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USGS2003

Nutrients in Shallow Ground Waters Beneath Relatively Undeveloped Areas in the Conterminous United States

Nutrient concentrations in shallow (well depth of 30 meters or less) ground waters of relatively undeveloped areas were evaluated to determine background conditions relative to agricultural and urban land uses. Lands comprising 67 percent or greater forest or range, 10 percent or less agricultural land, and 10 percent or less urban land were used to represent relatively undeveloped areas. Data subsets from the U.S. Geological Survey's National Water-Quality Assessment Program (81 wells) and retrospective studies (320 wells) yielded 75th percentile nitrate concentrations of 0.51 and 1.1 milligrams per liter, respectively, in shallow ground water beneath relatively undeveloped areas. The value of 1.1 milligrams per liter is a reasonable upper bound estimate of relative background concentration of nitrate in shallow ground waters in the United States and incorporates effects of nominal nitrogen load to susceptible aquifers. Relative background concentration of nitrate is variable and depends in part on land use, rock type, and climate. Median nitrate concentration was significantly greater in ground water beneath rangeland (1.20 milligrams per liter) than beneath forest land (0.06 milligram per liter). Median nitrate concentration in ground water beneath rangeland was 1.4-2.7 milligrams per liter in susceptible aquifers, which consist of coarse-textured deposits or fractured rock. Increased relative background concentration of nitrate in rangeland areas likely results from evaporative concentration of nominal nitrogen load associated with natural organic and inorganic sources in hydrogeologically susceptible settings.

Water-Resources Investigations Report↗
USGS2001

Ground-water age dating in community wells in Oswego County, New York

Officials in Oswego County, in north-central New York, have been concerned about potential contamination of community wells. Many of these wells are completed in unconfined glacial sand-and-gravel aquifers, although some are finished in till or in the underlying fractured and jointed bedrock of Late Ordovician and Early Silurian ages. Local shallow ground-water flow is affected by the orientation and hydraulic characteristics of the local topography and surficial sediments, whereas deeper regional flow is toward Lake Ontario. Concentrations of chlorofluorocarbons and tritium in water samples from 28 wells in the county were measured in 1999 for ground-water-age dating; results yield recharge dates ranging from about 1955 to 1994. The presence of water older than about 15 years in the sand-and-gravel aquifers differs from previous concepts of recharge sources and ground-water movement that were based on numerical modeling of ground-water flow. Young ground water (1 to 5 years old) probably represents recharge from recent precipitation and seepage from streams, whereas the oldest ground water (more than 40 years old) probably is derived from the fractured bedrock that underlies the glacial sediments or has moved along long flow paths in unconsolidated deposits, or through poorly permeable material. Some sand-and-gravel aquifers in Oswego County contain mixtures of old and young water. Wellhead-protection efforts need to focus on protection of the quality of young water in the sand-and-gravel aquifers because young water is more likely to be contaminated than old water.

Open-File Report↗
USGS1956

Ground-water resources of the Ainsworth unit, Cherry and Brown Counties, Nebraska

The Ainsworth unit, so named by the U. S. Bureau of Reclamation, is in north-central Nebraska and is in the drainage basin of the Niobrara River. It is an area of about 1,000 square miles in the east-central part of Cherry County and northern part of Brown County. The east-west length of the area is about 60 miles and the width ranges from 9 to 21 miles. About 80 percent of the area consists of grass-covered sandhills; the remainder is the Ainsworth tableland, which is flat to gently rolling farmland between Plum and Long Pine Creeks in the eastern part of the area. The average annual precipitation is about 23 inches. Although most of the C).ops are raised by dry-farming methods, some farmland is irrigated with water pumped from wells. The U. S. Bureau of Reclamation has proposed to irrigate much of the Ainsworth tableland with surface water to be stored in a reservoir on the Snake River at the west border of the Ainsworth unit. The rocks exposed in the Ainsworth unit range in age from Tertiary (Pliocene) to Quaternary (Recent). The Ogallala formation of Pliocene age is exposed along the lower part of the Snake River valley and underlies the entire Ainsworth unit. It is composed of silt, sand, and gravel, and contains layers of sandstone and conglomerate, much of which is cross bedded and cemented with lime; coarser sediments generally are more prominent in the lower part. Overlying the Ogallala formation are deposits of Pleistocene age consisting in part of layers of saturated sand and gravel which are the most important sources of ground water in the Ainsworth unit. Throughout most of the area the ground water is under watertable conditions, but locally it is confined by lenses of clay or silty clay. Some wells tap only the sand and gravel of Pleistocene age, some tap both the deposits of Pleistocene age and the underlying Ogallala formation, and some tap only the Ogallala formation; no wells are known to extend into rocks older than the Ogallala. Dune sand mantles the deposits of Pleistocene age in about 80 percent of the Ainsworth unit and a thin deposit of loess covers the surface elsewhere. Terrace deposits border the flood plain of the principal streams, and alluvium underlies the flood plain of most of the stream valleys in the area. Precipitation and underflow from the southwest are the principal sources of the ground water in the Ainsworth unit. As most of the precipitation in the sandhills evaporates, is utilized by growing plants, or penetrates to the zone of saturation, the overland runoff from this part of the area is small. In the vicinity of Ainsworth a minor amount of recharge probably is derived from the return of irrigation water pumped from wells. Where the water table is near the surface in the valleys of the sandhills, ground water is discharged directly from the zone of saturation to the atmosphere by evapotranspiration; and, as the surface of the lakes in the sandhills area is an extension of the water table, evaporation from the lake surface also constitutes ground-water discharge. In addition, ground water is discharged by the streams that are incised below the water table and by subsurface outflow. The yield of wells accounts for only a small part of the discharge of ground water from the area. In the Ainsworth unit the water table slopes northeastward from the region of favorable recharge, the sandhills, toward the Niobrara River and its principal tributaries. The average gradient of the water table is about 10 feet per mile. In the sandhills the water table is at or near the surface in the valleys and as much as 100 feet, or a little more, beneath the higher sandhills. In the vicinity of Ainsworth the water level in wells ranges from less than 1 foot to about 40 feet below the land surface, but nearer the Niobrara River and close to its deeply entrenched tributaries the depth to the water table is as much as, or a little more than, 200 feet.

Brown County, Cherry County↗
USGS2001

Ground-water flow to Death Valley, as inferred from the chemistry and geohydrology of selected springs in Death Valley National Park, California and Nevada

Death Valley lies downgradient from adjacent valleys to the north, south, east, and west in California and Nevada, and is the site of substantial ground-water discharge. The sources of the discharging waters have been discussed by several investigators in the past and are of heightened concern because of the potential disposal of high-level radioactive waste at Yucca Mountain, Nevada, and because of ground-water withdrawals attendant to commercial mining in the northwestern Amargosa Valley region. This report describes high- and low-discharge springs in and along the Amargosa Range that were sampled to augment the level of understanding of the extent and distribution of westward ground-water flow through the range. The Black Mountains do not seem to be part of a significant path of ground-water flow from the Amargosa region. This is attributed to the complex lithology and geologic history of the Black Mountains structural block and to the presence of the intervening Furnace Creek fault zone. The only ground-water discharge associated with the Black Mountains where water chemistry reflects an external source or sources is Saratoga Spring, for which δ 2 H and δ 18 O data indicate recharge in the Spring Mountains to the east. The southern part of the Funeral Mountains transmits a large volume of water through faulted and fractured rocks of Cambrian age that lie at or along the distal part of the northeast -oriented Spotted Range-Mine Mountain structural zone. Waters discharging from springs in the Furnace Creek Ranch vicinity (Travertine and Nevares) both compositionally and isotopically resemble waters from the Ash Meadows spring group in the Amargosa Desert. The Ash Meadows springs and water in the Amargosa Valley alluvium likely are chemically representative of ground water entering the southern Funeral Mountains. Much less ground water flows through the central and northern Funeral Mountains than flows through the southern part, as indicated by the geologic setting and chemistry of Keane Wonder Spring. The northern one-half of the mountains comprises early-to-middle Proterozoic metamorphic rocks that are the core of the Funeral Mountains anticlinorium. The core is largely unfaulted, plunges to the northeast and southwest, and is truncated to some extent on the east by the shallow-dipping Boundary Canyon fault. This structural setting and the paucity of springs in the northern one-half of the Funeral Mountains indicate a long traveltime from the Amargosa region to the western margin of the northern and central parts of the mountains. The Grapevine Mountains include the highest elevations in the Amargosa Range. Substantial precipitation and recharge above about 2,000 meters are evinced by numerous small springs and seeps along the east and west margins. The local nature of the recharge is reflected in δ 2 H and δ 18 O values and in the spring chemistries that indicate control by Tertiary volcanic rocks. The highest spring discharges associated with the Grapevine Mountains are near the north end of the mountains in the Grapevine Ranch area. The springs in this area are similar chemically and isotopically, except for one or two order-of-magnitude differences in calcium, magnesium, and strontium concentrations and a 1.2 per mil difference in δ 13 C values. These differences can be attributed to differences in the distal parts of the respective flow paths. The springs also lie at the end of a northeast -oriented structural zone in the Walker Lane Belt, and their δ 2 H, δ 13 C, and δ 18 O values indicate a recharge area likely to the northeast, outside of the Grapevine Mountains.

California;Nevada↗
USGS1999

Distribution of salinity in ground water from the interpretation of borehole-geophysical logs and salinity data, Calf Pasture Point, Davisville, Rhode Island

The distribution of salinity in ground water at Calf Pasture Point, a small coastal peninsula bounded by Narragansett Bay on the east and Allen Harbor on the west, in Davisville, Rhode Island, was interpreted from borehole-geophysical data and previously collected salinity data to help identify potential flowpaths of contaminated ground water to surface-water bodies. The surficial material at this 40-acre site, which ranges in thickness from about 30 to 85 feet, is composed of an upper sand unit, a silt unit, and a till unit overlying bedrock. Borehole-geophysical data indicate that fresh ground water is present in all surficial units in the northern and northwestern part of the site. In the central and eastern parts of the site, where most of the current land surface is composed of dredged fill placed in a small saltwater embayment, brackish and saline ground water predominate. Fresh ground water moving into this area from upgradient and recharge to this extended land surface from precipitation is diluting the saline groundwater in the upper sand and till units, and to a lesser extent in the silt unit. In this area, the freshwater-flow system is slowly expanding towards Narragansett Bay and the entrance channel to Allen Harbor.

Rhode Island↗
USGS2005

MODFLOW/MT3DMS-based simulation of variable-density ground water flow and transport

This paper presents an approach for coupling MODFLOW and MT3DMS for the simulation of variable-density ground water flow. MODFLOW routines were modified to solve a variable-density form of the ground water flow equation in which the density terms are calculated using an equation of state and the simulated MT3DMS solute concentrations. Changes to the MODFLOW and MT3DMS input files were kept to a minimum, and thus existing data files and data files created with most pre- and postprocessors can be used directly with the SEAWAT code. The approach was tested by simulating the Henry problem and two of the saltpool laboratory experiments (low- and high-density cases). For the Henry problem, the simulated results compared well with the steady-state semianalytic solution and also the transient isochlor movement as simulated by a finite-element model. For the saltpool problem, the simulated breakthrough curves compared better with the laboratory measurements for the low-density case than for the high-density case but showed good agreement with the measured salinity isosurfaces for both cases. Results from the test cases presented here indicate that the MODFLOW/MT3DMS approach provides accurate solutions for problems involving variable-density ground water flow and solute transport. ?? 2006 National Ground Water Association.

Ground Water↗
USGS1999

Ground-water occurrence and contribution to streamflow, northeast Maui, Hawaii

The study area lies on the northern flank of theEast Maui Volcano (Haleakala) and covers about129 square miles between the drainage basins ofMaliko Gulch to the west and Makapipi Stream tothe east. About 989 million gallons per day of rain-fall and 176 million gallons per day of fog dripreaches the study area and about 529million gal-lons per day enters the ground-water system asrecharge. Average annual ground-water with-drawal from wells totals only about 3 million gal-lons per day; proposed (as of 1998) additionalwithdrawals total about 18 million gallons per day.Additionally, tunnels and ditches of an extensiveirrigation network directly intercept at least 10 mil-lion gallons per day of ground water.The total amount of average annual stream-flow in gaged stream subbasins upstream of 1,300feet altitude is about 255 million gallons per dayand the total amount of average annual base flow isabout 62 million gallons per day. Six major sur-face-water diversion systems in the study area havediverted an average of 163 million gallons per dayof streamflow (including nearly all base flow ofdiverted streams) for irrigation and domestic sup-ply in central Maui during 1925–97.Fresh ground water is found in two mainforms. West of Keanae Valley, ground-water flowappears to be dominated by a variably saturatedsystem. A saturated zone in the uppermost rockunit, the Kula Volcanics, is separated from a fresh-water lens near sea level by an unsaturated zone inthe underlying Honomanu Basalt. East of KeanaeValley, the ground-water system appears to befully saturated above sea level to altitudes greaterthan 2,000 feet. The total average annual streamflow of gagedstreams west of Keanae Valley is about 140 milliongallons per day at 1,200 feet to 1,300 feet altitude.It is not possible to estimate the total averageannual streamflow at the coast. All of the base flowmeasured in the study area west of Keanae Valleyrepresents ground-water discharge from the high-elevation saturated zone. Total average dailyground-water discharge from the high-elevationsaturated zone upstream of 1,200 feet altitude isgreater than 38 million gallons per day, all of whichis eventually removed from the streams by surface-water diversion systems. Perennial streamflow hasbeen measured at altitudes greater than 3,000 feetin several of the streams. Discharge from the high-elevation saturated zone is persistent even duringperiods of little rainfall. The total average annual streamflow of the gaged streams east of Keanae Valley is about 109million gallons per day at about 1,300 feet altitude. It is not possible to estimate the total average annual streamflow at the coast nor at higher altitudes. All of the base flow measured east of Keanae Valley represents ground-water discharge from the vertically extensive freshwater-lens system. Total average daily ground-water discharge to gaged streams upstream of 1,200 feet altitude is about 27million gallons per day. About 19 million gallons per day of ground water discharges through the Kula and Hana Volcanics between about 500 feet and 1,300 feet altitude in the gaged stream sub-basins. About 13 million gallons per day of this discharge is in Hanawi Stream. The total ground-water discharge above 500 feet altitude in this part of the study area is greater than 56 million gallons per day.

Hawaii↗
USGS1994

Hydrogeology and simulation of ground-water flow at Arnold Air Force Base, Coffee and Franklin counties, Tennessee

The U.S. Air Force at Arnold Air Force Base (AAFB), in Coffee and Franklin Counties, Tennessee, is investigating ground-water contamination in selected areas of the base. This report documents the results of a comprehensive investigation of the regional hydrogeology of the AAFB area. Three aquifers within the Highland Rim aquifer system, the shallow aquifer, the Manchester aquifer, and the Fort Payne aquifer, have been identified in the study area. Of these, the Manchester aquifer is the primary source of water for domestic use. Drilling and water- quality data indicate that the Chattanooga Shale is an effective confining unit, isolating the Highland Rim aquifer system from the deeper, upper Central Basin aquifer system. A regional ground-water divide, approximately coinciding with the Duck River-Elk River drainage divide, underlies AAFB and runs from southwest to northeast. The general direction of most ground-water flow is to the north- west or to the northwest or to the southeast from the divide towards tributary streams that drain the area. Recharge estimates range from 4 to 11 inches per year. Digital computer modeling was used to simulate and provide a better understanding of the ground-water flow system. The model indicates that most of the ground-water flow occurs in the shallow and Manchester aquifers. The model was most sensitive to increases in hydraulic conductivity and changes in recharge rates. Particle-tracking analysis from selected sites of ground-water contamination indicates a potential for contami- nants to be transported beyond the boundary of AAFB.

Water-Resources Investigations Report↗
USGS1978

Hydrogeologic considerations for an interstate ground-water compact on the Madison aquifer, northern Great Plains

The development of an interstate ground-water compact for the Madison aquifer in the Northern Great Plains may provide a framework to allocate equitably this large ground-water resource while avoiding possible future interstate legal conflicts. However, some technical problems will have to be resolved first. A compact designed to regulate or to allocate the available ground water will have to be written in very precise, legally acceptable definitions. The required definitions may infer a degree of measurement accuracy that cannot be technically or economically provided. Therefore, a trade off may be required between preserving natural conditions and allowing beneficial use of the ground-water resource.

Montana;North Dakota;South Dakota;Wyoming↗
USGS1998

Geohydrology and simulated ground-water flow in northwestern Elkhart County, Indiana

In 1994, the U.S. Geological Survey, in cooperation with the U.S. Environmental Protection Agency and the City of Elkhart, developed a ground-water model of the Elkhart, Indiana, area to determine the avail-ability and source of water at potential new well fields. The modeled area covered 190 square miles of northwestern Elkhart County and a small part of southern Michigan. Three Superfund sites and several other sites in this area are undergoing environmental cleanup. The model would be used to guide the location of well fields so that Superfund sites and environmental cleanup areas would not be within recharge areas for the well fields. The City of Elkhart obtains its water supply from two aquifers separated by a generally continuous confining unit. The upper aquifer is composed primarily of sand and gravel of glacial origin. Thickness of the upper aquifer ranges from 0 to 116 feet and averages 47 feet. The lower aquifer is composed of sand and gravel with interbedded lenses of silt and clay. Thickness of the lower aquifer ranges from 1 to 335 feet and averages 35 feet. The intervening confining unit is composed of silt and clay with interbedded sand and gravel; the confining unit ranges from 0 to 177 feet, with an average thickness of 27 feet. Flow through the aquifers is generally horizontal vertically downward from the upper aquifer, through the confining unit, and into the lower aquifer, except where flow is vertically upward at the St. Joseph River and other large streams. The hydraulic characteristics of the aquifers and confining unit were estimated by analyzing aquifer-test data from well drillers? logs and by calibration of the model. The horizontal hydraulic conductivity of the upper aquifer is 170 feet per day within about 1 mile of the St. Joseph and Elkhart Rivers and 370 feet per day at distances greater than about 1 mile. The horizontal hydraulic conductivity of the lower aquifer is 370 feet per day throughout the modeled area, with the exception of an area near the center of the modeled area where the horizontal hydraulic conductivity is 170 feet per day. Transmissivity of the lower aquifer increases generally from southwest to northeast; transmissivity values range from near 0 where the lower aquifer is absent to 57,000 square feet per day and average about 8,100 square feet per day. The vertical hydraulic conductivity of the confining unit is 0.07 feet per day; the vertical conductivity of the streambeds commonly is 1.0 foot per day and ranges from 0.05 foot per day to 50 feet per day. The areal recharge rate to the outwash deposits was determined by a base-flow separation technique to be 16 inches per year, and the areal recharge rate to the till was assumed to be 4 inches per year. A two-layer digital model was used to simulate flow in the ground-water system. The model was calibrated on the basis of historical water-use data, water-level records, and gain/loss data for streams during May and June 1979. The model was recalibrated with water-use data and water-level records from 1988. For 1979 data, 49 percent of the inflow to the model area is from precipitation and 46 percent is ground-water inflow across the model boundaries. Most of the ground-water inflow across the model boundary is from the north and east, which corresponds to high values of transmissivity?as high as 57,000 feet squared per day?in the model layers in the northern and eastern areas. Eighty-two percent of the ground-water discharge is to the streams; 5 percent of the ground-water discharge is to wells. Source areas and flow paths to the City of Elkhart public well fields are affected by the location of streams and the geology in the area. Flow to the North Well Field originates north-west of the well field, forms relatively straight flow paths, and moves southeast toward the well field and the St. Joseph River. Flow to the South Well Field begins mostly in the out-wash along Yellow Creek south of the well field, moves northward, and t

Indiana↗
USGS2008

Decadal-scale changes of pesticides in ground water of the United States, 1993-2003

Pesticide data for ground water sampled across the United States between 1993-1995 and 2001-2003 by the U.S. Geological Survey National Water-Quality Assessment Program were evaluated for trends in detection frequency and concentration. The data analysis evaluated samples collected from a total of 362 wells located in 12 local well networks characterizing shallow ground water in agricultural areas and six local well networks characterizing the drinking water resource in areas of variable land use. Each well network was sampled once during 1993-1995 and once during 2001-2003. The networks provide an overview of conditions across a wide range of hydrogeologic settings and in major agricultural areas that vary in dominant crop type and pesticide use. Of about 80 pesticide compounds analyzed, only six compounds were detected in ground water from at least 10 wells during both sampling events. These compounds were the triazine herbicides atrazine, simazine, and prometon; the acetanilide herbicide metolachlor; the urea herbicide tebuthiuron; and an atrazine degradate, deethylatrazine (DEA). Observed concentrations of these compounds generally were <0.12 ??g L-1. At individual wells, changes in concentrations typically were <0.02 ??g L-1. Data analysis incorporated adjustments for changes in laboratory recovery as assessed through laboratory spikes. In wells yielding detectable concentrations of atrazine, DEA, and prometon, concentrations were significantly lower (?? = 0.1) in 2001-2003 than in 1993-1995, whereas detection frequency of these compounds did not change significantly. Trends in atrazine concentrations at shallow wells in agricultural areas were found to be consistent overall with recent atrazine use data. Copyright ?? 2008 by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. All rights reserved.

Conference Paper↗
USGS1995

A preliminary assessment of the occurrence and possible sources of MTBE in ground water of the United States, 1993-94

The 1990 Clean Air Act Amendments require fuel oxygenates to be added to gasoline used in some metropolitan areas to reduce atmospheric concen- trations of carbon monoxide or ozone. Methyl tert-butyl ether (MTBE), is the most commonly used fuel oxygenate and is a relatively new gasoline additive. Nevertheless, out of 60 volatile organic chemicals analyzed, MTBE was the second most frequently detected chemical in samples of shallow ambient ground water from urban areas that were collected during 1993-94 as part of the U.S. Geological Survey's National Water-Quality Assessment program. Samples were collected from 5 drinking-water wells, 12 springs, and 193 monitoring wells in urban areas. No MTBE was detected in drinking-water wells. At a reporting level of 0.2 ug/L (micrograms per liter), MTBE was detected most frequently in shallow ground water from urban areas (27 percent of 210 wells and springs sampled in 8 areas) as compared to shallow ground water from agricultural areas (1.3 percent of 549 wells sampled in 21 areas) or deeper ground water from major aquifers (1 percent of 412 wells sampled in 9 areas). Only 3 percent of the shallow wells sampled in urban areas had concentrations of MTBE that exceed 20 ug/L, which is the estimated lower limit of the U.S. Environmental Protection Agency draft drinking-water health advisory. Because MTBE is persistent and mobile in ground water, it can move from shallow to deeper aquifers with time. In shallow urban ground water, MTBE generally was not found with benzene, toluene, ethylbenzene, or xylenes (BTEX) compounds which commonly are associated with gasoline spills. This disassociation causes uncertainty as to the source of MTBE. Possible sources of MTBE in ground water include point sources, such as leaking storage tanks, and nonpoint sources, such as recharge of precipitation and storm-water runoff.

Open-File Report↗
USGS1983

Ground-water resources in the Hood Basin, Oregon

The Hood Basin, an area of 1,035 square miles in north-central Oregon, includes the drainage basins of all tributaries of the Columbia River between Eagle Creek and Fifteenmile Creek. The physical characteristics and climate of the basin are diverse. The Wasco subarea, in the eastern half of the basin, has moderate relief, mostly intermittent streams, and semiarid climate. The Hood subarea, in the western half, has rugged topography, numerous perennial streams, and a humid climate. Water-bearing geologic units that underlie the basin include volcanic, volcaniclastic, and sedimentary rocks of Miocene to Holocene age, and unconsolidated surficial deposits of Pleistocene and Holocene age. The most important water-bearing unit, the Columbia River Basalt Group, underlies almost the entire basin. Total thickness probably exceeds 2,000 feet, but by 1980 only the upper 1,000 feet or less had been developed by wells. Wells in this unit generally yield from 15 to 1,000 gallons per minute and a few yield as much as 3,300 gallons per minute. The most productive aquifer in the Columbia River Basalt Group is The Dalles Ground Water Reservoir, a permeable zone of fractured basalt about 25 to 30 square miles in extent that underlies the city of The Dalles. During the late 1950's and mid-1960's, withdrawals of 15,000 acre-feet per year or more caused water levels in the aquifer to decline sharply. Pumpage had diminished to about 5,000 acre-feet per year in 1979 and water levels have stabilized, indicating that ground water recharge and discharge, including the pumping, are in balance. The other principal geologic units in the basin have more limited areal distribution and less saturated thickness than the Columbia River Basalt Group. Generally, these units are capable of yielding from a few to a hundred gallons per minute to wells. Most of the ground water in the basin is chemically suitable for domestic, irrigation, or other uses. Some ground water has objectionable concentrations of iron (0.3 to 6.4 milligrams per liter) and manganese (0.05 to 1.2 milligrams per liter) or is moderately hard to very hard (60 to 260 milligrams per liter as CaCO 3 ). The principal use of ground water in the Hood Basin is for irrigation of crops, with an estimated withdrawal of 7,700 acre-feet in 1979. Additional ground-water withdrawals in 1979 were estimated as: Industrial, 2,600 acre-feet; public supply, 2,100 acre-feet; and domestic and stock supply, 200 acre-feet.

Oregon↗
USGS1946

Ground water in Pavant Valley

The users of wells for irrigation in Pavant Valley, particularly in the Flowell district, have long been cognizant of their utter dependency upon ground water for livelihood, and were among the first in the State to make an organized effort to conserve supplies by prevention of waste. Since passage of the State ground-water law in 1935, the State Engineer has not approved applications for new wells in the areas of most concentrated development, and has deferred adjudication of existing water rights until adequate data concerning the ground-water resources become available. The investigation of ground-water resources in Pavant Valley was suggested by the State Engineer and constitutes one of a series that are being made in the important groundwater basins of Utah by the Federal Geological Survey in cooperation with the State Engineer. The investigation was under the general supervision of Oscar E. Meinzer, geologist in charge of the ground-water division of the Federal Geological Survey. H. E. Thomas, in charge of groundwater investigations in Utah, returned from military service overseas in time to assist in the completion of the manuscript, and edited the report.

Utah↗
USGS1996

Ground-water quality assessment of the Georgia-Florida Coastal Plain study unit — Analysis of available information on nutrients, 1972-92

The U.S. Geological Survey is conducting an assessment of water quality in the Georgia-Florida Coastal Plain study unit as part of the National Water-Quality Assessment Program. An initial activity of the program is to compile and analyze existing water-quality data for nutrients in each study unit. Ground-water quality data were compiled from three data sources, the U.S. Geological Survey, Florida Department of Environmental Protection, and Georgia Geologic Survey. A total of 2,246 samples of ground water nutrient data for nitrogen and phosphorus species were compiled from these three data sources. Estimates of 1990 nitrogen and phosphorus inputs by county in the study area were calculated from livestock manure, fertilizers, septic tanks, and rainfall. Data for nitrate nitrogen concentrations in ground water were available from the greatest number of wells; samples from 1,233 wells were available in the U.S. Geological Survey, 820 wells from the Florida Department of Environmental Protection, and 680 wells from the Georgia Geologic Survey. The maximum contaminant level for nitrate nitrogen in drinking water of 10 milligrams per liter was exceeded in a higher percentage of samples from the U.S. Geological Survey, mostly because this data contained numerous samples near known contamination areas. The maximum contaminant level for nitrate nitrogen was exceeded in 3 percent of samples from Upper Floridan aquifer and 12 percent of samples from surficial aquifer system in U.S. Geological Survey data and less than 1 percent and 2 percent of samples from the Upper Floridan aquifer and surficial aquifer system, respectively, in Florida Department of Environmental Protection data. In Georgia Geologic Survey data, 1 percent of samples had concentrations of nitrate nitrogen exceeding 10 milligrams per liter. Nutrient concentration data were grouped into categories based on land use, hydrogeology (aquifer and confinement of the Upper Floridan aquifer), and land resource provinces (Central Florida Ridge, Coastal Flatwoods and Southern Coastal Plain) for the surficial aquifer system. The highest median nitrate nitrogen concentrations in the U.S. Geological Survey data were 0.4 milligrams per liter in ground-water samples from the unconfined Upper Floridan aquifer in agricultural areas and 9.0 milligrams per liter in samples from the surficial aquifer system in agricultural areas in the Central Florida Ridge. In Florida Department of Environmental Protection data, the highest median nitrate nitrogen concentrations were much lower and did not exceed 0.2 milligrams per liter in either the Upper Floridan aquifer or the surficial aquifer system. In Georgia Geologic Survey data the highest median nitrate nitrogen concentration was 1.4 milligrams per liter in agricultural areas in the Coastal Flatwoods. Highest median concentrations of total nitrogen of 10 milligrams per liter (includes nitrate, ammonia, and organic nitrogen) were in U.S. Geological Survey data in the surficial aquifer system in agricultural areas in the Central Florida Ridge. Median concentrations of ammonia nitrogen, orthophosphate phosphorus, and total phosphorus did not exceed 0.5 milligrams per liter in all categories from the Upper Floridan aquifer or the surficial aquifer system.

Florida, Georgia↗
USGS1981

Digital models of ground-water flow in the Cape Cod aquifer system, Massachusetts

The Cape Cod aquifer system was simulated with three-dimensional finite-difference ground-water-flow models. Five areas were modeled to provide tools that can be used to evaluate the hydrologic impacts of regional water development and waste disposal. The model boundaries were selected to represent the natural hydrologic boundaries of the aquifer. The boundary between fresh and saline ground water is treated as an interface along which there is no dispersion. The saline-water zone is treated as static (nonflowing). Comparisons of calculated and observed values of head, position of the boundary between fresh and saline water, and ground-water discharge at locations where data are available indicate that the simulated ground-water reservoirs generally agree with field conditions. Model analyses indicate that the total steady-state freshwater-flow rate through the five modeled areas is approximately 412 cubic feet per second.

Massachusetts↗
USGS1996

Quality of ground water and surface water in intermontane basins of the northern Rocky Mountains, Montana and Idaho

The Regional Aquifer-System Analysis (RASA) program is a series of studies by the U.S. Geological Survey (USGS) to analyze regional ground-water systems that compose a major portion of the Nations water supply (Sun, 1986). The Northern Rocky Mountains Intermontane Basins is one of the study regions in this national program. The main objectives of the RASA studies are to: (1) describe the ground-water systems as they exist today, (2) analyze the known changes that have led to the system's present condition, (3) combine results of previous studies in a regional analysis, where possible, and (4) provide means by which effects of future ground-water development can be estimated. The purpose of this study, which began in 1990, was to increase understanding of the hydrogeology of the intermontane basins of the Northern Rocky Mountains area. This report is Chapter C of a three-part series and describes the quality of ground water and surface water in the study area. Chapter A (Tuck and others, 1996) describes the geologic history and generalized hydrogeologic units. Chapter B (Briar and others, 1996) describes the general distribution of ground-water levels in basin-fill deposits. Water-quality data illustrated in this report represent the distribution of concentrations and composition of dissolved solids in ground water and surface water in the intermontane areas. The chemistry of ground and surface water in the intermontane areas is influenced by the chemical and physical nature of the rocks in the basin deposits of the valleys and surrounding bedrock in the mountains.

Hydrologic Atlas↗
USGS1996

Simulation of ground-water flow in the Saginaw Aquifer, Clinton, Eaton, and Ingham counties, Michigan

A numerical model was developed to simulate ground-water flow in the Tri-County region, which consists of Clinton, Eaton, and Ingham Counties, Michigan. This region includes a nine-township area surrounding Lansing, Michigan. The model simulates the regional response of the Saginaw aquifer to major groundwater withdrawals associated with public-supply wells. The Saginaw aquifer, which is in the Grand River and Saginaw Formations of Pennsylvanian age, is the primary source of ground water for Tri-County residents. The Saginaw aquifer is overlain by glacial deposits, which also are important ground-water sources in some locations. Flow in the Saginaw aquifer and the glacial deposits is simulated by discretizing the flow system into model cells arranged in two layers. Each cell, which corresponds to a land area of 0.0625 square mile, represents the locally averaged properties of the system. The spatial variation of hydraulic properties controlling ground-water flow was estimated by geostatistical analysis of 4,947 well logs. Parameter estimation, a form of nonlinear regression, was used to calibrate the flow model. Results of steady-state ground-water-flow simulations show close agreement between water flowing into and out of the model area for 1992 pumping conditions; standard error of the difference between simulated and measured heads is 14.7 feet. Simulation results for three alternative pumping scenarios for the year 2020 show that the glacial aquifer could be dewatered in places if hypothetical increases in pumping are not distributed throughout the Tri-County region. Contributing areas to public-supply wells in the nine-township area were delineated by a particle-tracking analysis. These areas cover about 121 square miles. Contributing areas for particles having travel times of 40 years or less cover about 42 square miles. Results of tritium sampling support results of model simulations to delineate contributing areas.

Michigan↗