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At least 469 records · Page 26Linked to original sources
USGS1982

Dissolved solids and sodium in water from the High Plains aquifer in parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming

In 1978, the U.S. Geological Survey initiated a 5-year study of the High Plains regional aquifer to provide: (1) Hydrologic information needed to evaluate the effects of continued ground-water development; and (2) computer models to predict aquifer response to changes in ground-water development. The plan of study for the High Plains Regional Aquifer-System Analysis was described by Weeks (1978). A description of the High Plains aquifer and a map of the 1978 water table were presented by Gutentag and Weeks (1980). Maps of the bedrock geology, altitude of aquifer base, and saturated thickness of the High Plains aquifer were published by Weeks and Gutentag (1981). Water-level and saturated-thickness changes, from predevelopment to 1980, were mapped by Luckey, Gutentag, and Weeks (1981). This report describes the areal distribution of dissolved solids and sodium in the water of the High Plains aquifer. Data used in this study were provided by the U.S. Geological Survey and State agencies in each of the eight States in the High Plains. Their contribution is an integral part of this investigation.

Hydrologic Atlas↗
USGS1988

Ground-water flow and solute transport at a municipal landfill site on Long Island, New York — Part 2: Simulation of ground-water flow

Data on the hydrogeology of a 26-sq-mi area surrounding the Brookhaven landfill site in central Suffolk County were collected as part of a hydrologic investigation of solute transport from the site. These data were used to develop a steady-state groundwater flow model of the upper glacial (water table) aquifer in the area. The model accounts for the leakage through confining units underlying the aquifer, seepage to streams, recharge from precipitation, and pumpage and redistribution of water. Refined estimates of aquifer and confining-unit properties were obtained through model calibrations. Water table altitudes generated by the calibrated model were used to determine groundwater velocities and probable flow paths in the vicinity of the site under long-term average hydrologic conditions. Groundwater velocities and probable flow paths in the study area were calculated from simulated water table altitudes generated by the calibrated flow model. Groundwater at the center of the site flows southeastward at a velocity of 1.1 ft/d. The report is the second in a three part series describing the hydrologic conditions and groundwater quality, groundwater flow, and solute transport in the vicinity of the Brookhaven landfill.

New York↗
USGS2002

Three-dimensional hydrogeologic framework model for use with a steady-state numerical ground-water flow model of the Death Valley regional flow system, Nevada and California

The U.S. Geological Survey, in cooperation with the Department of Energy and other Federal, State, and local agencies, is evaluating the hydrogeologic characteristics of the Death Valley regional ground-water flow system. The ground-water flow system covers an area of about 100,000 square kilometers from latitude 35° to 38°15' North to longitude 115° to 118° West, with the flow system proper comprising about 45,000 square kilometers. The Death Valley regional ground-water flow system is one of the larger flow systems within the Southwestern United States and includes in its boundaries the Nevada Test Site, Yucca Mountain, and much of Death Valley. Part of this study includes the construction of a three-dimensional hydrogeologic framework model to serve as the foundation for the development of a steady-state regional ground-water flow model. The digital framework model provides a computer-based description of the geometry and composition of the hydrogeologic units that control regional flow. The framework model of the region was constructed by merging two previous framework models constructed for the Yucca Mountain Project and the Environmental Restoration Program Underground Test Area studies at the Nevada Test Site. The hydrologic characteristics of the region result from a currently arid climate and complex geology. Interbasinal regional ground-water flow occurs through a thick carbonate-rock sequence of Paleozoic age, a locally thick volcanic-rock sequence of Tertiary age, and basin-fill alluvium of Tertiary and Quaternary age. Throughout the system, deep and shallow ground-water flow may be controlled by extensive and pervasive regional and local faults and fractures. The framework model was constructed using data from several sources to define the geometry of the regional hydrogeologic units. These data sources include (1) a 1:250,000-scale hydrogeologic-map compilation of the region; (2) regional-scale geologic cross sections; (3) borehole information, and (4) gridded surfaces from a previous three-dimensional geologic model. In addition, digital elevation model data were used in conjunction with these data to define ground-surface altitudes. These data, properly oriented in three dimensions by using geographic information systems, were combined and gridded to produce the upper surfaces of the hydrogeologic units used in the flow model. The final geometry of the framework model is constructed as a volumetric model by incorporating the intersections of these gridded surfaces and by applying fault truncation rules to structural features from the geologic map and cross sections. The cells defining the geometry of the hydrogeologic framework model can be assigned several attributes such as lithology, hydrogeologic unit, thickness, and top and bottom altitudes.

California, Nevada↗
USGS2009

Hydrologic conditions and a firm-yield assessment for J.B. Converse Lake, Mobile County, Alabama, 1991-2006

J.B. Converse (Converse) Lake is the primary source of drinking water for the city of Mobile, Alabama. Concerns regarding the ability of the reservoir to meet current and future water demands during drought conditions have prompted this study. The 1991 through 2006 water years included a drought that occurred during 2000, and drought conditions currently (2007) are affecting the area. To assist officials of the Mobile Area Water and Sewer System in planning for future demands for drinking water in the Mobile metropolitan area, the firm yield for Converse Lake was estimated by the U.S. Geological Survey. The firm yield of Converse Lake was estimated using the Massachusetts Department of Environmental Protection’s firm-yield-estimator (FYE) model, which recently was refined by the U.S. Geological Survey. The model uses a mass-balance approach to determine the maximum average daily withdrawal rate that can be sustained during a period of record that includes a drought of record. If the reservoir is in contact with an aquifer, the FYE also includes routines that estimate the volume of ground-water and surface-water exchange between the aquifer and the reservoir. The average daily firm yield for Converse Lake was estimated to be 79 million gallons per day using the FYE routine that does not include ground-water exchange between the reservoir and the adjacent aquifer. Observed lake levels and withdrawals during the drought of 2000 indicate that more than 74 million gallons per day of water were withdrawn without complete depletion of reservoir storage. Therefore, it is likely that ground-water exchange with the reservoir may supplement available reservoir storage. If water exchange occurs between the aquifer and the reservoir, an increase in the volume of water available to the reservoir may occur during a drought. To quantify the potential ground-water contribution to reservoir storage, an analytical solution was applied to the FYE simulation of Converse Lake to estimate ground-water exchange between the reservoir and the aquifer. Aquifer properties required by the FYE were estimated by model calibration to observed water levels that occurred during the drought of 2000. When ground-water exchange between the reservoir and adjacent aquifer is included, the average daily firm yield increased to 83 million gallons per day. The estimate of 83 million gallons per day incorporates both total surface-water flow and ground-water exchange components. This analysis indicated that direct ground-water interaction contributes about 5 percent of the firm yield of Converse Lake. However, the average daily firm yield of 83 million gallons per day, based in part on calibrated values for aquifer transmissivity and storage, can be used only as a guideline until these aquifer properties can be defined better by field investigation in the Converse Lake watershed.

Alabama↗
USGS2004

Evaluation of ground-water contribution to streamflow in coastal Georgia and adjacent parts of Florida and South Carolina

Stream-aquifer relations in the coastal area of Georgia and adjacent parts of Florida and South Carolina were evaluated as part of the Coastal Georgia Sound Science Initiative, the Georgia Environmental Protection Division's strategy to protect the Upper Floridan aquifer from saltwater intrusion. Ground-water discharge to streams was estimated using three methods: hydrograph separation, drought-streamflow measurements, and linear-regression analysis of streamflow duration. Ground-water discharge during the drought years of 1954, 1981, and 2000 was analyzed for minimum ground-water contribution to streamflow. Hydrograph separation was used to estimate baseflow at eight streamflow gaging stations during the 31-year period 1971?2001. Six additional streamflow gaging stations were evaluated using linear-regression analysis of flow duration to determine mean annual baseflow. The study area centers on three major river systems ? the Salkehatchie?Savannah?Ogeechee, Altamaha?Satilla?St Marys, and Suwannee ? that interact with the underlying ground-water system to varying degrees, largely based on the degree of incision of the river into the aquifer and on the topography. Results presented in this report are being used to calibrate a regional ground-water flow model to evaluate ground-water flow and stream-aquifer relations of the Upper Floridan aquifer. Hydrograph separation indicated decreased baseflow to streams during drought periods as water levels declined in the aquifer. Average mean annual baseflow ranged from 39 to 74 percent of mean annual streamflow, with a mean contribution of 58 percent for the period 1971?2001. In a wet year (1997), baseflow composed from 33 to 70 percent of mean annual streamflow. Drought-streamflow analysis estimated baseflow contribution to streamflow ranged from 0 to 24 percent of mean annual streamflow. Linear-regression analysis of streamflow duration estimated the Q35 (flow that is equaled or exceeded 35 percent of the time) as the most reasonable estimate of baseflow. The Q35, when compared to mean annual streamflow, estimated a baseflow contribution ranging from 65 to 102 percent of streamflow. The Q35 estimate tends to overestimate baseflow as evidenced by the baseflow contribution greater than 100 percent. Ground-water contributions to streamflow are greatest during winter when evapotranspiration is low, and least during summer when evapotranspiration is high. Baseflow accounted for a larger percentage of streamflow at gaging stations in the Salkehatchie?Savannah?Ogeechee River Basin than in the other two basins. This difference is due largely to the availability of data, proximity to the Piedmont physiographic province where the major rivers originate and are by supplied ground water, and proximity to the upper Coastal Plain where there is greater topographic relief and interconnection between streams and aquifers.

Florida, Georgia, South Carolina↗
USGS1983

Ground-water-use trends in the Twin Cities metropolitan area, Minnesota, 1880-1980

Detailed ground-water-use information from 1880 to 1980 has been collected and analyzed for the Twin Cities metropolitan area. Interpretation of historic water-use data was required to help water-resource planners and managers assess future trends in water use. Data were also needed for three computer-simulation models of ground-water flow being developed in the Twin Cities area. Methods were developed to collect, evaluate, store, and retrieve information from all local, State, and Federal agencies1 records and publications. A computerized water-use data base was constructed to facilitate generation of statistics on water-use trends and to store and retrieve data for the flow models. Ground-water use was analyzed by use category and aquifer and several trends were observed. Eighty percent of ground water currently withdrawn is from wells in the Prairie du Chien-Jordan aquifer. Ground-water use increased from 1880 until the early 1970's, and then declined slightly in the late 1970's. Industrial use in particular declined during the 1970's as a result of conservation that was prompted by increased sewagetreatment and energy costs. The intensity of pumping has decreased within the St. Paul and Minneapolis city limits and increased outside the city limits. The seasonal variability of ground-water use became more pronounced as the percentage of water used for irrigation and air conditioning increased.

Minneosta↗
USGS1988

Geohydrology, water quality, and preliminary simulations of ground-water flow of the alluvial aquifer in the upper Black Squirrel Creek basin, El Paso County, Colorado

The upper Black Squirrel Creek basin in eastern El Paso County, Colorado, is underlain by an alluvial aquifer and four bedrock aquifers. The climate of the area is semiarid, and streamflow is irregular. The alluvial aquifer has supplied water to wells since the late 1800's when ranchers first pumped water from shallow wells to grow grass hay for livestock. Ground-water pumpage from the alluvial aquifer has increased since the mid-1950's, and water-level declines have been substantial; the bedrock aquifers virtually are undeveloped. Ground-water pumpage for domestic, stock, agricultural, and municipal uses has exceeded recharge for the past 25 years. The present extent of the effect of pumpage on the alluvial aquifer was evaluated, and a ground-water flow model was used to simulate the future effect of continued pumpage on the aquifer. Measured water-level declines from 1974 through 1984 were as much as 30 feet in an area north of Ellicott, Colorado. On the basis of the simulations, water-level declines from October 1984 to April 1999 north of Ellicott might be as much as 20 to 30 feet and as much as 1 to 10 feet in most of the aquifer. Flow from the bedrock aquifers to the alluvial aquifer may account for a substantial volume of the recharge to the alluvial aquifer. The ground-water flow models provided a means of evaluating the importance of ground-water evapotranspiration at various stages of aquifer development. Simulated ground-water evapotranspiration decreased from 1949 to 1984; prior to 1950 ground-water evapotranspiration was about 43.5 percent of the total outflow, but in 1984 it was less than 3 percent. Thirty-six ground-water samples were collected during 1984. Chemical analyses indicated that concentrations of dissolved nitrite plus nitrate as nitrogen generally were large. Samples from 5 of the 36 wells had concentrations of dissolved nitrite plus nitrate as nitrogen that exceeded drinking-water standards; these concentrations could pose a health threat toinfants. Water from the alluvial aquifer generally is of suitable quality forother uses.

Colorado↗
USGS2007

Areas Contributing Recharge to Wells in the Tafuna-Leone Plain, Tutuila, American Samoa

To address the concerns about the potential for contamination of drinking-water wells in the Tafuna-Leone Plain, Tutuila, American Samoa, a numerical ground-water flow model was developed and used to delineate areas contributing recharge to the wells (ACRWs). Surveys and analyses were conducted to obtain or compile certain essential hydrogeologic information needed for the model, such as groundwater production statistics, ground-water levels under current production, and an assessment of the distribution of groundwater recharge. The ground-water surveys indicate that total production from all wells in the Tafuna-Leone Plain between 1985 and 2005 averaged 6.1 Mgal/d and showed a gradual increase. A synoptic survey indicates that current water levels in the Tafuna-Leone Plain are highest near its inland boundary, decrease toward the coast, and are slightly depressed in high-production well fields. Ground-water levels showed little effect from the increased production because hydraulic conductivites are high and withdrawal is small relative to recharge. Analysis of ground-water recharge using a soil water-budget analysis indicates that the Tafuna-Leone Plain and adjacent areas receive about 280 Mgal/d of water from rainfall, of which 24 percent runs off to the ocean, 26 percent is removed by evapotranspiration, and 50 percent goes to ground-water recharge. Ground-water recharge per unit area is generally higher at the mountain crests than at the coast, but the highest recharge per unit area is in the mountain-front recharge zone at the juncture between the Tafuna-Leone Plain and the adjacent mountains. Surface water from the mountains also contributes to ground-water recharge in the eastern Tafuna-Leone Plain, in a process analogous to mountain-front recharge described in arid areas. Analysis of stream-gage data indicates that in the mountains of Tutuila, ground water discharges and contributes substantially to the total flow of the streams. In contrast, multiple lines of evidence indicate that in the eastern Tafuna-Leone Plain, surface water recharges the highly permeable underlying aquifer. Steady-state model simulations representing current ground-water production conditions in the Tafuna-Leone Plain indicate that most ACRWs extend less than a mile from the production wells; thus, travel distance between any point within an ACRW and its well is short. A simulation representing a condition in which all wells are operating at maximum capacity resulted in larger ACRWs, which demonstrates that increasing ground-water withdrawal from existing wells, or building and developing new wells, increases the surface area that could potentially contribute contaminants. In some places, such as in Malaeimi Valley, water can travel quickly via surface-water routes to an area where the water can infiltrate within the ACRWs of a well field.

Scientific Investigations Report↗
USGS2000

Probability of detecting atrazine/desethyl-atrazine and elevated concentrations of nitrate plus nitrate as nitrogen in ground water in the Idaho part of the western Snake River Plain

As ground water continues to provide an ever-growing proportion of Idaho?s drinking water, concerns about the quality of that resource are increasing. Pesticides (most commonly, atrazine/desethyl-atrazine, hereafter referred to as atrazine) and nitrite plus nitrate as nitrogen (hereafter referred to as nitrate) have been detected in many aquifers in the State. To provide a sound hydrogeologic basis for atrazine and nitrate management in southern Idaho—the largest region of land and water use in the State—the U.S. Geological Survey produced maps showing the probability of detecting these contaminants in ground water in the upper Snake River Basin (published in a 1998 report) and the western Snake River Plain (published in this report). The atrazine probability map for the western Snake River Plain was constructed by overlaying ground-water quality data with hydrogeologic and anthropogenic data in a geographic information system (GIS). A data set was produced in which each well had corresponding information on land use, geology, precipitation, soil characteristics, regional depth to ground water, well depth, water level, and atrazine use. These data were analyzed by logistic regression using a statistical software package. Several preliminary multivariate models were developed and those that best predicted the detection of atrazine were selected. The multivariate models then were entered into a GIS and the probability maps were produced. Land use, precipitation, soil hydrologic group, and well depth were significantly correlated with atrazine detections in the western Snake River Plain. These variables also were important in the 1998 probability study of the upper Snake River Basin. The effectiveness of the probability models for atrazine might be improved if more detailed data were available for atrazine application. A preliminary atrazine probability map for the entire Snake River Plain in Idaho, based on a data set representing that region, also was produced. In areas where this map overlaps the 1998 map of the upper Snake River Basin, the two maps show broadly similar probabilities of detecting atrazine. Logistic regression also was used to develop a preliminary statistical model that predicts the probability of detecting elevated nitrate in the western Snake River Plain. A nitrate probability map was produced from this model. Results showed that elevated nitrate concentrations were correlated with land use, soil organic content, well depth, and water level. Detailed information on nitrate input, specifically fertilizer application, might have improved the effectiveness of this model.

Idaho↗
USGS1998

Simulation of ground-water flow, Dayton area, southwestern Ohio

A numerical model was used simulate the regional ground-water-flow system in the Dayton area in southwestern Ohio. Ground water is the primary source of drinking water for the Dayton area. The aquifer consists of glacial sands and gravels in a buried bedrock valley. The shale bed rock in the area is poorly permeable, but the glacial deposits can yield up to 2,000 gallons per minute to wells. Interaction with surface water is an important component of the ground-water-flow system. A steady-state, three dimensional, three-layer MODFLOW model of the glacial deposits was constructed to simulate the ground-water-flow system. The modeled area encompasses about 241 mi2 in Montgomery, Greene, and Clark Counties. The model simulated steady-state conditions of September 1993 and included 187 pumped wells. Hydraulic conductivities in the model ranged from less than 1 foot per day to 450 feet per day. Simulated recharge rates ranged from 6 inches per year to 12.2 inches per year. Recharge was used in select areas to simulate inflow from the bed rock-valley walls. Measured water levels from 579 wells and streamflow gain-loss data from six river reaches were used to evaluate the model. Ninety-one percent of simulated heads were within 15 feet of the measured heads. The root-mean-square error and mean absolute difference between measured and simulated heads were 7.3 feet and 4.5 feet respectively for layer 1, 10.1 feet and 6.5 feet for layer 2, and 8.8 feet and 6.8 feet for layer 3. Recharge and river leakage accounts for 81 percent of the water entering the model; pumped wells and river leakage accounts for almost 91 percent of the ground water leaving the model. Interaction of the ground-water system and the major rivers, which include the Great Miami, Mad, Stillwater, and Little Miami Rivers, is known from previous investigations in the area; however, the model simulation indicates that the smaller streams also may have a significant local influence. The vertical hydraulic conductivity of the glacial deposits appears to have more effect on ground-water flow in some areas near the bed rock-valley walls than in the central areas of the valley. At a local scale, simulated heads in the central areas of the valley were generally insensitive to changes in aquifer parameters. The sensitivity of the model to changes in simulated hydraulic properties of the aquifer was assessed by systematically changing model parameters in four subareas of the model. All areas of the model were sensitive to changes in recharge. Changes in other parameters, such as hydraulic conductivity or riverbed conductance, had variable effects. The sensitivity of the model can be used to indicate the types of additional hydrogeologic data that would be most useful to future investigations.

Water-Resources Investigations Report↗
USGS2009

Assessment of Ground-Water Resources in the Seacoast Region of New Hampshire

Numerical ground-water-flow models were developed for a 160-square-mile area of coastal New Hampshire to provide insight into the recharge, discharge, and availability of ground water. Population growth and increasing water use prompted concern for the sustainability of the region's ground-water resources. Previously, the regional hydraulic characteristics of the fractured bedrock aquifer in the Seacoast region of New Hampshire were not well known. In the current study, the ground-water-flow system was assessed by using two different models developed and calibrated under steady-state seasonal low-flow and transient monthly conditions to ground-water heads and base-flow discharges. The models were, (1) a steady-state model representing current (2003-04) seasonal low-flow conditions used to simulate current and future projected water use during low-flow conditions; and (2) a transient model representing current average and estimated future monthly conditions over a 2-year period used to simulate current and future projected climate-change conditions. The analysis by the ground-water-flow models indicates that the Seacoast aquifer system is a transient flow system with seasonal variations in ground-water flow. A pseudosteady- state condition exists in the fall when the steady-state model was calibrated. The average annual recharge during the period analyzed, 2000-04, was approximately 51 percent of the annual precipitation. The average net monthly recharge rate between 2003 and 2004 varied from 5.5 inches per month in March, to zero in July, and to about 0.3 inches per month in August and September. Recharge normally increases to about 2 inches per month in late fall and early winter (November through December) and declines to about 1.5 inches per month in late winter (January and February). About 50 percent of the annual recharge coincides with snowmelt in the spring (March and April), and 20 percent occurs in the late fall and early winter (November through February). Net recharge, calculated as infiltration of precipitation minus evapotranspiration, can be negative during summer months (particularly July). Regional bulk hydraulic conductivities of the bedrock aquifer were estimated to be about 0.1 to 1.0 feet per day. Estimated hydraulic conductivities in model areas representing the Rye Complex and the Kittery Formation were higher (0.5 to 1 foot per day) than in areas representing the Eliot Formation, the Exeter Diorite, and the Newburyport Complex, which have estimated hydraulic conductivities of 0.1 to 0.2 foot per day. A northeast-southwest regional anisotropy of about 5:1 was estimated in some areas of the model; this pattern is parallel to the regional structural trend and predominant fracture orientation. In areas of the model with more observation data, the upper and lower 95-percent confidence intervals for the estimated bedrock hydraulic conductivity were about half an order of magnitude above and below the parameter, respectively, and the estimated confidence intervals for estimated specific storage were within an order of magnitude of the parameter. In areas of the model with few data points, or few stresses, confidence intervals were several orders of magnitude. Estimated model parameters and their confidence intervals are a function of the conceptual model design, observation data, and the weights placed on the data. The amount of recharge that enters the bedrock aquifer at a specific point depends on (1) the location of the point in the flow field; (2) the hydraulic conductivity of the bedrock (or the connectivity of fractures); and (3) the stresses within the bedrock aquifer. In addition, ground water stored in unconsolidated overburden sediments, including till and other fine-grained sediments, may constitute a large percentage of the water available from storage to the bedrock aquifer. Recharge into the bedrock aquifer at a point can range from zero to nearly all the recharge at the surface dependin

Scientific Investigations Report↗
USGS1998

Surface-water/ground-water relations in the Lemhi River Basin, east-central Idaho

This report summarizes work carried out in cooperation with the Bureau of Reclamation to provide hydrologic information to help Federal, State, and local agencies meet the goals of the Lemhi River Model Watershed Project. The primary goal of the project is to maintain, enhance, and restore anadromous and resident fish habitat in the Lemhi River, while maintaining a balance between resource protection and established water uses. The main objectives of the study were to carry out seepage measurements to determine seasonal distributed gains and losses in the Lemhi River and to estimate annual ground-water underflow from the basin to the Salmon River. In 1997, seepage measurements were made during and after the irrigation season along a 60-mile reach of the Lemhi River between Leadore and Salmon. Except for one 4-mile reach that lost 1.3 cubic feet per second per mile, the river gained from ground water in early August when ground-water levels were high. Highest flows in the Lemhi River in early August were about 400 cubic feet per second. In October, when ground-water levels were low, river losses to ground water were about 1 to 16 cubic feet per second per mile. In October, highest flows in the Lemhi River were about 500 cubic feet per second, near the river's mouth. Annual ground-water underflow from the Lemhi River Basin to the Salmon River was estimated by using a simplified water budget and by using Darcy's equation. The water-budget method contained large uncertainties associated with estimating precipitation and evapotranspiration. Results of both methods indicate that the quantity of ground water leaving the basin as underflow is small, probably less than 2 percent of the basin's total annual water yield.

Idaho↗
USGS2009

Estimating Locations of Perennial Streams in Idaho Using a Generalized Least-Squares Regression Model of 7-Day, 2-Year Low Flows

Many State and Federal agencies use information regarding the locations of streams having intermittent or perennial flow when making management and regulatory decisions. For example, the application of some Idaho water quality standards depends on whether streams are intermittent. Idaho Administrative Code defines an intermittent stream as one having a 7-day, 2-year low flow (7Q2) less than 0.1 ft3/s. However, there is a general recognition that the cartographic representation of perennial/intermittent status of streams on U.S. Geological Survey (USGS) topographic maps is not as accurate or consistent as desirable from one map to another, which makes broad management and regulatory assessments difficult and inconsistent. To help resolve this problem, the USGS has developed a methodology for predicting the locations of perennial streams based on regional generalized least-squares (GLS) regression equations for Idaho streams for the 7Q2 low-flow statistic. Using these regression equations, the 7Q2 streamflow may be estimated for naturally flowing streams in most areas in Idaho. The use of these equations in conjunction with a geographic information system (GIS) technique known as weighted flow accumulation allows for an automated and continuous estimation of 7Q2 streamflow at all points along stream reaches. The USGS has developed a GIS-based map of the locations of streams in Idaho with perennial flow based on a 7Q2 of 0.1 ft3/s and a transition zone of plus or minus 1 standard error. Idaho State cooperators plan to use this information to make regulatory and water-quality management decisions. Originally, 7Q2 equations were developed for eight regions of similar hydrologic characteristics in the study area, using long-term data from 234 streamflow-gaging stations. Equations in five of the regions were revised based on spatial patterns observed in the initial perennial streams map and unrealistic behavior of the equations in extrapolation. The standard errors of prediction for the final equations ranged from a minimum of +75.0 to -42.9 percent in the central part of the study area to a maximum of +277 to -73.5 percent in the southern part of the study area. The equations are applicable only to unregulated, naturally-flowing streams and may produce unreliable results outside the range of explanatory variables used for equation development. Extrapolation outside the range of available data was necessary, however, to predict perennial flow initiation points and transition zones along stream reaches. The map of perennial streams was evaluated by comparing predicted stream classifications with four independent datasets, including field observations by other government agencies. Overall, 81 percent of the comparison data points agreed with the USGS perennial streams model. Regions with the highest number of disagreements had a high percentage of mountainous and forested area with potential mountain front recharge zones, and regions with the highest agreements had a high percentage of low gradient, low elevation area. As a whole, the USGS model predicted a higher number of perennial streams than predictions made with the independent datasets. Some disagreements were due to poor site location coordinates, timing of the comparison site visits during unusually wet or dry years, discrepancies in classification criteria, and variable ground water contributions to flow in some areas. The Idaho Department of Environmental Quality Beneficial Use Reconnaissance Program (BURP) dataset is considered the most representative dataset for comparison because it covered a range of climate conditions and the number of sites visited were consistent from year to year during the study period. Eighty-five percent of BURP comparison data points agreed with the USGS perennial streams model. Although site-specific flow data may be needed to correctly classify streams in some areas, this information rarely is available and is not always practical to o

Scientific Investigations Report↗
USGS1994

An updated numerical simulation of the ground-water flow system for the Castle Lake debris dam, Mount St. Helens, Washington, and implications for dam stability against heave

A numerical simulation of the ground-water flow system in the Castle Lake debris dam, calibrated to data from the 1991 and 1992 water years, was used to estimate factors of safety against heave and internal erosion. The Castle Lake debris dam, 5 miles northwest of the summit of Mount St. Helens, impounds 19,000 acre-ft of water that could pose a flood hazard in the event of a lake breakout. A new topographic map of the Castle Lake area prior to the 1980 eruption of Mount St. Helens was prepared and used to calculate the thickness of the debris avalanche deposits that compose the dam. Water levels in 22 piezometers and discharges from seeps on the dam face measured several times per year beginning in 1990 supplemented measurements in 11 piezometers and less frequent seep discharge measurements made since 1983. Observations in one group of piezometers reveal heads above the land surface and head gradients favoring upward flow that correspond to factors of safety only slightly greater than 2. The steady-state ground-water flow system in the debris dam was simulated using a threedimensional finite difference computer program. A uniform, isotropic model having the same shape as the dam and a hydraulic conductivity of 1.55 ft/day simulates the correct water level at half the observation points, but is in error by 10 ft or more at other points. Spatial variations of hydraulic conductivity were required to calibrate the model. The model analysis suggests that ground water flows in both directions between the debris dam and Castle Lake. Factors of safety against heave and internal erosion were calculated where the model simulated upward flow of ground water. A critical gradient analysis yields factors of safety as low as 2 near the piezometers where water level observations indicate low factors of safety. Low safety factors are also computed near Castle Creek where slumping was caused by a storm in January, 1990. If hydraulic property contrasts are present in areas of the debris dam unsampled by piezometers, then low safety factors may exist that are not evident in the numerical model analysis. Numerical model simulations showed that lowering Castle Lake by 40 feet increases many factors of safety by 0.1, but increases greater than 1 are limited to the area of 1990 slumping.

Washington↗
USGS1989

Effect of urban runoff on the quality of lakes in Eagan, Minnesota

Sixteen lakes in the city of Eagan, Minnesota, were sampled during 1982-83 to detect water-quality changes that might have occurred because of urbanization since a previous study conducted during 1972-78. Each of the lakes was sampled five times to determine pH, specific conductance, dissolved oxygen, water temperature, transparency, and concentration of dissolved chloride. Three determinations of chlorophyll were made for each of the lakes near the end of the study, and additional determinations were made for a few lakes of particular interest. Most of the lakes have been incorporated into the city's storm-runoff system for use as retention basins during large storms. The chemistry of the lakes appears to be degraded by urban runoff. Chloride concentrations were significantly higher in six lakes than during the previous study, and remained elevated (about 30 milligrams per liter) in four other lakes. The association between increased chloride and specific conductance with runoff from urban areas suggests that the lakes are subject to contamination by chloride commonly present in urban runoff, and chloride concentrations harmful to aquatic life may have been associated with high specific conductance measured in one of the lakes. Analysis of the data collected for this study indicate that the chemistry of the lakes changes, adjusting to a variety of influences including (1) alternate loading and flushing by runoff, (2) excessive average-annual precipitation, (3) changing ground-water and lake interactions, and (4) changing land use in the lake watersheds. Some lakes affected by urban runoff had reduced concentrations of total phosphorus; however, other lakes unaffected by urban runoff also had significantly lower phosphorus, suggesting that increased precipitation may have diluted the lake water. Ten phosphorus-loading models tested or developed during the previous study generally were found to be inadequate for describing the results of this study. The trophic status of 12 lakes improved but declined in the other 4 lakes, and productivity increased 38 percent in what had been the least-eutrophic lake.

Minnesota↗
USGS1992

Geohydrology of, and simulation of ground-water flow in, the Milford-Souhegan glacial-drift aquifer, Milford, New Hampshire

Hydrogeologic data collected since 1990 were assessed and a ground-water-flow model was refined in this study of the Milford-Souhegan glacial-drift aquifer in Milford, New Hampshire. The hydrogeologic data collected were used to refine estimates of hydraulic conductivity and saturated thickness of the aquifer, which were previously calculated during 1988-90. In October 1990, water levels were measured at 124 wells and piezometers, and at 45 stream-seepage sites on the main stem of the Souhegan River, and on small tributary streams overlying the aquifer to improve an understanding of ground-water-flow patterns and stream-seepage gains and losses. Refinement of the ground-water-flow model included a reduction in the number of active cells in layer 2 in the central part of the aquifer, a revision of simulated hydraulic conductivity in model layers 2 and representing the aquifer, incorporation of a new block-centered finite-difference ground-water-flow model, and incorporation of a new solution algorithm and solver (a preconditioned conjugate-gradient algorithm). Refinements to the model resulted in decreases in the difference between calculated and measured heads at 22 wells. The distribution of gains and losses of stream seepage calculated in simulation with the refined model is similar to that calculated in the previous model simulation. The contributing area to the Savage well, under average pumping conditions, decreased by 0.021 square miles from the area calculated in the previous model simulation. The small difference in the contrib- uting recharge area indicates that the additional data did not enhance model simulation and that the conceptual framework for the previous model is accurate.

Water-Resources Investigations Report↗
USGS1984

Water-table contours, directions of ground-water movement, and measurements of inflow to American Falls Reservoir, southeastern Idaho, April 1984

In 1978 the U.S. Geological Survey began a 5-year study of the High Plains regional aquifer system to provide hydrologic information for evaluating the effects of long-term development of the aquifer and to develop a capability for predicting aquifer response to alternative changes in ground-water management. By use of a digital model, this report presents a quantitative description of the High Plains aquifer in Oklahoma. The High Plains aquifer consists predominantly of the Tertiary Ogallala Formation and overlying Quaternary alluvium and terrace deposits which are hydraulically connected to the High Plains aquifer. Much of the aquifer is underlain by formations of Permian through Cretaceous age, which generally have very small hydraulic conductivities. In some areas parts of underlying Triassic, Jurassic, or Cretaceous rocks are hydraulically connected with the aquifer. The High Plains aquifer is a water-table aquifer in which water moves generally to the east-southeast. Before the beginning of extensive irrigation of the 1960's, the aquifer was essentially in dynamic equilibrium with recharge from precipitation balanced by natural discharge from the aquifer. Ground-water discharge appeared in streams leaving the area or was returned to the atmosphere through evapotranspiration. Accurate records of irrigation pumpage are not available from the High Plains. In order to estimate irrigation pumpage, published records of crop distribution were used and a consumptive use was assigned to each principal irrigated crop. This method gave an estimated irrigation demand. Pumpage was taken as a percentage of the total irrigation demand. Irrigation has decreased ground-water discharge from the High Plains aquifer. Ground-water discharge was estimated as approximately 118 cubic feet per second in 1980. A finite-difference digital model was used to simulate flow in the High Plains aquifer. The recharge was adjusted so that 1980 ground-water discharge was 118 cubic feet per second, the estimated ground-water discharge for 1980. Recharge in the eastern half of the modeled area was 0.45 inch per year; one-half this value was used in the western half of the modeled area. Hydraulic conductivity was divided into three zones: 19.3 feet per day in the eastern zone; 16.2 feet per day in the central zone; and 8.28 feet per day in the western zone. A specific yield of 14.7 percent was used in the model. Using all these parameters, the model was calibrated so that the mean difference between predevelopment modeled and measured head was -0.044 foot. Following the calibration procedure, the model was used to predict the volumes of water in storage and distribution of saturated thickness in 1993 and 2020 using the 1980 pumping rates. The calculated quantity of water in storage in the aquifer in 1941 (predevelopment) was approximately 135.2 million acre-feet; in 1980, approximately 121.9 million acre-feet; in 1993 approximately 112.7 million acre-feet; and in 2020, approximately 96.2 million acre-feet. The High Plains aquifer in Oklahoma will continue to be an important source of water past the year 2000. As withdrawals continue from the aquifer at the present rate, the water table will continue to decline and when the water table drops below the streambed in any part of the area, ground-water discharge to streams will cease in that area. Based on the calculated volumes of water in storage, the volume of water remaining in storage as compared to the predevelopment volume is as follows: 90 percent in 1980, 83 percent in 1993, and 71 percent in 2020.

Idaho↗
USGS1981

Hydrogeology of the Buffalo aquifer, Clay and Wilkin Counties, West-Central Minnesota

The Buffalo aquifer is the principal source of ground-water supplies in the Moorhead, Minnesota area. The aquifer is an elongate deposit of sand and gravel, which locally contains water under confined conditions. Although the Buffalo aquifer contains about 270 billion gallons of water in storage, only 120 billion gallons could be withdrawn. Largest well yields occur along the deep trough in the center of the aquifer. Induced streambed infiltration may be possible in certain areas where the stream overlies the aquifer and where the intervening lake sediments are thin or absent. A numerical model constructed for aquifer evaluation has shown that a considerable amount of ground water is discharged through the confining bed to the stream or leaves the area as underflow to the west. Water from the Buffalo aquifer generally is very hard and of the calcium bicarbonate type. The average discharge of the Buffalo River for the base period 1946-78 ranges from 0.229 cubic foot per second per square mile near Hawley to 0.108 cubic foot per second per square mile at Sabin. Surface water in the Buffalo River drainage system is dominantly a calcium bicarbonate type similar to ground water of the area, especially at low flow in the upper reaches of the tributaries.

Minnesota↗