Geology Reports⌕ Search

SEARCH · Geology Reports

Results for “Southwest Hydrology”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 343 records · Page 19Linked to original sources
USGS2023

Expansive, positive changes to fish habitat diversity following the formation of a valley plug in a degraded desert river

Widespread hydrologic alterations have simplified in-stream habitats in rivers globally, driving population declines and extirpations of many native fishes. Here, we examine how rapid geomorphic change in a historically degraded desert river has influenced habitat diversification and ecosystem persistence. In 2010, a large reach of the degraded and simplified lower San Rafael River (SRR), Utah, was impacted by the formation of a valley plug and began to shift from a homogenous, single-thread channel to a complex, multi-threaded riverscape. We combined field measurements and drone-collected imagery to document changes in fish habitat due to the valley plug. Our results demonstrate that in 2021, the affected reach was more diverse than any other stream reach along the SRR, containing 641% more diverse habitat (e.g., pools, riffles, and backwaters) than what was measured in 2015. The plug reach also retained water for periods beyond what was expected during seasonal drying, with the total extent of inundation within the riverscape increasing by over 2800%. Since the formation of the valley plug, riparian habitat has increased by 230% and channel networks have expanded to more than 50 distinct channels throughout the zone of influence. Our results provide evidence of successful self-restoration in a formerly highly degraded reach of desert river, and encourage new methods of desert river restoration. We aim to inform the use of large-scale, disruptive restoration actions like intentional channel occlusions, with the goal of mitigating the impacts of simplification and increasing habitat persistence in the face of exacerbated aridity in the desert Southwest.

Utah↗
USGS1984

Carbonate geology and hydrology of the Edwards Aquifer in the San Antonio area, Texas

Regional differences in the porosity and permeability of the Edwards aquiifer are related to three major depositional areas, the Maverick basin the Devils River trend, and the San Marcos platform, that existed during Early Cretaceous time. The rocks of the Maverick basin are predominantly deep basinal deposits of dense, homogeneous mudstones of low primary porosity. Permeability is principally associated with cavernous voids in the upper part of the Salmon Peak Formation in the Maverick basin. The rocks of the Devils River trend are a complex of marine and supratidal deposits in the lower part and reefal or inter-reefal deposits in the upper part. Permeable zones, which occur in the upper part of the trend, are associated with collapse breccias and rudist reefs. The rocks of the San Marcos platform predominantly are micrites that locally contain collapse breccias, honeycombed, burrowed mudstones, and rudist reef deposits that are well leached and very permeable. The rocks of the San Marcos plat form form the most transmissive part of the Edwards aquifer in the San Antonio area. Karstification of the rocks on the San Marcos plat- form during Cretaceous time enhanced the permeability of the aquifer. Permeability of the Edwards aquifer is greatest in particular strata (lithofacies) which have been leached in the freshwater zone. Ground water moves along vertical or steeply inclined fractures that are passageways by which water can enter permeable strata. Water moves from the fractures into beds formed by collapse breccias, burrowed wackestones, and rudist grainstones that have significant secondary porosity and permeability. Water has selectively dissolved sedimentary features within those rocks to increase the size of the openings and the degree of interconnection between pore voids. Recognition of the hydrostratigraphic subdivisions provides a basis for defining the nonhomogeneity of the aquifer and determining its storage characteristics. The aquifer is considered to be a faulted and multilayered aquifer in which lateral circulation is mainly through very permeable, hydrostratigraphic subdivisions that are hydraulically connected at places by openings associated with steep-angle, normal faults. The Edwards aquifer is vertically displaced for its entire thickness at places along major northeastward trending faults. At these places, ground-water circulation is diverted either southwest or northeast.

Open-File Report↗
USGS1979

Summary appraisals of the nation's ground-water resources – Lower Colorado region

This report summarizes ground-water availability in the lower Colorado region and discusses the potential for greater ground-water development and increased efficiency of water use. The climate in the most highly developed southwestern part of the region is warm and dry and that in the northeastern part is cool and moist to dry. Although the regional average annual precipitation is only about 14 inches and most streambeds are dry during most of the year, about 1.5 billion acre-feet of ground water of moderate to good chemical quality is stored in aquifers of the region. Much of the water use is founded on pumped withdrawal of ground water. However, in most of the southwestern part of the region pumpage and consumptive use are in excess of replenishment, resulting in declining water levels. In the southwestern part of the region, water levels generally are from 200-500 feet below land surface and in large areas are less than 200 feet. Large-diameter water wells in this part of the region commonly produce 500-1 ,500 gallons per minute of water. In the northeastern part of the region, water levels generally are more than 500 feet below land surface, but in some large areas water levels are from 200-500 feet, and locally are less than 200 feet below the surface. In the northeastern part of the region, water wells generally have lower yields than those to the southwest. The yields range from a few to 2,000 gallons per minute, but most wells yield from 10 to less than 500 gallons per minute. In the southwestern part of the region about 1 billion acre-feet of ground water is recoverable from storage from the water table to a depth of 700 feet below the land surface. In the northeast about 150 million acre-feet can be recovered by dewatering a 100-foot-thick section of a typical aquifer (16 million-acre area). The estimated current annual rate of ground-water depletion, occurring almost entirely in the southwestern part of the region, is 2.4-3.2 million acre-feet per year. Almost 6 million acre-feet is pumped each year-about 90 percent for agriculture, 6 percent for public supply, and 3 percent for industrial use. The percentage of use for public supply and industry is increasing each year. The potential for greater development of ground water in the southwestern part of the region is constrained by land subsidence, earth cracks, increasing costs of pumping and transportation, and moderate to poor chemical quality of water. More ground water can be developed in the northeastern part of the region, where the major constraint is pumping cost owing to low to moderate well yields and depth to water. Some benefits can be realized everywhere in the region through changes in current use and greater efficiencies of use. Additional supplies may be made available by capture of natural evapotranspiration. Increasing the efficiency of use is possible hydrologically but, in the near term, is more expensive than increasing groundwater development. Decrease of irrigation, change to water-saving methods of irrigation, use of saline water, decrease of per capita public- supply use, and more reuse of water in almost every type of use could help extend the supply and thereby reduce the current rate of ground-water depletion. Financial problems have not yet caused an overall decrease in pumpage, but, locally, operating costs or partial dewatering of the aquifer has eliminated or decreased withdrawal. Current water laws in all States of the region, except Arizona, control or allocate the use of ground water.

Arizona, California, Nevada, New Mexico, Utah↗
USGS2007

Geologic map of Wupatki National Monument and vicinity, Coconino County, Northern Arizona

Introduction The geologic map of Wupatki National Monument is a cooperative effort between the U.S. Geological Survey, the National Park Service, and the Navajo Nation to provide geologic information for resource management officials of the National Park Service, U.S. Forest Service, Navajo Indian Reservation (herein the Navajo Nation), and visitor information services at Wupatki National Monument, Arizona. Funding for the map was provided in part by the Water Rights Branch of the Water Resources Division of the National Park Service. Field work on the Navajo Nation was conducted under a permit from the Navajo Nation Minerals Department. Any persons wishing to conduct geologic investigations on the Navajo Nation must first apply for, and receive, a permit from the Navajo Nation Minerals Department, P.O. Box 1910, Window Rock, Arizona 86515, telephone (928)-871-6587. Wupatki National Monument lies within the USGS 1:24,000-scale Wupatki NE, Wupatki SE, Wupatki SW, Gray Mountain, East of SP Mountain, and Campbell Francis Wash quadrangles in northern Arizona. The map is bounded approximately by longitudes 111? 16' to 111? 32' 30' W. and latitudes 35? 30' to 35? 37' 40' N. The map area is in Coconino County on the southern part of the Colorado Plateaus geologic province (herein Colorado Plateau). The map area is locally subdivided into three physiographic parts, the Coconino Plateau, the Little Colorado River Valley, and the San Francisco Volcanic Field as defined by Billingsley and others (1997) [fig. 1]. Elevations range from 4,220 ft (1,286 m) at the Little Colorado River near the northeast corner of the map area to about 6,100 ft (1,859 m) at the southwest corner of the map area. The small community of Gray Mountain is about 16 mi (26 km) northwest of Wupatki National Monument Visitor Center, and Flagstaff, Arizona, the nearest metropolitan area, is about 24 mi (38 km) southwest of the Visitor Center (fig. 1). U.S. Highway 89 provides access to the west entrance of Wupatki National Monument. A paved Coconino County road provides a loop from Wupatki National Monument south to Sunset Crater National Monument and back to U.S. Highway 89 about 10 mi (16 km) north of Flagstaff, Arizona. Access to Coconino National Forest is via dirt roads maintained by the National Forest Service. Several unimproved dirt roads on Babbitt Ranch lands provide limited access to remote areas north of Wupatki National Monument. Travel is mostly restricted to paved roads within Wupatki National Monument, and a dirt road that crosses the Little Colorado River provides access to the Navajo Nation area east and northeast of the Little Colorado River. The Little Colorado River crossing is not bridged and can be impassable when the river is flowing. Four-wheel-drive vehicles are recommended but not necessary for travel in remote parts of the Navajo Nation. Extra food and water are highly recommended for travel in this sandy area. Land ownership north of Wupatki National Monument forms a checkerboard pattern between private and State land. Coconino National Forest manages lands south of Wupatki National Monument and the National Park Service manages Wupatki National Monument. The Leupp and Tolani Lake Chapters of the Navajo Nation manage the area northeast and east of the Little Colorado River (see land management boundaries on map). The geologic map of Wupatki National Monument provides updated geologic framework information for this part of the Colorado Plateau. The geologic information supports Federal, State, and private land managers when conducting geologic, biologic, and hydrologic investigations and will support future and ongoing geologic and associated scientific investigations of all disciplines within the Wupatki National Monument area.

Arizona↗
USGS2010

The geochemistry of the Fox Hills-Basal Hell Creek Aquifer in southwestern North Dakota and northwestern South Dakota

The Late Cretaceous Fox Hills Formation and the basal portion of the overlying Hell Creek Formation constitute an important aquifer in the Fort Union coal region. Throughout most of southwestern North Dakota and northwestern South Dakota the aquifer is at depths ranging from 1000 to 2000 ft, except for exposures along the Cedar Creek anticline. Water flows in the aquifer from southwest to northeast, with flow rates of a few feet per year. The recharge and discharge areas of the aquifer are separated by a north-south trending transition zone in which significant changes in water chemistry occur. Dissolved constituents in the recharge area (the western part of the study area) are Na + = 18 mmol/l, Cl − = 0.7 mmol/1, SO 4 2− = 2.7 mmol/1, and HCO 3 − = 13 mmol/l (δ 13 C = −12‰) with p H = 8.5. Ca 2+ , Mg 2+ , and K + are each less than 0.1 mmol/l, dissolved O 2 = 0, and traces of H 2 S and CH 4 are present. Computer modeling and carbon isotope data suggest the following reactions in the recharge area. CO 2 derived from lignitic carbon reacts to dissolve carbonate minerals, with cations then being exchanged for Na + on clay minerals. The high p H in the aquifer is the result of buffering by carbonate-ion exchange equilibria. In the discharge area, p H values have declined to 8.3, Cl − has increased from 0.7 to 5.5 mmol/l, with a parallel increase in Na + SO 4 2− has essentially disappeared, HCO 3 − has increased from 13 to 21 mmol/l (δ 13 C = −9‰), CH 4 has attained concentrations greater than 0.5 mmol/l, and small amounts of He are present. Traces of H 2 S are present, and Ca 2+ , Mg 2+ , and K + concentrations remain low throughout the aquifer: These changes can be accounted for by reactions in the aquifer: (1) sulfate reduction to pyrite with lignitic material as the carbon source and (2) continuous buffering of p H by the carbonate-ion exchange equilibria. Chemical and hydrologic data suggest that the increase in NaCl results from upward movement of small volumes of water into the Fox Hills aquifer from the transition zone eastward. Redox reactions in the aquifer are closely analogous to those observed in pore waters of reducing marine sediments. Reactions approach but do not achieve true thermodynamic equilibrium. Measurements of redox potential suggest a downgradient decrease in redox potential. The measurements are not amenable to quantitative interpretation.

Water Resources Research↗
USGS2001

Restoration of biogeochemical function in mangrove forests

Forest structure of mangrove restoration sites (6 and 14 years old) at two locations (Henderson Creek [HC] and Windstar [WS]) in southwest Florida differed from that of mixed-basin forests (>50 years old) with which they were once contiguous. However, the younger site (HC) was typical of natural, developing forests, whereas the older site (WS) was less well developed with low structural complexity. More stressful physicochemical conditions resulting from incomplete tidal flushing (elevated salinity) and variable topography (waterlogging) apparently affected plant survival and growth at the WS restoration site. Lower leaf fall and root production rates at the WS restoration site, compared with that at HC were partly attributable to differences in hydroedaphic conditions and structural development. However, leaf and root inputs at each restoration site were not significantly different from that in reference forests within the same physiographic setting. Macrofaunal consumption of tethered leaves also did not differ with site history, but was dramatically higher at HC compared with WS, reflecting local variation in leaf litter processing rates, primarily by snails (Melampus coffeus). Degradation of leaves and roots in mesh bags was slow overall at restoration sites, however, particularly at WS where aerobic decomposition may have been more limited. These findings indicate that local or regional factors such as salinity regime act together with site history to control primary production and turnover rates of organic matter in restoration sites. Species differences in senescent leaf nitrogen content and degradation rates further suggest that restoration sites dominated by Laguncularia racemosa and Rhizophora mangle should exhibit slower recycling of nutrients compared with natural basin forests where Avicennia germinans is more abundant. Structural development and biogeochemical functioning of restored mangrove forests thus depend on a number of factors, but site-specific as well as regional or local differences in hydrology and concomitant factors such as salinity and soil waterlogging will have a strong influence over the outcome of restoration projects.

Restoration Ecology↗
USGS1978

Ground-water availability in the Hayes-Red Willow, Frenchman, and Meeker-Driftwood irrigation districts, southwest Nebraska

Surface-water supplies are diminishing in the Hitchcock-Red Willow and Frenchman Valley Irrigation Districts in southwest Nebraska. Stream depletions due to ground-water withdrawals upstream from Enders Reservoir (northwest of the study area) have resulted in a shortage of about 8,700 acre-feet per year. The availability of ground water to supply part of this deficit was examined. Two surficial aquifers that are currently economical to develop were investigated. The Ogallala aquifer of late Tertiary age which underlies all the area except the major stream valleys has a hydraulic conductivity averaging less than 200 gallons per day per square foot. Hydraulic conductivities and saturated thicknesses are greater north of and parallel to the Republican River from McCook to Culbertson. The most productive aquifer comprises alluvial deposits of Pleistocene and Holocene age which occupy the valleys of the Republican River and Frenchman Creek. Saturated thickness of this aquifer averages 5O feet, and hydraulic conductivity averages 1,000 gallons per day per square foot. The two aquifers are hydraulically connected except where the bedrock surface on both sides of the Republican River valley is higher than the water levels in the aquifers. Canal seepage and deep percolation have caused water levels to rise as much as 20 feet in the Hitchcock-Red Willow and Frenchman Valley Irrigation Districts north of the Republican River and as much as 40 feet in the Meeker-Driftwood Irrigation District south of the River. Streamflow accretions from ground water have not increased significantly on Frenchman, Blackwood, or Driftwood Creeks. Ground-water inflow to the Republican River has increased about 6 cubic feet per second between Trenton and McCook. A digital model of the stream aquifer system was used to simulate the pre-1976 hydrology and assess future conditions for two supply-well configurations as well as for existing private irrigation wells. The first supply-well configuration could sustain less than 37 percent of the well-field capacity of 5,970 acre-feet per year. Projected maximum stream depletions were 12 cubic feet per second on Frenchman Creek and 4 cubic feet per second on Blackwood Creek. The second supply-well configuration could sustain 79 percent of the well-field capacity of 2,780 acre-feet per year. Simulated stream depletions were less than 5 percent of mean annual flow at the end of 19 years. Model simulation indicated that existing wells could supply consumptive-irrigation requirements to district lands for at least 19 years, with well failures limited to areas in the Meeker-Driftwood District where the aquifer is thin.

Nebraska↗
USGS1963

Hydrology of upper Black Earth Creek basin, Wisconsin, with a section on surface water

The upper Black Earth Creek drainage basin has an area of 46 square miles and is in Dane County in south-central Wisconsin. The oldest rock exposed in the valley walls is the sandstone of Late Cambrian age. Dolomite of the Prairie du Chien Group of Ordovician age overlies the sandstone and forms the. resistant cap on the hills. The St. Peter Sandstone, Platteville and Decorah Formations, and Galena Dolomite, all Ordovician in age, form a narrow belt along the southern boundary of the area. Outwash and alluvium of Pleistocene and Recent age fill the valleys. The eastern half of the area was glaciated and is covered with till. The sandstone of Late Cambrian age and the sand and gravel of the outwash deposits are hydraulically connected. Ground water occurs under unconfined (water-table) conditions in the western unglaciated part of the basin and under artesian conditions beneath the till locally in the eastern part. The source of most of the ground water is direct infiltration of precipitation; however, some ground water enters the area as underflow from the south. About 7 inches of the 30 inches of average annual precipitation recharges the ground-water reservoir. The ground water generally moves toward Black Earth Creek where it is discharged. Some ground water moves out of the basin as underflow beneath the valley of Black Earth Creek, and some is discharged by evapotranspiration or is withdrawn by pumping from wells. Water levels in shallow nonartesian wells respond rapidly to precipitation. The effect of precipitation on water levels in artesian wells is slower and more subdued. Water levels are generally highest in spring and lowest in fall and winter. The flow of upper Black Earth Creek is derived mostly from ground-water discharge, except during short periods of and immediately after precipitation when most of the flow is derived from surface runoff. The runoff from upper Black Earth Creek basin decreased from an average of 8.72 inches per square mile of drainage area in 1955 to 5.55 inches in 1958; the decrease reflects the generally decreasing precipitation and declining water levels in the basin during that period. On July 10, 1958, the discharge from the basin was 0.367 cubic feet per second per square mile, and the greatest discharge was 0.84 cubic feet per second per square mile from the southwest subbasin. The ground water has an average temperature of about 50°F. It is a calcium magnesium bicarbonate type water and is very hard.

Wisconsin↗
USGS2021

Procedures for field data collection, processing, quality assurance and quality control, and archiving of relative- and absolute-gravity surveys

Repeat microgravity surveys carried out using relative- and absolute-gravity meters are useful for identifying changes in subsurface mass, such as the volume of water stored in an aquifer. These surveys require careful field procedures to achieve the part-per-billion accuracy required to measure the small changes in gravity relevant for hydrologic studies. This chapter describes techniques and methods for carrying out gravity surveys, requirements for assuring high-quality survey results, and data processing and archival procedures. The focus is on acquiring and documenting repeat gravity surveys for monitoring changes in groundwater storage. Similar gravity surveys may be completed to evaluate other causes of mass change, such as those caused by magma movement below volcanoes. The methods are also useful for one-time surveys that map spatial gravity variations associated with geologic structures such as faults or sedimentary basins. Repeat microgravity surveys can be carried out using relative-gravity meters, absolute-gravity meters, or both. Specific locations, known as gravity stations, are visited during each survey. Most commonly, absolute- and relative-gravity are combined using the least-squares method of network adjustment, much like benchmark elevations and relative-height differences in a leveling network. This chapter primarily describes the use of the A-10 absolute-gravity meter manufactured by Micro-g LaCoste, Inc., and relative-gravity meters made by LaCoste & Romberg (no longer in production) and ZLS Corporation, Inc. Field and office procedures are similar for other instruments such as the FG-5 absolute-gravity meter and Scintrex relative-gravity meters, but some adaptation may be required. Quality control for absolute-gravity data focuses primarily on proper field procedures and maintaining the time and distance calibration of the instrument. Quality control for relative-gravity surveys requires careful field procedures, an understanding of how the meter is behaving while in the field, and appropriate postprocessing. The techniques and methods described in this chapter were developed over 30 years at the USGS Arizona Water Science Center and the Southwest Gravity Program and are the basis for many studies on groundwater-storage change and geologic structure. A description of the Program and complete bibliography is available at https://www.usgs.gov/centers/az-water/science/azwsc-capabilities-hydrologic-gravity-monitoring .

Techniques and Methods↗
USGS1989

Hydrogeology and water quality near a solid- and hazardous-waste landfill, Northwood, Ohio

Hydrogeology and water quality of ground water and selected streams were evaluated near a landfill in northwestern Ohio. The landfill is used for codisposal of solid and hazardous waste. Water-level and geologic data were collected from 36 wells and 3 surface-water sites during the period November 1983 to November 1985. Water-quality samples were collected from 18 wells and 3 surface-water sites this during this same period. The primary aquifers in the area are the Greenfield Dolomite and underlying Lockport Dolomite of Silurian age. These bedrock carbonates are overlain by two clay tills of Wisconsin age. The tills are capped by a glacial lake clay. The tills generally are saturated, but do not yield sufficient water to be considered an aquifer. Two wells in the study area yield water, in part, from discontinuous deposits of outwash sand and gravel at the lower till-bedrock interface. Regional ground-water flow is from southwest to northeast; local flow is influenced by a ground-water mound centered under the northernmost cells of the landfill. Water levels in wells penetrating refuse within the landfill and the presence of leachate seeps indicate that the refuse is saturated. Head relations among the landfill, till, and dolomite aquifer indicate a vertical component of flow downward from the landfill to the dolomite aquifer. Water levels near the landfill fluctuate as much as 14 feet per year, in contrast to fluctuations of less than 3 feet per year in wells upgradient landfill. Ground waters from wells completed in the dolomite aquifer and glacial till were found to have major-iron concentrations controlled, in large part, by reaction with calcite, dolomite, and other minerals in the aquifer. Only minor departures from equilibrium mineral saturation were noted for ground water, except in wells affected by cement/grout contamination. Molal ratios of calcuim:magnesium in ground water suggest a similar chemical evolution of waters throughout the dolomite aquifer in the study area. Stable-isotope ratios of oxygen and hydrogen indicate the source of water in the till unit and dolomite aquifer is atmospheric precipitation. Elevated levels of total dissolved solids, boron, ammonia, and iron in the leachate and in wells downgradient of the landfill may indicate mixing of ground water with leachate. Oxygen and hydrogen stable-isotope ratios were used to differentiate waters from the glacial till and dolomite aquifer. Isotope ratios also show a shift off the local mixing line for leachate and for a well just downgradient from the landfill. The shift to heavier values of o D in the well water may be indicative of leachate mixing with ground water. The effect of this mixing denoted by hydrologic, isotopic, and chemical-quality data is limited mostly to elevated levels of the common ions. Analysis did not indicate significant levels of toxic metals or organic contaminants except phenol, which was present at concentrations of from 1 to 5 micrograms per liter in six wells. Analysis of water-quality data from nearby streams suggest that surface leaching from the landfill does not significantly affect stream-water quality, but may contribute to higher level of trace metals in the streambed sediments.

Water-Resources Investigations Report↗
USGS2017

Groundwater levels, geochemistry, and water budget of the Tsala Apopka Lake system, west-central Florida, 2004–12

Tsala Apopka Lake is a complex system of lakes and wetlands, with intervening uplands, located in Citrus County in west-central Florida. It is located within the 2,100 square mile watershed of the Withlacoochee River, which drains north and northwest towards the Gulf of Mexico. The lake system is managed by the Southwest Florida Water Management District as three distinct “pools,” which from upstream to downstream are referred to as the Floral City Pool, Inverness Pool, and Hernando Pool. Each pool contains a mixture of deep-water lakes that remain wet year round, ephemeral (seasonal) ponds and wetlands, and dry uplands. Many of the major deep-water lakes are interconnected by canals. Flow from the Withlacoochee River, when conditions allow, can be diverted into the lake system. Flow thorough the canals can be used to control the distribution of water between the three pools. Flow in the canals is controlled using structures, such as gates and weirs. Hydrogeologic units in the study area include a surficial aquifer consisting of Quaternary-age sediments, a discontinuous intermediate confining unit consisting of Miocene- and Pliocene-age sediments, and the underlying Upper Floridan aquifer, which consists of Eocene- and Oligocene-age carbonates. The fine-grained quartz sands that constitute the surficial aquifer are generally thin, typically less than 25 feet thick, within the vicinity of Tsala Apopka Lake. A thin, discontinuous, sandy clay layer forms the intermediate confining unit. The Upper Floridan aquifer is generally unconfined in the vicinity of Tsala Apopka Lake because the intermediate confining unit is discontinuous and breached by numerous karst features. In the study area, the Upper Floridan aquifer includes the upper Avon Park Formation and Ocala Limestone. The Ocala Limestone is the primary source of drinking water and spring flow in the area. The objectives of this study are to document the interaction of Tsala Apopka Lake, the surficial aquifer, and the Upper Floridan aquifer; and to estimate an annual water budget for each pool and for the entire lake system for 2004–12. The hydrologic interactions were evaluated using hydraulic head and geochemical data. Geochemical data, including major ion, isotope, and age-tracer data, were used to evaluate sources of water and to distinguish flow paths. Hydrologic connection of the surficial environment (lakes, ponds, wetlands, and the surficial aquifer) was quantified on the basis of a conceptualized annual water-budget model. The model included the change in surface water and groundwater storage, precipitation, evapotranspiration, surface-water inflow and outflow, and net groundwater exchange with the underlying Upper Floridan aquifer. The control volume for each pool extended to the base of the surficial aquifer and covered an area defined to exceed the maximum inundated area for each pool during 2004–12 by 0.5 foot. Net groundwater flow was computed as a lumped value and was either positive or negative, with a negative value indicating downward or lateral leakage from the control volume and a positive value indicating upward leakage to the control volume. The annual water budget for Tsala Apopka Lake was calculated using a combination of field observations and remotely sensed data for each of three pools and for the composite three pool area. A digital elevation model at a 5-foot grid spacing and bathymetric survey data were used to define the land-surface elevation and volume of each pool and to calculate the changes in inundated area with change in lake stage. Continuous lake-stage and groundwater-level data were used to define the change in storage for each pool. The rainfall data used in the water-budget calculations were based on daily radar reflectance data and measured rainfall from weather stations. Evapotranspiration was computed as a function of reference evapotranspiration, adjusted to actual evapotranspiration using a monthly land-cover coefficient (based on evapotranspiration measurements at stations located in representative landscapes). Surface-water inflows and outflows were determined using stage data collected at a series of streamgages installed primarily at the water-control structures. Discharge was measured under varying flow regimes and ratings were developed for the water-control structures. The discharge data collected during the study period were used to calibrate a surface-water flow model for 2004–12. Flows predicted by the model were used in the water-budget analysis. Net groundwater flow was determined as the residual term in the water-budget equation. The results of the water-budget analysis indicate that rainfall was the largest input of water to Tsala Apopka Lake, whereas evapotranspiration was the largest output. For the 2004–12 analysis period, surface-water inflow accounted for 11 percent of the inputs, net groundwater inflow accounted for 1 percent of inputs (annual periods with positive net groundwater flow were included as inputs, while annual periods with negative net groundwater flow were counted as outputs), and rainfall accounted for the remaining 88 percent. For the same period, the outputs consisted of 2 percent surface-water outflow, 12 percent net groundwater outflow, and 86 percent evapotranspiration. Net groundwater inflows and surface-water/groundwater storage were negligible during the water-budget period but could be important components of the budget in individual years. The net groundwater flow was negative (downward) for 8 out of the 9 years modeled (2004–12), indicating that the Tsala Apopka Lake study area was primarily a recharge area for the underlying Upper Floridan aquifer during this time period. Groundwater-level elevation in paired wells (adjacent wells completed in the surficial aquifer and Upper Floridan aquifer) typically was higher in the surficial aquifer than the Upper Floridan aquifer. However, hydraulic head data indicate that the surficial aquifer often has discharge potential to the surface-water system, especially in the low lying areas near the major lakes. Surficial-aquifer water levels were often higher than lake stages, especially during wet periods, which is likely an indication of aquifer-to-lake seepage in these areas. East of the major lakes, hydraulic head data were nearly equal in the surficial aquifer and Upper Floridan aquifer, which is an indication that the Upper Floridan aquifer is unconfined. Based on deuterium and oxygen stable isotope data collected in December 2011 and December 2012, there was no evidence of recharge to the Upper Floridan aquifer from the wetlands east of the major lakes; aquifer isotopic ratios did not indicate an enriched source, which is typical of lake and wetland sources. West of the major lakes, there was evidence of enriched isotopic ratios in water samples from the Upper Floridan aquifer. Differences in hydraulic head at paired wells in the surficial aquifer and Upper Floridan aquifer indicated that the surficial aquifer has the potential to recharge the Upper Floridan aquifer in the western part of the pools and west of the major lakes.

Florida↗
USGS1973

Geologic appraisal of Paradox basin salt deposits for water emplacement

Thick salt deposits of Middle Pennsylvanian age are present in an area of 12,000 square miles in the Paradox basin of southeast Utah and southwest Colorado. The deposits are in the Paradox Member of the Hermosa Formation. The greatest thickness of this evaporite sequence is in a troughlike depression adjacent to the Uncompahgre uplift on the northeast side of the basin. The salt deposits consist of a cyclical sequence of thick halite units separated by thin units of black shale, dolomite, and anhydrite. Many halite units are several hundred feet thick and locally contain economically valuable potash deposits. Over much of the Paradox basin the salt deposits occur at depths of more than 5,000 feet. Only in a series of salt anticlines located along the northeastern side of the basin do the salt deposits rise to relatively shallow depths. The salt anticlines can be divided geographically and structurally into five major systems. Each system consists of a long undulating welt of thickened salt over which younger rocks are arched in anticlinal form. Locally there are areas along the axes of the anticlines where the Paradox Member was never covered by younger sediments. This allowed large-scale migration of Paradox strata toward and up through these holes in the sediment cover forming diapiric anticlines. The central or salt-bearing cores of the anticlines range in thickness from about 2,500 to 14,000 feet. Structure in the central core of the salt anticlines is the result of both regional-compression and flowage of the Paradox Member into the anticlines from adjacent synclines. Structure in the central cores of the salt anticlines ranges from relatively undeformed beds to complexly folded and faulted masses, in which stratigraphic continuity is undemonstrable. The presence of thick cap rock .over many of the salt anticlines is evidence of removal of large volumes of halite by groundwater. Available geologic and hydrologic information suggests that this is a relatively slow process and that any waste-storage or disposal sites in these structures should remain dry for hundreds of thousands of years. Trace to commercial quantities of oil and gas are found in all of the black shale-dolomite-anhydrite interbeds of the Paradox Member. These hydrocarbons constitute a definite hazard in the construction and operation of underground waste-storage or disposal facilities. However, many individual halite beds are of. sufficient thickness that a protective seal of halite can be left between the openings and the gassy beds. A total of 12 different localities were considered to be potential waste-storage or disposal sites in the Paradox basin. Two Sharer dome and Salt Valley anticline, were considered to have the most favorable characteristics.

Utah↗
USGS2003

Seasonal movement of the Slumgullion landslide determined from global positioning system surveys and field instrumentation, July 1998-March 2002

Measurements of landslide movement made by global positioning system surveys and extensometers over a 3.5-year period show that the Slumgullion landslide in the San Juan Mountains of southwest Colorado moved throughout the monitoring period, but that daily velocities varied on a seasonal basis. Landslide velocities peaked in the early spring and summer in response to snowmelt and summer thunderstorms, respectively. Velocities were slowest in mid-winter when air and soil temperatures were coldest and precipitation was generally low and/or in the form of snow with a low water content. We hypothesize that the seasonal variability in velocities is due to ground-water levels and corresponding pore pressures that decrease when surface water is unavailable or cannot infiltrate frozen landslide material, and increase when surface water from melting snow or rainfall infiltrates unfrozen landslide material. We also suggest that patches of bouldery debris and fractures (created by continuous movement of the landslide) are conduits through which surface water can infiltrate, regardless of the frozen or unfrozen state of the landslide matrix material. Therefore, the availability of surface water is more important than landslide temperature in controlling the rate of landslide movement. This hypothesis is supported by field instrumentation data that show (1) landslide velocities coinciding with precipitation amounts regardless of the depth of freezing of landslide material, (2) spring and annual landslide velocities that were greatest when the depth of freezing was also the greatest, and (3) a rapid (several weeks or less) velocity and pore pressure response to rainfall. The persistent, but seasonally variable movement of the landslide, fits the bathtub model for landslide movement described by Baum and Reid [Baum, R.L., Reid, M.E., 2000. Ground water isolation by low-permeability clays in landslide shear zones. In: Bromhead, E.N., Dixon, N., Ibsen, M.-L. (Eds.), Landslides in Research, Theory and Practice. Proc. 8th Int. Symp. on Landslides, Cardiff, Wales, vol. 1, 139-144]. In their model, the landslide is isolated both mechanically and hydrologically from adjacent materials by low permeability clays. These clays cause the landslide to retain water. Our data support this model by suggesting that pore pressures at the basal landslide surface are always adequate to maintain landslide movement and that any infiltration of water at the surface of the landslide is adequate to rapidly increase landslide velocity. ?? 2002 Elsevier Science B.V. All rights reserved.

Engineering Geology↗
USGS2024

Hydrologic investigations and a preliminary conceptual model of the groundwater system at North Penn Area 1 Superfund Site, Souderton, Montgomery County, Pennsylvania

The U.S. Geological Survey (USGS) conducted hydrogeologic investigations, reviewed existing data, and developed a preliminary conceptual model of the groundwater system as part of technical support of the U.S. Environmental Protection Agency (EPA) at the North Penn Area 1 Superfund Site (hereafter, the NP1 Site) located within the Borough of Souderton in Montgomery County, Pennsylvania. Field work and monitoring took place during 2012–18. The area is underlain by sedimentary formations that form a fractured-rock aquifer used for drinking water and industrial supply. The EPA placed the Site on the National Priorities List in 1989, identifying tetrachloroethylene (PCE) and trichloroethylene (TCE) as contaminants of concern. During 2012–18, the USGS conducted field activities that included drilling an 82-foot (ft)-deep monitoring well (MG 2220) in 2016, reconstructing a 208-ft-deep former industrial production well (MG 668 [Granite Knitting Mill]), and collecting borehole geophysical and video logs and water levels from those and five additional wells, which ranged in depth from about 50 to 200 ft below land surface. Continuous water levels were collected during 2014–17, and a synoptic set of water levels were measured in April 2018 in the seven wells. The borehole geophysical logs (caliper, acoustic televiewer, natural gamma, single-point resistance, vertical flow, and fluid temperature and resistivity) and borehole video logs in the seven wells were evaluated to assess potential for lithologic correlation and to identify and describe water-bearing features, which included both low- and high-angle fractures and other openings oriented along dipping bedding planes, joints, or possible faults. Borehole geophysical logs collected by USGS in 1992 in a 300-ft-deep former production well near the Site were also evaluated. Few to no distinctive features were identified on geophysical logs (natural gamma and single-point resistance) that could be used for correlation, thus limiting this approach to determining local geologic structure. Extensive fracturing in the upper 62 ft of monitoring well MG 2220 indicates that the well was likely drilled through a zone of faulting, and other evidence of faulting is present in the area near the Site. Assessment of continuous water levels showed hydraulic connections among some wells as indicated by rising or falling water levels in response to changes in pumping rates at nearby wells. A map of water levels measured in April 2018 indicates potential for groundwater flow generally toward the stream to the south and southwest of the Site, but the limited water-level data are insufficient to describe vertical groundwater gradients or lateral gradients in any detail. Review of 1999–2022 volatile organic compound (VOC) monitoring data collected by the Pennsylvania Department of Environmental Protection for five monitoring wells indicates that the highest groundwater concentrations of PCE and TCE were found in samples from extraction well MG 2201 (S-1) downgradient from, and nearest to, the previously identified Site contaminant source area, and these concentrations fluctuated through time. PCE concentrations were higher than TCE concentrations in samples from all five monitoring wells and were much higher than TCE concentrations in samples from extraction well MG 2201 (S-1). Temporally variable recharge is a possible factor affecting observed fluctuations in PCE concentrations in groundwater samples from well extraction MG 2201 (S-1), as indicated by a general inverse relation between PCE concentrations and water levels in a nearby long-term observation well. The PCE concentration of 1,830 micrograms per liter (μg/L) in a May 2018 water sample from monitoring well MG 2220 was more than four times the PCE concentration of 444 μg/L in a December 2017 sample from the nearby extraction well MG 2201 (S-1), which is open to fewer fractures. Low concentrations of VOCs were measured in surface water at two stream sites downgradient from wells with the highest groundwater VOC concentrations at the Site, indicating that discharge of contaminated groundwater to the stream is likely. Development of a conceptual model of the groundwater system was constrained by limited data. In areas with no pumping, groundwater-flow directions generally are thought to be controlled by topography and geologic structure (bedding orientation) and likely to the south and southwest of the Site, with local flow directions affected by orientations of fractures, joints, and local faults. Additional investigations that could help improve the conceptual model of the groundwater system and help delineate the extent of groundwater contamination and its transport are discussed.

Pennsylvania↗
USGS2012

Water quality and the composition of fish and macroinvertebrate communities in the Devils and Pecos Rivers within and upstream from the Amistad National Recreation Area, Texas, 2005-7

To gain a better understanding of the water quality and status of fish and macroinvertebrate communities, the U.S. Geological Survey, in cooperation with the National Park Service and Amistad National Recreation Area, completed a reconnaissance-level survey of the water quality and fish and macroinvertebrate communities in the Devils and Pecos Rivers in and upstream from the Amistad National Recreation Area in southwest Texas during 2005–7. Water-quality conditions during the spring and summer months of 2005 in the Devils and Pecos Rivers were assessed at locations just upstream from the Amistad National Recreation Area, and the composition of fish and macroinvertebrate communities were assessed during 2006 and 2007 in and upstream from the Amistad National Recreation Area and Amistad Reservoir. Water-quality samples were collected at one site on both the Devils and Pecos Rivers. Fish and macroinvertebrates were collected at the water-quality sampling site on each river and at three additional sites on each river. The water-quality constituents of primary concern were total dissolved solids, chloride, sulfate, ammonia plus organic nitrogen, nitrate plus nitrite, orthophosphate, phosphorus, selenium, and selected pesticides. During the spring and summer of 2005, the concentrations of total dissolved solids ranged from 208 to 232 milligrams per liter (mg/L) in samples from the Devils River compared to 1,460 to 2,390 mg/L in samples from the Pecos River. Total dissolved solid concentrations measured in samples collected from the Devils River and Pecos River did not exceed the proposed State of Texas water-quality standard applicable for the segments of each river where samples were collected. During the spring and summer of 2005, chloride concentrations measured in samples collected in 2005 from the Devils River ranged from 11.6 to 12.9 mg/L, compared to chloride concentrations measured in samples collected from the Pecos River, which ranged from 519 to 879 mg/L. Chloride concentrations in samples collected from the Devils River in 2005 were less than the lower quartile (25th percentile) value of 14.0 mg/L reported for chloride concentrations in water-quality samples collected at the same sampling location during 1978–95 by the U.S. Geological Survey as part of the Hydrologic Benchmark Network program. The chloride concentrations measured in samples collected from the Pecos River during the spring and summer of 2005 represented a range of values similar to the interquartile range of 548 to 942 mg/L reported for samples collected during 1974–2007 at the same sampling location by the U.S. Geological Survey as part of the National Stream Quality Accounting Network program. None of the chloride concentrations measured in samples collected from the Devils or Pecos Rivers in 2005 exceeded applicable proposed State of Texas water-quality standards for chloride. Sulfate concentrations ranged from 7.55 to 8.20 mg/L in samples from the Devils River compared to 298 to 503 mg/L in samples from the Pecos River. Concentrations of sulfate did not exceed applicable proposed State of Texas water-quality standards. Ammonia plus organic nitrogen concentrations were reported as nitrogen ranged from 0.12 to 0.14 mg/L of nitrogen in samples collected from the Devils River compared to 0.15 to 0.32 mg/L of nitrogen in samples collected from the Pecos River. Ammonia plus organic nitrogen concentrations measured in samples collected from the Devils River in 2005 were less than the lower quartile (25th percentile) value of 0.23 mg/L of nitrogen for concentrations in water-quality samples collected at the same sampling location during 1978–95 by the U.S. Geological Survey as part of the Hydrologic Benchmark Network program. Ammonia plus nitrogen concentrations measured in samples collected from the Pecos River were similar to the range of historical concentrations measured in samples collected from the same Pecos River sampling location by the U.S. Geological Survey National Stream Quality Accounting Network program. Nitrate plus nitrite concentrations in samples from the Devils River and Pecos Rivers were within the historical range of concentrations for samples collected at the same locations on each river. Total phosphorous and orthophosphate concentrations were less than the laboratory reporting levels in the water samples from the Devils and Pecos Rivers. None of the selenium concentrations measured in samples collected during the spring and summer of 2005 from the Devils or Pecos Rivers exceeded the Texas Surface Water Quality Standards (chronic criterion of 5 μg/L or the acute criterion of 20 μg/L) established by the State for the protection of aquatic life. Concentrations of pesticides in the samples collected from the Devils and Pecos Rivers during March–August 2005 were very low and not present in detectable amounts (all reported concentrations were below laboratory reporting levels). The total number of fish species collected was the same in the Devils River and Pecos River, but the species found in the two rivers varied slightly. The number of fish species generally increased from the site farthest upstream to the site farthest downstream in the Devils River, and decreased between the site farthest upstream and site farthest downstream in the Pecos River. The redbreast sunfish was the most abundant species collected in the Devils River, and the blacktail shiner was the most abundant species collected in the Pecos River. Comparing the species from each river, the percentage of omnivorous fish species was larger at the more downstream sites closer to Amistad Reservoir, and the percentage of species tolerant of environmental stressors was larger in the Pecos River. The fish community, assessed on the basis of the number of shared species among the sites sampled, was more similar to the fish community at the other sites on the same river than it was to the fish community from any other site in the other river. More macroinvertebrate taxa were collected in the Devils River than in the Pecos River. The largest number of macroinvertebrate taxa were from the site second farthest downstream on the Devils River, and the smallest numbers of macroinvertebrate taxa were from the farthest downstream site on the Pecos River. Mayflies were more common in the Devils River, and caddisflies were less common than mayflies at most sites. Net-spinning caddisflies were more common at the Devils River sites. The combined percent of mayfly, caddisfly, and stonefly taxa was generally larger at the Pecos River sites. Riffle beetles were the most commonly collected beetle taxon among all sites, and water-penny beetles were only collected at the Pecos River sites. A greater number of true midge taxa were collected more than any other taxa at the genus and species taxonomic level. Non-insect macroinvertebrate taxa were more common at the Devils River sites. Corbicula sp. (presumably the introduced Asian clam) was found at sites in both rivers, and amphipods were more abundant in the Devils River. The Margalef species richness index, based on aquatic insect taxa only, was larger at the Devils River sites than at the Pecos River sites. The Hilsenhoff's biotic index was largest at the site farthest downstream in the Devils River and smallest at the site second farthest downstream in the Pecos River. Overall similarity among sites based on the number of shared macroinvertebrate taxa indicated that each site is more similar to other sites on the same river than to sites on the other river.

Texas↗
USGS1985

Hydrology of the alluvial, buried channel, basal Pleistocene and Dakota aquifers in west-central Iowa

A ground-water resources investigation in west-central Iowa indicates that water is available from alluvial, buried channel, basal Pleistocene, and Dakota aquifers. The west-central Iowa area includes Audubon, Carrol1, Crawford, Greene, Guthrie, Harrison, Monona, and Shelby Counties. Nine alluvial aquifers consisting of sand and gravel are in the valleys of the Little Sioux, Maple, Soldier, Boyer, West Nishnabotna, East Nishnabotna, South Raccoon, Middle Raccoon, and North Raccoon Rivers. These aquifers contain about 870,000 acre-feet of water that is potentially available to wells. Potential well yields generally are less than 50 gallons per minute. The water generally is very hard (greater than 180 milligrams per liter hardness as calcium carbonate), is a calcium bicarbonate type, and has an average dissolved-solids concentration of less than 600 milligrams per liter. Seven buried channel aquifers Anthon, Denison, Fremont, Hardin Creek, Adaza, Beaver, and Bagley consisting of sand and gravel, underlie about 594 square miles in west-central Iowa and contain about 65,000 acre-feet of water potentially available to wells. Potential well yields of as much as 1,000 gallons per minute are possible in a few of the deeper and thicker parts of some of the buried channel aquifers, but well yields of 10 to 100 gallons per minute are more common. Water generally is very hard, is a calcium bicarbonate type, and has an average dissolved-solids concentration of 400 to 800 milligrams per liter in the shallow buried channel aquifers in Carrol1, Greene, and Guthrie Counties. In the deep buried channel aquifer in Audubon, Crawford, Harrison, Monona, and Shelby counties, the water generally is very hard, is a sodium sulfate or calcium sulfate type, and has an average dissolved-solids concentrations of 3,000 milligrams per liter. The basal Pleistocene aquifer is at the base of the Pleistocene deposits on many bedrock ridges and consists of sand and gravel. Estimated well yields of as much as 500 gallons per minute can be obtained from the aquifer; however, 5 to 50 gallons per minute are more common. Water from the basal Pleistocene aquifer generally is very hard, is a calcium bicarbonate or calcium sulfate type, and has an average dissolved-solids concentration of 1,000 milligrams per liter. The Dakota aquifer consists of the saturated sandstone and gravel units in the Dakota Formation. Isolated erosional remnants of the Dakota Formation form the caps of many bedrock ridges. The Dakota Formation is thickest where the bedrock surface is relatively high and flat, forming an ancient, buried, surface-water divide between southwest and southeast trending buried drainages in Audubon, Carroll, and Guthrie Counties. Sandstone thickness of as much as 150 feet exists in Guthrie County, but an average thickness of 30 feet is more common. Water from wells less than 200 feet deep generally is a calcium bicarbonate type and has an average dissolved-solids concentration of 650 milligrams per liter. Water from wells more than 200 feet deep generally is a calcium sulfate or sodium bicarbonate type and has an average dissolved-solids concentrations of 2,200 milligrams per liter.

Iowa↗
USGS2009

Magnetotelluric and audiomagnetotelluric groundwater survey along the Humu'ula portion of Saddle Road near and around the Pohakuloa Training Area, Hawaii

The Pohakuloa Training Area (PTA), operated by the U.S. Army on the Big Island of Hawaii, is in need of a reliable potable water supply to sustain ongoing operations by staff and trainees. In an effort to acquire baseline hydrologic data with which to develop a plan for providing that water, a series of magnetotelluric (MT) geophysical surveys was performed that spanned the Mauna Loa/Mauna Kea Saddle region of Hawaii Island. These surveys provided electrical resistivity profiles and resistivity maps at several elevations along the axis of the field measurements that can be interpreted to yield information on the depth to the water table. In 2004 a preliminary sequence of 23 audiomagnetotelluric (AMT) soundings was collected along Saddle Road extending from the Waikii Ranch area, west of the PTA, to Department of Hawaiian Home Lands Humu'ula properties east of the Mauna Kea access road. The results of those soundings showed that highly resistive rocks, consistent with dry basalts, were present to depths of at least one kilometer, the maximum depth to which the AMT technique can reliably reach in Hawaii's rocks. A second survey was conducted in 2008 using MT instruments capable of recovering resistivity data to depths of several kilometers below sea level where saturated formations are known to exist. A total of 30 MT soundings was performed along a roughly east to west transect that extended from the (recently acquired) Keamuku PTA lands on the west to as far as the County of Hawaii's upper Kaumana water supply well to the east. Inversion and processing of the field data yielded an electrical cross-section following the Saddle that roughly parallels the geologic contact between the Mauna Kea and Mauna Loa lavas. Several additional electrical sections were constructed normal to the main transect to investigate the three-dimensional nature of the contact. These resistivity data and models suggest that the elevation of saturated rock in places are 400 to 600 meters above mean sea level beneath the surveyed region. Highest elevations for water-saturated zones based upon preferred electrical models are located between training area 3 and training area 6 southwest of training area 4.

Hawai'i↗
USGS2008

An Update of Hydrologic Conditions and Distribution of Selected Constituents in Water, Snake River Plain Aquifer and Perched-Water Zones, Idaho National Laboratory, Idaho, Emphasis 2002-05

Radiochemical and chemical wastewater discharged since 1952 to infiltration ponds, evaporation ponds, and disposal wells at the Idaho National Laboratory (INL) has affected water quality in the Snake River Plain aquifer and perched-water zones underlying the INL. The U.S. Geological Survey, in cooperation with the U.S. Department of Energy, maintains ground-water monitoring networks at the INL to determine hydrologic trends, and to delineate the movement of radiochemical and chemical wastes in the aquifer and in perched-water zones. This report presents an analysis of water-level and water-quality data collected from aquifer and perched-water wells in the USGS ground-water monitoring networks during 2002-05. Water in the Snake River Plain aquifer primarily moves through fractures and interflow zones in basalt, generally flows southwestward, and eventually discharges at springs along the Snake River. The aquifer is recharged primarily from infiltration of irrigation water, infiltration of streamflow, ground-water inflow from adjoining mountain drainage basins, and infiltration of precipitation. From March-May 2001 to March-May 2005, water levels in wells declined throughout the INL area. The declines ranged from about 3 to 8 feet in the southwestern part of the INL, about 10 to 15 feet in the west central part of the INL, and about 6 to 11 feet in the northern part of the INL. Water levels in perched water wells declined also, with the water level dropping below the bottom of the pump in many wells during 2002-05. For radionuclides, concentrations that equal 3s, wheres s is the sample standard deviation, represent a measurement at the minimum detectable concentration, or 'reporting level'. Detectable concentrations of radiochemical constituents in water samples from wells in the Snake River Plain aquifer at the INL generally decreased or remained constant during 2002-05. Decreases in concentrations were attributed to decreased rates of radioactive-waste disposal, radioactive decay, changes in waste-disposal methods, and dilution from recharge and underflow. In October 2005, reportable concentrations of tritium in ground water ranged from 0.51+or-0.12 to 11.5+or-0.6 picocuries per milliliter and the tritium plume extended south-southwestward in the general direction of ground-water flow. Tritium concentrations in water from several wells southwest of the Idaho Nuclear Technology and Engineering Center (INTEC) decreased or remained constant as they had during 1998-2001, with the exception of well USGS 47, which increased a few picocuries per milliliter. Most wells completed in shallow perched water at the Reactor Technology Complex (RTC) were dry during 2002---05. Tritium concentrations in deep perched water exceeded the reporting level in nine wells at the RTC. The tritium concentration in water from one deep perched water well exceeded the reporting level at the INTEC. Concentrations of strontium-90 in water from 14 of 34 wells sampled during October 2005 exceeded the reporting level. Concentrations ranged from 2.2+or-0.7 to 33.1+or-1.2 picocuries per liter. However, concentrations from most wells remained relatively constant or decreased since 1989. Strontium-90 has not been detected within the eastern Snake River Plain aquifer beneath the RTC partly because of the exclusive use of waste-disposal ponds and lined evaporation ponds rather than the disposal well for radioactive-wastewater disposal at RTC. At the RTC, strontium-90 concentrations in water from six wells completed in deep perched ground water exceeded the reporting level during 2002-05. At the INTEC, the reporting level was exceeded in water from three wells completed in deep perched ground water. During 2002-05, concentrations of plutonium-238, and plutonium-239, -240 (undivided), and americium-241 were less than the reporting level in water samples from all wells sampled at the INL. During 2002-05, concentrations of cesium-137 in water from all wells sa

Scientific Investigations Report↗