared to test the plausibility of a hypothesis that the brine was derived from a relict source of halite that was dissolved by glacial melt water. The 2D models used parameter estimates obtained with the calibrated 3D model. Model results indicated the brine could have migrated from the bedded-halite subcrop area and remained in the glacial sediments at the northern end of trough for over 16,000 years, as suggested by radiocarbon dating. The 2D models also indicated that slow dissipation of brine occurs through a mixing zone formed by upward flow of freshwater over the southern end of the brine pool. The simulated depletion rate is controlled by the rate of mixing, which is limited by the specified grid resolution and the accuracy of the numerical method used to solve the advection-dispersion equation. A numerical solution obtained by using an implicit finite-difference method with upstream weighting and a 2D grid containing a column and layer spacing of 76 m and 3 m, respectively, provided an acceptable match to chloride concentration profiles measured at three locations within the Onondaga Trough.

New York↗
USGS1985

Effects of climate, vegetation, and soils on consumptive water use and ground-water recharge to the Central Midwest Regional aquifer system, mid-continent United States

The Central Midwest aquifer system, in parts of Arkansas, Colorado, Kansas, Missouri, Nebraska, New Mexico, South Dakota, and Texas, is a region of great hydrologic diversity. This study examines the relationships between climate, vegetation, and soil that affect consumptive water use and recharge to the groundwater system. Computations of potential recharge and consumptive water use were restricted to those areas where the aquifers under consideration were the immediate underlying system. The principal method of analysis utilized a soil moisture computer model. This model requires four types of input: (1) hydrologic properties of the soils, (2) vegetation types, (3) monthly precipitation, and (4) computed monthly potential evapotranspiration (PET) values. The climatic factors that affect consumptive water use and recharge were extensively mapped for the study area. Nearly all the pertinent climatic elements confirmed the extreme diversity of the region. PET and those factors affecting it--solar radiation, temperature, and humidity--showed large regional differences; mean annual PET ranged from 36 to 70 inches in the study area. The seasonal climatic patterns indicate significant regional differences in those factors affecting seasonal consumptive water use and recharge. In the southern and western parts of the study area, consumptive water use occurred nearly the entire year; whereas, in northern parts it occurred primarily during the warm season (April through September). Results of the soil-moisture program, which added the effects of vegetation and the hydrologic characteristics of the soil to computed PET values, confirmed the significant regional differences in consumptive water use or actual evapotranspiration (AET) and potential groundwater recharge. Under two different vegetative conditions--the 1978 conditions and pre-agricultural conditions consisting of only grassland and woodland--overall differences in recharge were minimal. Mean annual recharge under both conditions averaged slightly more than 4.5 inches for the entire study area, but ranged from less than 0.10 inches in eastern Colorado to slightly more than 15 inches in Arkansas.

Arkansas, Colorado, Kansas, Missouri, Nebraska, Ne↗
USGS2000

Simulation of projected water demand and ground-water levels in the Coffee Sand and Eutaw-McShan aquifers in Union County, Mississippi, 2010 through 2050

Ground water from the Eutaw-McShan and the Coffee Sand aquifers is the major source of supply for residential, commercial, and industrial purposes in Union County, Mississippi. Unbiased, scientifically sound data and assessments are needed to assist agencies in better understanding and managing available water resources as continuing development and growth places more stress on available resources. The U.S. Geological Survey, in cooperation with the Tennessee Valley Authority, conducted an investigation using water-demand and ground-water models to evaluate the effect of future water demand on groundwater levels. Data collected for the 12 public-supply facilities and the self-supplied commercial and industrial facilities in Union County were used to construct water-demand models. The estimates of water demand to year 2050 were then input to a ground-water model based on the U.S. Geological Survey finite-difference computer code, MODFLOW. Total ground-water withdrawals for Union County in 1998 were estimated as 2.85 million gallons per day (Mgal/d). Of that amount, municipal withdrawals were 2.55 Mgal/d with about 1.50 Mgal/d (59 percent) delivered to residential users. Nonmunicipal withdrawals were 0.296 Mgal/d. About 80 percent (2.27 Mgal/d) of the total ground-water withdrawal is produced from the Eutaw-McShan aquifer and about 13 percent (0.371 Mgal/d) from the Coffee Sand aquifer. Between normal- and high-growth conditions, total water demand could increase from 72 to 131 percent (2.9 Mgal/d in 1998 to 6.7 Mgal/d in year 2050) with municipal demand increasing from 77 to 146 percent (2.6 to 6.4 Mgal/d). Increased pumping to meet the demand for water was simulated to determine the effect on water levels in the Coffee Sand and Eutaw- McShan aquifers. Under baseline-growth conditions, increased water use by year 2050 could result in an additional 65 feet of drawdown in the New Albany area below year 2000 water levels in the Coffee Sand aquifer and about 120 feet of maximum drawdown in the Eutaw-McShan aquifer. Under normal-growth conditions, increased water use could result in an additional 65 feet of drawdown in the New Albany area below year 2000 water levels in the Coffee Sand aquifer and about 135 feet of maximum drawdown in the Eutaw-McShan aquifer. Under high-growth conditions, increased water use could result in 75 feet of drawdown in the New Albany area below year 2000 water levels in the Coffee Sand aquifer and about 190 feet of maximum drawdown in the Eutaw-McShan aquifer. The resulting highgrowth projected water level for the year 2050 at the center of the drawdown cone in the New Albany area is between 450 and 500 feet above the top of the Eutaw-McShan aquifer.

Mississippi↗
USGS1997

Ambient quality of ground water in the vicinity of Naval Submarine Base Bangor, Kitsap County, Washington, 1995

A study of the ambient ground-water quality in the vicinity of Naval Submarine Base (SUBASE) Bangor was conducted to provide the U.S. Navy with background levels of selected constituents. The Navy needs this information to plan and manage cleanup activities on the base. During March and April 1995, 136 water-supply wells were sampled for common ions, trace elements, and organic compounds; not all wells were sampled for all constituents. Man-made organic compounds were detected in only two of fifty wells, and the sources of these organic compounds were attributed to activities in the immediate vicinities of these off-base wells. Drinking water standards for trichloroethylene, iron, and manganese were exceeded in one of these wells, which was probably contaminated by an old local (off-base) dump. Ground water from wells open to the following hydrogeologic units (in order from shallow to deep) was investigated: the Vashon till confining unit (Qvt, three wells); the Vashon aquifer (Qva, 54 wells); the Upper confining unit (QC1, 16 wells); the Permeable interbeds within QC1 (QC1pi, 34 wells); and the Sea-level aquifer (QA1, 29 wells).The 50th and 90th percentile ambient background levels of 35 inorganic constituents were determined for each hydrogeologic unit. At least ten measurements were required for a constituent in each hydrogeologic unit for determination of ambient background levels, and data for three wells determined to be affected by localized activities were excluded from these analyses. The only drinking water standards exceeded by ambient background levels were secondary maximum contaminant levels for iron (300 micrograms per liter), in QC1 and QC1pi, and manganese (50 micrograms per liter), in all of the units. The 90th percentile values for arsenic in QC1pi, QA1, and for the entire study area are above 5 micrograms per liter, the Model Toxics Control Act Method A value for protecting drinking water, but well below the maximum contaminant level of 50 micrograms per liter for arsenic. The manganese standard was exceeded in 38 wells and the standard for iron was exceeded in 12 wells. Most of these wells were in QC1 or QC1pi and had dissolved oxygen concentrations of less than 1 milligram per liter and dissolved organic carbon concentrations greater than 1 milligram per liter. The dissolved oxygen concentration is generally lower in the deeper units, while pH increases; the recommended pH range of 6.5-8.5 standard units was exceeded in 9 wells. The common-ion chemistry was similar for all of the units.

Washington↗
USGS2010

Geophysical investigations at Hidden Dam, Raymond, California: Summary of fieldwork and data analysis

Geophysical field investigations have been carried out at the Hidden Dam in Raymond, California for the purpose of better understanding the hydrogeology and seepage-related conditions at the site. Known seepage areas on the northwest right abutment area of the downstream side of the dam are documented by Cedergren. Subsequent to the 1980 seepage study, a drainage blanket with a subdrain system was installed to mitigate downstream seepage. Flow net analysis provided by Cedergren suggests that the primary seepage mechanism involves flow through the dam foundation due to normal reservoir pool elevations, which results in upflow that intersects the ground surface in several areas on the downstream side of the dam. In addition to the reservoir pool elevations and downstream surface topography, flow is also controlled by the existing foundation geology as well as the presence or absence of a horizontal drain within the downstream portion of the dam. The purpose of the current geophysical work is to (1) identify present-day seepage areas that may not be evident due to the effectiveness of the drainage blanket in redirecting seepage water, and (2) provide information about subsurface geologic structures that may control subsurface flow and seepage. These tasks are accomplished through the use of two complementary electrical geophysical methods, self-potentials (SP) and direct-current (DC) electrical resistivity, which have been commonly utilized in dam-seepage studies. SP is a passive method that is primarily sensitive to active subsurface groundwater flow and seepage, whereas DC resistivity is an active-source method that is sensitive to changes in subsurface lithology and groundwater saturation. The focus of this field campaign was on the downstream area on the right abutment, or northwest side of the dam, as this is the main area of interest regarding seepage. Two exploratory self-potential lines were also collected on the downstream left abutment of the dam to identify potential seepage in that area. This report is primarily a summary of the field geophysical data acquisition, with some preliminary results and interpretation. Further work will involve a more rigorous analysis of the geophysical datasets and an examination of a large dataset of historical observations of water levels in a number of observation wells and piezometers compared with reservoir elevation. In addition, a partially saturated flow model will be developed to better understand seepage patterns given the available information about dam construction, geophysical results, and data from installed observation wells and piezometers.

California↗
USGS1995

Regression models of monthly water-level change in and near the Closed Basin Division of the San Luis Valley, south-central Colorado

The Bureau of Reclamation is developing a water-resource project, the Closed Basin Division, in the San Luis Valley of south-central Colorado that is designed to salvage unconfined ground water that currently is discharged as evapotranspiration. The water table in and near the 130,000-acre Closed Basin Division area will be lowered by an annual withdrawal of as much as 100,000 acre-feet of ground water from the unconfined aquifer. The legislation authorizing the project limits resulting drawdown of the water table in preexisting irrigation and domestic wells outside the Closed Basin Division to a maximum of 2 feet. Water levels in the closed basin in the northern part of the San Luis Valley historically have fluctuated more than 2 feet in response to water-use practices and variation of climatically controlled recharge and discharge. Declines of water levels in nearby wells that are caused by withdrawals in the Closed Basin Division can be quantified if water-level fluctuations that result from other water-use practices and climatic variations can be estimated. This study was done to evaluate water-level change at selected observation wells in and near the Closed Basin Division. Regression models of monthly water-level change were developed to predict monthly water-level change in 46 selected observation wells. Predictions of monthly water-level change are based on one or more of the following: elapsed time, cosine and sine functions with an annual period, streamflow depletion of the Rio Grande, electrical use for agricultural purposes, runoff into the closed basin, precipitation, and mean air temperature. Regression models for five of the wells include only an intercept term and either an elapsed-time term or terms determined by the cosine and sine functions. Regression models for the other 41 wells include 1 to 4 of the 5 other variables, which can vary from month to month and from year to year. Serial correlation of the residuals was detected in 24 of the regression models. These models also include an autoregressive term to account for serial correlation in the residuals. The adjusted coefficient of determination (Ra2) for the 46 regression models range from 0.08 to 0.89, and the standard errors of estimate range from 0.034 to 2.483 feet. The regression models of monthly water- level change can be used to evaluate whether post-1985 monthly water-level change values at the selected observation wells are within the 95-percent confidence limits of predicted monthly water-level change.

Water-Resources Investigations Report↗
USGS2001

Numerical Simulation of Ground-Water Flow and Assessment of the Effects of Artificial Recharge in the Rialto-Colton Basin, San Bernardino County, California

The Rialto?Colton Basin, in western San Bernardino County, California, was chosen for storage of imported water because of the good quality of native ground water, the known storage capacity for additional ground-water storage in the basin, and the availability of imported water. To supplement native ground-water resources and offset overdraft conditions in the basin during dry periods, artificial-recharge operations during wet periods in the Rialto?Colton Basin were begun in 1982 to store surplus imported water. Local water purveyors recognized that determining the movement and ultimate disposition of the artificially recharged imported water would require a better understanding of the ground-water flow system. In this study, a finite-difference model was used to simulate ground-water flow in the Rialto?Colton Basin to gain a better understanding of the ground-water flow system and to evaluate the hydraulic effects of artificial recharge of imported water. The ground-water basin was simulated as four horizontal layers representing the river- channel deposits and the upper, middle, and lower water-bearing units. Several flow barriers bordering and internal to the Rialto?Colton Basin influence the direction of ground-water flow. Ground water may flow relatively unrestricted in the shallow parts of the flow system; however, the faults generally become more restrictive at depth. A particle-tracking model was used to simulate advective transport of imported water within the ground-water flow system and to evaluate three artificial-recharge alternatives. The ground-water flow model was calibrated to transient conditions for 1945?96. Initial conditions for the transient-state simulation were established by using 1945 recharge and discharge rates, and assuming no change in storage in the basin. Average hydrologic conditions for 1945?96 were used for the predictive simulations (1997?2027). Ground-water-level measurements made during 1945 were used for comparison with the initial-conditions simulation to determine if there was a reasonable match, and thus reasonable starting heads, for the transient simulation. The comparison between simulated head and measured water levels indicates that, overall, the simulated heads match measured water levels well; the goodness-of-fit value is 0.99. The largest differences between simulated head and measured water level occurred between Barrier H and the Rialto?Colton Fault. Simulated heads near the Santa Ana River and Warm Creek, and simulated heads northwest of Barrier J, generally are within 30 feet of measured water levels and five are within 20 feet. Model-simulated heads were compared with measured long-term changes in hydrographs of composite water levels in selected wells, and with measured short-term changes in hydrographs of water levels in multiple-depth observation wells installed for this project. Simulated hydraulic heads generally matched measured water levels in wells northwest of Barrier J (in the northwestern part of the basin) and in the central part of the basin during 1945?96. In addition, the model adequately simulated water levels in the southeastern part of the basin near the Santa Ana River and Warm Creek and east of an unnamed fault that subparallels the San Jacinto Fault. Simulated heads and measured water levels in the central part of the basin generally are within 10 feet until about 1982?85 when differences become greater. In the northwestern part of the basin southeast of Barrier J, simulated heads were as much as 50 feet higher than measured water levels during 1945?82 but matched measured water levels well after 1982. In the compartment between Barrier H and the Rialto?Colton Fault, simulated heads match well during 1945?82 but are comparatively low during 1982?96. Near the Santa Ana River and Warm Creek, simulated heads generally rose above measured water levels except during 1965?72 when simulated heads compared well with measured water levels. Average

Water-Resources Investigations Report↗
USGS1991

Field experiments and simulations of infiltration-rate response to changes in hydrologic conditions for an artificial-recharge test basin near Oakes, southeastern North Dakota

Ponded depth in an artificial-recharge basin was used as a management option to conduct turbid water from the James River to the Oakes aquifer. Infiltration-rate response to changes in ponded depth was evaluated for a 15xl5-meter artificial-recharge test basin constructed in a medium-sandy soil in the irrigation area near Oakes, southeastern North Dakota. Field experiments conducted at the test basin indicated that the clogged soil surface was easily scoured by currents caused by the addition of turbid water to increase ponded depth. Field measurements and computer-model simulations indicated that infiltration-rate response to an increase in ponded depth would be large for a clogged soil condition consisting of a surface filter-cake layer 0.1 centimeter in depth underlain by a sediment-clogged layer extending from 0.1 to 23 centimeters beneath the basin floor. Simulated infiltration-rate response to changes in ponded depth for surface filter-cake layer impedance values ranging from 0 to 1,000 hours indicated that infiltration-rate response would approach and remain near the maximum value for impedance values greater than 10 hours. The smallest infiltration-rate response would occur for impedance values less than 1 hour. For the case of a ground-water mound intersecting the basin floor, the percentage of infiltration-rate response to changes in ponded depth was not influenced by basin geometry, basin surface area, or underlying aquifer hydraulic conductivity. The simulated infiltration-rate response to changes in ponded depth increased when the water-table depth was shallow. Total recharge per unit area was greater for artificial-recharge basins having a less compact geometry than for artificial-recharge basins having a more compact geometry, for artificial-recharge basins having a small surface area than for artificial-recharge basins having a large surface area, and for artificial-recharge basins where underlying aquifer hydraulic conductivity was large rather than small. Artificial-recharge basin conditions most conducive to an effective infiltration-rate response to changes in ponded depth were least conducive to an enhanced total recharge.

Water-Resources Investigations Report↗
USGS1999

Use of environmental tracers to evaluate ground-water age and water-quality trends in a buried-valley aquifer, Dayton area, southwestern Ohio

Chlorofluorocarbons (CFC method) and tritium and helium isotopes (3H-3He method) were used as environmental tracers to estimate ground-water age in conjunction with efforts to develop a regional ground-water flow model of the buried-valley aquifer in the Dayton area, southwestern Ohio. This report describes results of CFC and water-quality sampling, summarizes relevant aspects of previously published work, and describes the use of 3H-3He ages to characterize temporal trends in ground-water quality of the buried-valley aquifer near Dayton, Ohio. Results of CFC sampling indicate that approximately 25 percent of the 137 sampled wells were contaminated with excess CFC's that rendered the ground water unsuitable for age dating. Evaluation of CFC ages obtained for the remaining samples indicated that the CFC compounds used for dating were being affected by microbial degradation. The degradation occurred under anoxic conditions that are found in most parts of the buried-valley aquifer. As a result, ground-water ages derived by the CFC method were too old and were inconsistent with measured tritium concentrations and independently derived 3H-3He ages. Limited data indicate that dissolved methane may play an important role in the degradation of the CFC's. In contrast, the 3H-3He technique was found to yield ground-water ages that were chemically and hydrologically reasonable. Ground-water ages derived by the 3H-3He technique were compared to values for selected water- quality characteristics to evaluate temporal trends in ground-water quality in the buried- valley aquifer. Distinct temporal trends were not identified for pH, alkalinity, or calcium and magnesium because of rapid equilibration of ground-water with calcite and dolomite in aquifer sediments. Temporal trends in which the amount of scatter and the number of outlier concentrations increased as ground-water age decreased were noted for sodium, potassium, boron, bromide, chloride, ammonia, nitrate, phosphate, sulfate, and organic carbon. Elevated concentrations of these constituents in shallow ground water are probably related to human activities. Temporal trends in which concentrations declined as ground-water age increased may reflect natural processes that reduce constituent concentrations to low levels. For example, the absence of nitrate detections in ground water recharged before 1980 may indicate natural removal of nitrate by bacterially mediated denitrification. Temporal trends observed for dissolved oxygen, iron, nitrate and silica indicate that these constituents may help identify recently (post-1990) recharged ground water.

Ohio↗
USGS2007

Simulation of streamflow and estimation of ground-water recharge in the Upper Cibolo Creek Watershed, south-central Texas, 1992-2004

A watershed model (Hydrological Simulation Program?FORTRAN) was developed, calibrated, and tested by the U.S. Geological Survey, in cooperation with the U.S. Army Corps of Engineers, San Antonio River Authority, San Antonio Water System, and Guadalupe-Blanco River Authority, to simulate streamflow and estimate ground-water recharge in the upper Cibolo Creek watershed in south-central Texas. Rainfall, evapotranspiration, and streamflow data were collected during 1992?2004 for model calibrations and simulations. Estimates of average ground-water recharge during 1992?2004 from simulation were 79,800 acre-feet (5.47 inches) per year or about 15 percent of rainfall. Most of the recharge (about 74 percent) occurred as infiltration of streamflow in Cibolo Creek. The remaining recharge occurred as diffuse infiltration of rainfall through the soil and rock layers and karst features. Most recharge (about 77 percent) occurred in the Trinity aquifer outcrop. The remaining 23 percent occurred in the downstream part of the watershed that includes the Edwards aquifer recharge zone (outcrop). Streamflow and recharge in the study area are greatly influenced by large storms. Storms during June 1997, October 1998, and July 2002 accounted for about 11 percent of study-area rainfall, 61 percent of streamflow, and 16 percent of the total ground-water recharge during 1992?2004. Annual streamflow and recharge also were highly variable. During 1999, a dry year with about 16 inches of rain and no measurable runoff at the watershed outlet, recharge in the watershed amounted to only 0.99 inch compared with 13.43 inches during 1992, a relatively wet year with about 54 inches of rainfall. Simulation of flood-control/recharge-enhancement structures showed that certain structures might reduce flood peaks and increase recharge. Simulation of individual structures on tributaries showed relatively little effect. Larger structures on the main stem of Cibolo Creek were more effective than structures on tributaries, both in terms of flood-peak reduction and recharge enhancement. One simulated scenario that incorporated two main-stem structures resulted in a 37-percent reduction of peak flow at the watershed outlet and increases in stream-channel recharge of 6.6 percent in the Trinity aquifer outcrop and 12.6 percent in the Edwards aquifer (recharge zone) outcrop.

Texas↗
USGS1996

Geochemical and isotopic composition of ground water with emphasis on sources of sulfate in the upper Floridan Aquifer in parts of Marion, Sumter, and Citrus counties, Florida

In inland areas of northwest central Florida, sulfate concentrations in the Upper Floridan aquifer are extremely variable and sometimes exceed drinking water standards (250 milligrams per liter). This is unusual because the aquifer is unconfined and near the surface, allowing for active recharge. The sources of sulfate and geochemical processes controlling ground-water composition were evaluated in this area. Water was sampled from thirty-three wells in parts of Marion, Sumter, and Citrus Counties, within the Southwest Florida Water Management District; these included at least a shallow and a deep well at fifteen separate locations. Ground water was analyzed for major ions, selected trace constituents, dissolved organic carbon, and stable isotopes (sulfur-34 of sulfate and sulfide, carbon-13 of inorganic carbon, deuterium, and oxygen-18). Sulfate concentrations ranged from less than 0.2 to 1,400 milligrams per liter, with higher sulfate concentrations usually in water from deeper wells. The samples can be categorized into a low sulfate group (less than 30 milligrams per liter) and a high sulfate group (greater than 30 milligrams per liter). For the high sulfate water, concentrations of calcium and magnesium increased concurrently with sulfate. Chemical and isotopic data and mass-balance modeling indicate that the composition of high sulfate waters is controlled by dedolomitization reactions (dolomite dissolution and calcite precipitation, driven by dissolution of gypsum). Gypsum occurs deeper in the aquifer than open intervals of sampled wells. Upward flow has been documented in deeper parts of the aquifer in the study area, which may be driven by localized discharge areas or rapid flow in shallow parts of the aquifer. Mixing between shallow ground water and sulfate-rich water that dissolved gypsum at the base of the aquifer is probably responsible for the range of concentrations observed in the study area. Other solutes that increased with sulfate apparently originate from the gypsum itself, from other mineral assemblages found deeper in the aquifer in association with gypsum, and from residual seawater from less- flushed, deeper parts of the aquifer. These ions are subsequently transported with sulfate to shallower parts of the aquifer where gypsum is not present. The composition of low sulfate ground water is controlled by differences in the extent of microbially mediated reactions, which produce carbon dioxide. This, in turn, influences the extent of calcite dissolution. Ground waters which underwent limited microbial reactions contained dissolved oxygen and were usually in ridge areas where recharge typically is rapid. Anaerobic waters were in lower lying areas of Sumter County, where soils are poorly drained and aquifer recharge is slow. Anaerobic waters had higher concentrations of calcium, bicarbonate, sulfide, dissolved organic carbon, iron, manganese, and silica, and had lower concentrations of nitrate than aerobic ground waters. For low sulfate waters, sulfate generally originates from meteoric sources (atmospheric precipitation), with variable amounts of oxidation of reduced sulfur and sulfate reduction. Sulfide is sometimes removed from solution, probably by precipitation of a sulfide minerals such as pyrite. In areas where deep ground water has low sulfate concentrations, the shallow flow system is apparently deeper than where high sulfate concentrations occur, and upwelling sulfate-rich water is negligible. The range of sulfate concentrations observed in the study areas and differences in sulfate concentrations with depth indicate a complex interaction between shallow and deep ground-water flow systems.

Water-Resources Investigations Report↗
USGS1996

Assessment of intrinsic bioremediation of gasoline contamination in the shallow aquifer, Laurel Bay Exchange, Marine Corps Air Station Beaufort, South Carolina

Laboratory, field, and digital solute-transport- modeling studies demonstrate that microorganisms indigenous to the shallow ground-water system at Laurel Bay Exchange, Marine Corps Air Station Beaufort, South Carolina, can degrade petroleum hydrocarbons in gasoline released at the site. Microorganisms in aquifer sediments incubated in the laboratory under aerobic and anaerobic conditions mineralized radiolabeled carbon 14-toluene to 14 C-carbon dioxide with first-order rate constants of K bio = -0.640 per day and K bio = -0.003 per day, respectively. Digital solute- transport modeling using the numerical code SUTRA revealed that anaerobic biodegradation of benzene occurs with a first-order rate constant near K bio = -0.00025 per day. Sandy aquifer material beneath Laurel Bay Exchange is characterized by relatively high hydraulic conductivities (K aq = 8.9 to 17.3 feet per day), average ground-water flow rate of about 60 feet per year, and a relatively uniform hydraulic gradient of 0.004 feet per foot. The sandy aquifer material also has low adsorptive potentials for toluene and benzene (both about K ad = 2.0 x 10 -9 cubic feet per milligram), because of the lack of natural organic matter in the aquifer. The combination of this ground-water-flow rate and absence of significant adsorptive capacity in the aquifer permits toluene and benzene concentrations to be detected downgradient from the source area in monitoring wells, even though biodegradation of these compounds has been demonstrated. Solute-transport simulations, however, indicate that toluene and benzene will not reach the Broad River, the nearest point of contact with wildlife or human populations, about 3,600 feet west of the site boundary. These simulations also show that contamination will not be transported to the nearest Marine Corps property line about 2,400 feet south of the site. This is primarily because the source of contaminants has essentially been removed, and the low adsorptive capacity of the aquifer sediments has prevented the occurrence of an adsorbed, continuous source of petroleum hydrocarbons. Therefore, digital simulations of toluene and benzene transport at Laurel Bay Exchange indicate that intrinsic bioremediation could be a successful remediation alternative for prohibiting transport of dissolved toluene and benzene to the Broad River.

South Carolina↗
USGS2002

Simulation of Transient Ground-Water Flow in the Valley-Fill Aquifers of the Upper Rockaway River Basin, Morris County, New Jersey

More than 90 percent of the public water supply in the upper Rockaway River Valley in Morris County, New Jersey, is obtained from ground-water withdrawals from the valley-fill aquifers. During 1997, an average of 9.6 million gallons per day of ground water was withdrawn from these aquifers. The aquifer system consists of an unconfined aquifer (upper aquifer) and a locally confined aquifer (lower aquifer), which are composed of sands and gravels. These aquifers are separated by a discontinuous confining unit that consists mostly of silt and clay. Increases in ground-water withdrawals can induce movement of water from streams to wells, increase flow from the upper aquifer to the lower aquifer, and reduce base flow in the Rockaway River downstream. A ground-water-flow model was used to simulate and quantify the effects of current withdrawals on the valley-fill aquifer system under transient monthly conditions. Recharge over the model area varies both spatially and temporally. Part of model calibration consisted of adjusting percentages of monthly precipitation that recharges the valley-fill aquifer system. More recharge occurs during winter and spring than during summer and fall. This seasonal variation affects ground-water discharge to the Rockaway River. Ground-water withdrawals from the valleyfill aquifers also affect ground-water discharge to the Rockaway River. Three scenarios were simulated to observe the effects of ground-water withdrawals on ground-water discharge to the Rockaway River and to determine the extent to which variations in rates of withdrawals correspond to variations in rates of streamflow depletion. Streamflow depletion was estimated by comparing model-computed ground-water discharge for the three scenarios with the modelcomputed ground-water discharge under transient conditions. In scenario 1, all pumpage was removed from the model. In scenarios 2 and 3, 1 million gallons per day of ground-water withdrawals in excess of the current pumpage was withdrawn from the valley-fill aquifers. In scenario 2, the additional 1 million gallons per day of withdrawals were made from a hypothetical well located in the upper aquifer about 250 feet from the river. In scenario 3, the additional withdrawals were made from a hypothetical well located in the lower aquifer about 1,750 feet from the river. Results of scenario 1 indicated that the difference between the streamflow depletion and withdrawals is small; increases in ground-water withdrawals from the valley-fill aquifers correspond to decreases in ground-water discharge to the Rockaway River of approximately the same amount. Results of scenario 2 and 3 indicated that a lag time could occur between the introduction of withdrawals and the full magnitude of the effects of the withdrawals on streamflow depletion. A lag time of about seven months occurred for scenario 2 with the well placed in the upper aquifer. A longer lag time of more than 1.5 years occurred with the well placed in the lower aquifer and separated from the upper aquifer by a confining unit (scenario 3). Extreme low flow in the Rockaway River is mostly base flow. A flow-duration analysis of the Rockaway River at the surface-water gaging station upstream from the Boonton Reservoir during the drought of 1961-66 indicated that streamflow from the upper Rockaway River Basin alone might not be sufficient to meet the minimum passing flow of 7 million gallons per day during a drought. Under similar drought conditions today, during 3.2 percent of the drought time, streamflow at this station upstream from the reservoir would be less than the minimum passing flow requirement downstream from the reservoir.

Water-Resources Investigations Report↗
USGS2000

Investigation of the geology and hydrology of the Mogollon Highlands of central Arizona: a project of the Arizona Rural Watershed Initiative

The Mogollon Highlands of east central Arizona is a region of forested plateau and mountains, deep, sheerwalled canyons, and desert valleys. Known for its scenic beauty and characterized by a generally mild climate, the area, though still sparsely populated, attracts an increasing number of tourists and summer residents. Furthermore, the permanent population is expected to nearly double over the next 50 years. Consequently, there is increased pressure on the water resources of this area for several sometimes conflicting uses. Rational management of water resources is necessary to meet increased domestic requirements while ensuring an adequate supply of water for commercial and agricultural use, for Indian lands, and for preservation of valued environmental elements, including surface waters, riparian woodlands, forest and grassland areas, and wildlife and aquatic habitat. Such management requires an understanding of the relations among different components of the hydrologic system—recharge areas, surface flows, shallow aquifers, deep aquifers, discharge areas, and the regional ground-water flow system—and how each is affected by geology, climate, topography, and human use. The U.S. Geological Survey (USGS) is conducting an assessment of the hydrogeology of the Mogollon Highlands in cooperation with the Arizona Department of Water Resources. The study, funded through the State’s Rural Watershed Initiative program, is one of three assessments being conducted by the USGS. Assessments also are underway in the Upper-Middle Verde River watershed and on the Coconino Plateau. Each study has as its objectives: (1) the collection, compilation, and evaluation of all existing geologic, hydrologic, and related data pertaining to the study area and the creation of a data base that is readily accessible to the public and (2) developing an understanding of the hydrogeologic framework, which is the relation between geologic and hydrologic properties, that can be used for water-- resources management purposes and that will support the development of an interpretive and predictive model to estimate the effects of climate and water use on the sustainability of regional water resources. Although the three contiguous areas in north-central Arizona are being studied separately, a single data base is being constructed from which data on each area can be extracted separately.

Arizona↗
USGS2000

Computer-model analysis of ground-water flow and simulated effects of contaminant remediation at Naval Weapons Industrial Reserve Plant, Dallas, Texas

In June 1993, the Department of the Navy, Southern Division Naval Facilities Engineering Command (SOUTHDIV), began a Resource Conservation and Recovery Act (RCRA) Facility Investigation (RFI) of the Naval Weapons Industrial Reserve Plant (NWIRP) in north-central Texas. The RFI has found trichloroethene, dichloroethene, vinyl chloride, as well as chromium, lead, and other metallic residuum in the shallow alluvial aquifer underlying NWIRP. These findings and the possibility of on-site or off-site migration of contaminants prompted the need for a ground-water-flow model of the NWIRP area. The resulting U.S. Geological Survey (USGS) model: (1) defines aquifer properties, (2) computes water budgets, (3) delineates major flowpaths, and (4) simulates hydrologic effects of remediation activity. In addition to assisting with particle-tracking analyses, the calibrated model could support solute-transport modeling as well as help evaluate the effects of potential corrective action. The USGS model simulates steadystate and transient conditions of ground-water flow within a single model layer. The alluvial aquifer is within fluvial terrace deposits of Pleistocene age, which unconformably overlie the relatively impermeable Eagle Ford Shale of Late Cretaceous age. Over small distances and short periods, finer grained parts of the aquifer are separated hydraulically; however, most of the aquifer is connected circuitously through randomly distributed coarser grained sediments. The top of the underlying Eagle Ford Shale, a regional confining unit, is assumed to be the effective lower limit of ground-water circulation and chemical contamination. The calibrated steady-state model reproduces long-term average water levels within +5.1 or –3.5 feet of those observed; the standard error of the estimate is 1.07 feet with a mean residual of 0.02 foot. Hydraulic conductivity values range from 0.75 to 7.5 feet per day, and average about 4 feet per day. Specific yield values range from 0.005 to 0.15 and average about 0.08. Simulated infiltration rates range from 0 to 2.5 inches per year, depending mostly on local patterns of ground cover. Computer simulation indicates that, as of December 31, 1998, remediation systems at NWIRP were removing 7,375 cubic feet of water per day from the alluvial aquifer, with 3,050 cubic feet per day coming from aquifer storage. The resulting drawdown prevented 1,800 cubic feet per day of ground water from discharging into Cottonwood Bay, as well as inducing another 1,325 cubic feet per day into the aquifer from the bay. An additional 1,200 cubic feet of water per day (compared to pre-remediation conditions) was prevented from discharging into the west lagoon, east lagoon, Mountain Creek Lake, and Mountain Creek swale. Particle-tracking simulations, assuming an aquifer porosity of 0.15, were made to delineate flowpath patterns, or contaminant “capture zones,” resulting from 2.5- and 5-year periods of remediation activity at NWIRP. The resulting flowlines indicate three such zones, or areas from which ground water is simulated to have been removed during July 1996–December 1998, as well as extended areas from which ground water would be removed during the next 2.5 years (January 1999– June 2001). Simulation indicates that, as of December 31, 1998, the recovery trench was intercepting about 827 cubic feet per day of ground water that—without the trench—would have discharged into Cottonwood Bay. During this time, the trench is simulated to have removed about 3,221 cubic feet per day of water from the aquifer, with about 934 cubic feet per day (29 percent) coming from the south (Cottonwood Bay) side of the trench.

Texas↗
USGS2005

Questa baseline and premining ground-water quality investigation. 8. Lake-sediment geochemical record from 1960 to 2002, Eagle Rock and Fawn Lakes, Taos County, New Mexico

Geochemical studies of lake sediment from Eagle Rock Lake and upper Fawn Lake were conducted to evaluate the effect of mining at the Molycorp Questa porphyry molybdenum deposit located immediately north of the Red River. Two cores were taken, one from each lake near the outlet where the sediment was thinnest, and they were sampled at 1-cm intervals to provide geochemical data at less than 1-year resolution. Samples from the core intervals were digested and analyzed for 34 elements using ICP–AES (inductively coupled plasma–atomic emission spectrometry). The activity of 137 Cs has been used to establish the beginning of sedimentation in the two lakes. Correlation of the geochemistry of heavy-mineral suites in the cores from both Fawn and Eagle Rock Lakes has been used to develop a sedimentation model to date the intervals sampled. The core from upper Fawn Lake, located upstream of the deposit, provided an annual sedimentary record of the geochemical baseline for material being transported in the Red River, whereas the core from Eagle Rock Lake, located downstream of the deposit, provided an annual record of the effect of mining at the Questa mine on the sediment in the Red River. Abrupt changes in the concentrations of many lithophile and deposit-related metals occur in the middle of the Eagle Rock Lake core, which we correlate with the major flood-of-record recorded at the Questa gage at Eagle Rock Lake in 1979. Sediment from the Red River collected at low flow in 2002 is a poor match for the geochemical data from the sediment core in Eagle Rock Lake. The change in sediment geochemistry in Eagle Rock Lake in the post-1979 interval is dramatic and requires that a new source of sediment be identified that has substantially different geochemistry from that in the pre-1979 core interval. Loss of mill tailings from pipeline breaks are most likely responsible for some of the spikes in trace-element concentrations in the Eagle Rock Lake core. Enrichment of Al 2 O 3 , Cu, and Zn occurred as a result of chemical precipitation of these metals from ground water upstream in the Red River. Comparisons of the geochemistry of the post-1979 sediment core with both mine wastes and with premining sediment from the vicinity of the Questa mine indicate that both are possible sources for this new component of sediment. Existing data have not resolved this enigma.

New Mexico↗
USGS1993

Geology and ground-water resources in the Zebulon area, Georgia

The current (1991) surface-water source of drinking-water supply for the city of Zebulon, Pike County, Georgia, no longer provides an adequate water supply and periodically does not meet water-quality standards. The hydrogeology of crystalline rocks in the Zebulon area was evaluated to assess the potential of ground-water resources as a supplemental or alternative source of water to present surface-water supplies. As part of the ground-water resource evaluation, well location and construction data were compiled, a geologic map was constructed, and ground water was sampled and analyzed. Three mappable geologic units delineated during this study provide a basic understanding of hydrogeologic settings in the Zebulon area. Rock types include a variety of aluminosilicate schists, granitic rocks, amphibolites/honblende gneisses, and gondites. Several geologic features that may enhance ground-water availability were identified in the study area. These features include contacts between contrasting rock types, where a high degree of differential weathering has occurred, and well-developed structural features, such as foliation and jointing are present. High-yielding wells (greater than 25 gallons per minute) and low-yielding wells (less than one gallon per minute) were located in all three geologic units in a variety of topographic settings. Well yields range from less than one gallon per minute to 250 gallons per minute. The variable total depths and wide ranges of casing depths of the high-yielding wells are indicative of variations in depths to water-bearing zones and regolith thicknesses, respectively. The depth of water-bearing zones is highly variable, even on a local scale. Analyses of ground-water samples indicate that the distribution of iron concentration is as variable as well yield in the study area and does not seem to be related to a particular rock type. Iron concentrations in ground-water samples ranged from 0.02 to 5.3 milligrams per liter. Both iron concentration and well yield vary substantially over a relatively small area. Implementation and Verification of a One-Dimensional, Unsteady-Flow Model for Spring Brook near Warrenville, Illinois By Mary J. Turner, Anthony P. Pulokas, and Audrey L. Ishii Abstract A one-dimensional, unsteady-flow model, Full EQuations (FEQ) model, based on de Saint-Venant equations for dynamic flow in open channels, was calibrated and verified for a 0.75-mile reach of Spring Brook, a tributary to the West Branch Du Page River, near Warrenville in northeastern Illinois. The model was used to simulate streamflow in a small urban stream reach with two short culverts, one with overbank flow around the culvert during high flows. Streamflow data were collected on the reach during three high-flow periods. Data from one period were used to calibrate the model, and data from the other two periods were used to verify the model. Stages and discharges over the periods were simulated, and the results were compared graphically with stage and discharge data collected at 10 sites in the study reach. Errors in simulated stage and discharge were small except when debris, not represented in the model, clogged the culvert. The effects of changes in physical and computational model parameters also were studied. The model was insensit'lve to replacement of measured cross sections with interpolated cross sections, especially if the measured thalweg elevation was preserved. Variation of the roughness, slope, and length of the culvert over-bank section, as well as the chosen representative measured cross section, caused only slight changes in the simulated peak stage and discharge. Changes in the modeled culvert area caused large differences in the simulated highflows in the vicinity of the culvert, whereas simulated low flows were unaffected. At all flows, the misrepresentation of the culvert area caused the simulated water-surface elevations to deviate from the measured elevations, especially on the falling

Georgia↗
USGS2007

Estimated infiltration, percolation, and recharge rates at the Rillito Creek focused recharge investigation site, Pima County, Arizona

A large fraction of ground water stored in the alluvial aquifers in the Southwest is recharged by water that percolates through ephemeral stream-channel deposits. The amount of water currently recharging many of these aquifers is insufficient to meet current and future demands. Improving the understanding of streambed infiltration and the subsequent redistribution of water within the unsaturated zone is fundamental to quantifying and forming an accurate description of streambed recharge. In addition, improved estimates of recharge from ephemeral-stream channels will reduce uncertainties in water-budget components used in current ground-water models. This chapter presents a summary of findings related to a focused recharge investigation along Rillito Creek in Tucson, Arizona. A variety of approaches used to estimate infiltration, percolation, and recharge fluxes are presented that provide a wide range of temporal- and spatial-scale measurements of recharge beneath Rillito Creek. The approaches discussed include analyses of (1) cores and cuttings for hydraulic and textural properties, (2) environmental tracers from the water extracted from the cores and cuttings, (3) seepage measurements made during sustained streamflow, (4) heat as a tracer and numerical simulations of the movement of heat through the streambed sediments, (5) water-content variations, (6) water-level responses to streamflow in piezometers within the stream channel, and (7) gravity changes in response to recharge events. Hydraulic properties of the materials underlying Rillito Creek were used to estimate long-term potential recharge rates. Seepage measurements and analyses of temperature and water content were used to estimate infiltration rates, and environmental tracers were used to estimate percolation rates through the thick unsaturated zone. The presence or lack of tritium in the water was used to determine whether or not water in the unsaturated zone infiltrated within the past 40 years. Analysis of water-level and temporal-gravity data were used to estimate recharge volumes. Data presented in this chapter were collected from 1999 though 2002. Precipitation and streamflow during this period were less than the long-term average; however, two periods of significant streamflow resulted in recharge—one in the summer of 1999 and the other in the fall/winter of 2000. Flux estimates of infiltration and recharge vary from less than 0.1 to 1.0 cubic meter per second per kilometer of streamflow. Recharge-flux estimates are larger than infiltration estimates. Larger recharge fluxes than infiltration fluxes are explained by the scale of measurements. Methods used to estimate recharge rates incorporate the largest volumetric and temporal scales and are likely to have fluxes from other nearby sources, such as unmeasured tributaries, whereas the methods used to estimate infiltration incorporate the smallest scales, reflecting infiltration rates at individual measurement sites.

Arizona↗