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Groundwater & aquifers

Find USGS groundwater and aquifer publications. Search an aquifer name or place, compare source-supplied dates and open reports for hydrogeology, recharge and water-level studies.

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At least 1,711 records · Page 95Linked to original sources

USGS2013

Water-table and Potentiometric-surface altitudes in the Upper Glacial, Magothy, and Lloyd aquifers beneath Long Island, New York, April-May 2010

The U.S. Geological Survey (USGS), in cooperation with State and local agencies, systematically collects groundwater data at varying measurement frequencies to monitor the hydrologic conditions on Long Island, New York. Each year during April and May, the USGS conducts a synoptic survey of water levels to define the spatial distribution of the water table and potentiometric surfaces within the three main water-bearing units underlying Long Island—the upper glacial, Magothy, and Lloyd aquifers (Smolensky and others, 1989)—and the hydraulically connected Jameco (Soren, 1971) and North Shore aquifers (Stumm, 2001). These data and the maps constructed from them are commonly used in studies of Long Island’s hydrology and are used by water managers and suppliers for aquifer management and planning purposes. Water-level measurements made in 503 monitoring wells, a network of observation and supply wells, and 16 streamgage locations across Long Island during April–May 2010 were used to prepare the maps in this report. Measurements were made by the wetted-tape method to the nearest hundredth of a foot. Water-table and potentiometric-surface altitudes in these aquifers were contoured by using these measurements. The water-table contours were interpreted by using water-level data collected from 16 streamgages, 349 observation wells, and 1 supply well screened in the upper glacial aquifer and (or) shallow Magothy aquifer; the Magothy aquifer’s potentiometric-surface contours were interpreted from measurements at 67 observation wells and 27 supply wells screened in the middle to deep Magothy aquifer and (or) the contiguous and hydraulically connected Jameco aquifer. The Lloyd aquifer’s potentiometric-surface contours were interpreted from measurements at 55 observation wells and 4 supply wells screened in the Lloyd aquifer or the contiguous and hydraulically connected North Shore aquifer. Many of the supply wells are in continuous operation and, therefore, were turned off for a minimum of 24 hours before measurements were made so that the water levels in the wells could recover to the level of the potentiometric head in the surrounding aquifer. Full recovery time at some of these supply wells can exceed 24 hours; therefore, water levels measured at these wells are assumed to be less accurate than those measured at observation wells, which are not pumped (Busciolano, 2002). In this report, all water-level altitudes are referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29). Hydrographs are included on these maps for selected wells that are instrumented with recording equipment. These hydrographs are representative of the 2010 water year1 to show the changes that have occurred throughout that period. The synoptic survey water level measured at the well is included on each hydrograph.

New York↗

USGS2023

Geologic framework and hydrostratigraphy of the Edwards and Trinity aquifers within northern Bexar and Comal Counties, Texas

During 2020–22, the U.S. Geological Survey, in cooperation with the Edwards Aquifer Authority, revised a previous publication that described the geologic framework and hydrostratigraphy of the Edwards and Trinity aquifers within northern Bexar and Comal Counties, Texas. This report presents the refined maps and descriptions of geologic framework and hydrostratigraphy of the Edwards and Trinity aquifers within northern Bexar and Comal Counties that resulted from additional field data. Two informal geologic units and their corresponding informal hydrostratigraphic unit (HSU) names are introduced in this report; these informal units were identified during geologic mapping work done in counties adjoining the study area. Hydrostratigraphically, the rocks exposed in the study area represent a section of the upper confining unit to the Edwards aquifer, the Edwards aquifer, the upper zone of the Trinity aquifer, the middle zone of the Trinity aquifer, and the lower confining unit to the middle zone of the Trinity aquifer. The Washita, Eagle Ford, Austin, and Taylor Groups are generally considered to be the upper confining unit to the Edwards aquifer. The Edwards aquifer was subdivided into nine informally named HSUs (from top to bottom) as follows: I, II, III, IV, V, VI, VII, Seco Pass, and VIII. The upper zone of the Trinity aquifer was subdivided into five informal HSUs and two subunits (from top to bottom) as follows: cavernous, Camp Bullis, upper evaporite, fossiliferous (subunits: upper and lower), and lower evaporite. The middle zone of the Trinity aquifer was subdivided into nine named HSUs (from top to bottom) as follows: Bulverde, Little Blanco, Twin Sisters, Doeppenschmidt, Herff Falls (where present), Rust, Honey Creek, Hensell, and Cow Creek. The middle zone of the Trinity aquifer is underlain by the confining Hammett HSU. Groundwater recharge and flow paths in the study area are influenced not only by the hydrostratigraphic characteristics of the individual HSUs but also by faults and fractures.

Texas↗

USGS2008

Occurrence, Distribution, Sources, and Trends of Elevated Chloride Concentrations in the Mississippi River Valley Alluvial Aquifer in Southeastern Arkansas

Water-quality data from approximately 2,500 sites were used to investigate the distribution of chloride concentrations in the Mississippi River Valley alluvial aquifer in southeastern Arkansas. The large volume and areal distribution of the data used for the investigation proved useful in delineating areas of elevated (greater than 100 milligrams per liter) chloride concentrations, assessing potential sources of saline water, and evaluating trends in chloride distribution and concentration over time. Irrigation water containing elevated chloride concentrations is associated with negative effects to rice and soybeans, two of the major crops in Arkansas, and a groundwater chloride concentration of 100 milligrams per liter is recommended as the upper limit for use on rice. As such, accurately delineating areas with high salinity ground water, defining potential sources of chloride, and documenting trends over time is important in assisting the agricultural community in water management. The distribution and range of chloride concentrations in the study area revealed distinct areas of elevated chloride concentrations. Area I includes an elongated, generally northwest-southeast trending band of moderately elevated chloride concentrations in the northern part of the study area. This band of elevated chloride concentrations is approximately 40 miles in length and varies from approximately 2 to 9 miles in width, with a maximum chloride concentration of 360 milligrams per liter. Area II is a narrow, north-south trending band of elevated chloride concentrations in the southern part of the study area, with a maximum chloride concentration of 1,639 milligrams per liter. A zone of chloride concentrations exceeding 200 milligrams per liter is approximately 25 miles in length and 5 to 6 miles in width. In Area I, low chloride concentrations in samples from wells completed in the alluvial aquifer next to the Arkansas River and in samples from the upper Claiborne aquifer, which underlies the alluvial aquifer, indicate that leakage from the river and upward flow of saline water in underlying aquifers are not likely sources for the saline water in the alluvial aquifer in Area I. A good comparison was noted for chloride concentrations in Area I and surface geomorphology. In the majority of cases, elevated chloride concentrations occurred in backswamp deposits, with low concentrations (less than 50 milligrams per liter) in areas of active or abandoned channel deposits. The fine-grained, clay-rich deposits associated with backswamp areas likely restrict recharge, induce increased ratios between evapotranspiration and recharge, and experience minimal flushing of salts concentrated during evapotranspiration. In Area II, chloride isoconcentration maps of the underlying upper Claiborne aquifer, in addition to samples from wells completed in the middle and lower Claiborne aquifers, showed a similar chloride distribution to that of the alluvial aquifer with decreasing chloride concentrations to the east of the zone of elevated chloride concentrations, which suggests a deeper source of saline water that affects Tertiary and Quaternary aquifer systems. Mixing curves developed from bromide/chloride ratios in water samples from the alluvial aquifer, Tertiary aquifers, and samples of brine water from the Jurrasic Smackover Formation additionally discounted upward flow of saline water from underlying Tertiary formations as a potential mechanism for salinity in the alluvial aquifer in Area II. A review of information on oil exploration wells in Chicot County revealed that most of these wells were drilled from 1960 to 1980, after the elevated chloride concentrations were detected in the early 1950s. The elongated nature of the zone of elevated chloride concentrations in Area II suggests a line source or linear conduit connection with the source. Maps of a fractured limestone in the Smackover Formation in Arkansas, Mississippi, and Louisiana for purpose

Scientific Investigations Report↗

USGS2015

Effects of groundwater pumping on agricultural drains in the Tule Lake subbasin, Oregon and California

Since 2001, irrigators in the upper Klamath Basin have increasingly turned to groundwater to compensate for reductions in surface-water allocation caused by shifts from irrigation use to instream flows for Endangered Species Act listed fishes. The largest increase in groundwater pumping has been in and around the Bureau of Reclamation’s Klamath Irrigation Project, which includes the Tule Lake subbasin in the southern part of the upper Klamath Basin. Agricultural drains on the Klamath Project are an important source of water for downstream irrigators and for the Tule Lake and Lower Klamath Lake National Wildlife Refuges. U.S. Geological Survey regional groundwater-flow model simulations and records of irrigation-return flow pumped from the Tule Lake subbasin into the adjacent Lower Klamath Lake subbasin have indicated that water-level declines from pumping may be causing decreased flow of shallow groundwater to agricultural drains. To better define the effect of increased pumping on drain flow and on the water balance of the groundwater system, the annual water volume pumped from drains in three subareas of the Tule Lake subbasin was estimated and a fine-grid, local groundwater model of the Tule Lake subbasin was constructed. Results of the agricultural-drain flow analysis indicate that groundwater discharge to drains has decreased such that flows in 2012 were approximately 32,400 acre-ft less than the 1997–2000 average flow. This decrease was concentrated in the northern and southeastern parts of the subbasin, which corresponds with the areas of greatest groundwater pumping. Model simulation results of the Tule Lake subbasin groundwater model indicate that increased supplemental pumping is the dominant stress to the groundwater system in the subbasin. Simulated supplemental pumping and decreased recharge from irrigation between 2000 and 2010 totaled 323,573 acre-ft, 234,800 acre-ft (73 percent) of which was from supplemental pumping. The response of the groundwater system to this change in stress included about 180,500 acre-ft (56 percent) of decreased groundwater discharge to drains and a 126,000 acre-ft (39 percent) reduction in aquifer storage. The remaining 5 percent came from reduced groundwater flow to other model boundaries, including the Lost River, the Tule Lake sumps, and interbasin flow.

California, Oregon↗

USGS2017

Baseline assessment of groundwater quality in Pike County, Pennsylvania, 2015

The Devonian-age Marcellus Shale and the Ordovician-age Utica Shale, which have the potential for natural gas development, underlie Pike County and neighboring counties in northeastern Pennsylvania. In 2015, the U.S. Geological Survey, in cooperation with the Pike County Conservation District, conducted a study that expanded on a previous more limited 2012 study to assess baseline shallow groundwater quality in bedrock aquifers in Pike County prior to possible extensive shale-gas development. Seventy-nine water wells ranging in depths from 80 to 610 feet were sampled during June through September 2015 to provide data on the presence of methane and other aspects of existing groundwater quality in the various bedrock geologic units throughout the county, including concentrations of inorganic constituents commonly present at low values in shallow, fresh groundwater but elevated in brines associated with fluids extracted from geologic formations during shale-gas development. All groundwater samples collected in 2015 were analyzed for bacteria, dissolved and total major ions, nutrients, selected dissolved and total inorganic trace constituents (including metals and other elements), radon-222, gross alpha- and gross beta-particle activity, dissolved gases (methane, ethane, and propane), and, if sufficient methane was present, the isotopic composition of methane. Additionally, samples from 20 wells distributed throughout the county were analyzed for selected man-made volatile organic compounds, and samples from 13 wells where waters had detectable gross alpha activity were analyzed for radium-226 on the basis of relatively elevated gross alpha-particle activity. Results of the 2015 study show that groundwater quality generally met most drinking-water standards for constituents and properties included in analyses, but groundwater samples from some wells had one or more constituents or properties, including arsenic, iron, manganese, pH, bacteria, sodium, chloride, sulfate, total dissolved solids, and radon-222, that did not meet (commonly termed failed or exceeded) primary or secondary maximum contaminant levels (MCLs) or Health Advisories (HA) for drinking water. Except for iron, dissolved and total concentrations of major ions and most trace constituents generally were similar. Only 1 of 79 well-water samples had any constituent that exceeded a MCL, with an arsenic concentration of about 30 micrograms per liter (µg/L) that was higher than the MCL of 10 µg/L. However, total arsenic concentrations were higher than the HA of 2 µg/L in samples from another 12 of 79 wells (about 15 percent). Secondary maximum contaminant levels (SMCLs) were exceeded most frequently by pH and concentrations of iron and manganese. The pH was outside of the SMCL range of 6.5–8.5 in samples from 24 of 79 wells (30 percent), ranging from 5.5 to 9.2; more samples had pH values less than 6.5 than had pH values greater than 8.5. Total iron concentrations typically were much greater than dissolved iron concentrations, indicating substantial presence of iron in particulate phase, and exceeded the SMCL of 300 µg/L more often [35 of 79 samples (44 percent)] than dissolved iron concentrations [samples from 8 of 79 wells (10 percent)]. Total manganese concentrations exceeded the SMCL of 50 µg/L in samples from 31 of 79 wells (39 percent) and the HA of 300 µg/L in samples from 13 of 79 wells (about 16 percent). A few (1–2) samples had concentrations of sodium, chloride, sulfate, or TDS higher than the SMCLs of 60, 250, 250, and 500 mg/L, respectively. However, dissolved sodium concentrations were higher than the HA of 20 mg/L in samples from 15 of 79 wells (nearly 20 percent). Total coliform bacteria were detected in samples from 25 of 79 wells (32 percent) but Escherichia coli were not detected in any sample. Radon-222 activities ranged from 11 to 5,100 picocuries per liter (pCi/L), with a median of 1,440 pCi/L, and exceeded the proposed and the alternate proposed drinking-water standards of 300 and 4,000 pCi/L, respectively, in samples from 60 of 79 wells (75 percent) and in samples from 2 of 79 wells (3 percent), respectively. Groundwater samples from all wells were analyzed for dissolved methane by one contract laboratory that determined water from 19 of the 79 wells (24 percent) had concentrations of methane greater than the reporting level of 0.010 milligrams per liter (mg/L) with a maximum methane concentration of 2.5 mg/L. Methane concentrations in 18 replicate samples submitted to a second laboratory for dissolved gas and isotopic analysis generally were higher by as much as a factor of 2.7 from those determined by the first laboratory, indicating potential bias related to combined sampling and analytical methods, and therefore, caution needs to be used when comparing methane results determined by different methods. The isotopic composition of methane in 9 of 10 samples with sufficient dissolved methane (about 0.3 mg/L) for isotopic analysis is consistent with values reported for methane of microbial origin produced through carbon dioxide reduction; an isotopic shift in 1 or 2 samples may indicate subsequent methane oxidation. The low concentrations of ethane relative to methane in these samples further indicate that the methane may be of microbial origin. Groundwater samples with relatively elevated methane concentrations (near or greater than 0.3 mg/L) also had chemical compositions that differed in some respects from groundwater with relatively low methane concentrations (less than 0.3 mg/L) by having higher pH (greater than 8) and higher concentrations of sodium, lithium, boron, fluoride, arsenic, and bromide and chloride/bromide ratios indicative of mixing with a small amount of brine of probable natural occurrence. The spatial distribution of groundwater compositions differs by topographic setting and lithology and generally shows that (1) relatively dilute, slightly acidic, oxygenated, calcium-carbonate type waters tend to occur in the uplands underlain by the undivided Poplar Gap and Packerton members of the Catskill Formation in southwestern Pike County; (2) waters of near neutral pH with the highest amounts of hardness (calcium and magnesium) generally occur in areas of intermediate altitudes underlain by other members of the Catskill Formation; and (3) waters with pH values greater than 8, low oxygen concentrations, and the highest arsenic, sodium, lithium, bromide, and methane concentrations can be present in deep wells in uplands but most frequently occur in stream valleys, especially at low altitudes (less than about 1,200 feet above North American Vertical Datum of 1988) where groundwater may be discharging regionally, such as to the Delaware River in northern and eastern Pike County. Thus, the baseline assessment of groundwater quality in Pike County prior to gas-well development shows that shallow (less than about 1,000 feet deep) groundwater generally meets primary drinking-water standards for inorganic constituents but varies spatially, with methane and some constituents present in high concentrations in brine (and connate waters from gas and oil reservoirs) present at low to moderate concentrations in some parts of Pike County.

Pennsylvania↗

USGS2025

Simulation of groundwater flow in Wake County, North Carolina, 2000 through 2070

In 2019, the U.S. Geological Survey and Wake County Environmental Services began a collaborative study to evaluate groundwater resources and long-term groundwater availability in the county’s fractured-rock groundwater system. Wake County, in central North Carolina, is experiencing rapid population growth, associated land development, and changing water use. Hydrogeologic data including groundwater levels, aquifer testing, borehole fracture flow measurements, water-quality samples, and groundwater age-dating tracers were collected, along with findings from previous investigations, to help inform a conceptual model of the flow system used to develop a modular three-dimensional finite-difference groundwater-flow model (MODFLOW) for simulating historical and future groundwater conditions from 2000 to 2070. Hydraulic conductivity and transmissivity ranges were estimated from 17 slug tests and 21 borehole-flow measurements. Groundwater-quality analytical results from 19 sampling sites indicate that oxidation-reduction (redox) conditions varied within the regolith and bedrock and that minimal evaporation occurred before recharge entered the groundwater system. Age dating revealed mixtures of older and younger water, ranging from the 1940s to the 1990s—indicating variable flow pathways of recharge within permeable bedrock fracture zones. To simplify the complex fractured-rock groundwater system, two layers representing the regolith and the fractured bedrock were used in the MODFLOW model. Model calibration included parameter estimation and provided a reasonable fit to observed groundwater levels and estimated stream base flows. The model forecast scenarios incorporated future climate-model data for two emissions scenarios with land cover change projections to simulate potential impacts to future groundwater levels, recharge, and base flows. Recharge and base flow projections were largely within historical ranges, with no apparent long-term trends, but did indicate a slight downward shift in median values—likely, in part, because of differences in spatial resolution of input climate datasets. Seasonal patterns were consistent with historical data, with projections of possible increases in future winter recharge. Model limitations are discussed, and additional monitoring and model refinement needs are highlighted to support decision making for local groundwater management.

North Carolina↗

USGS2026

Mapping groundwater recharge potential and an assessment of available hydrogeologic data within five selected focus areas in eastern Nebraska

Groundwater recharge potential was mapped using airborne electromagnetic survey data collected over the previously glaciated part of eastern Nebraska. To evaluate the appropriate resolution at which airborne electromagnetic data should be interpolated (or represented), we compared a 500-meter cell raster and a filled contour shapefile to known geologic features, point estimates of groundwater recharge, and selected water-quality data. The median groundwater recharge rate for the estimates collected in areas mapped as having high groundwater recharge potential were higher compared to the groundwater recharge rate estimates collected in areas mapped as having moderate and low recharge potential. Selected water-quality data were examined to determine the influence of near-surface geology on water quality, groundwater geochemistry, and groundwater age. The strongest statistical correlations were produced when examining specific conductance values measured from wells in areas mapped as high, moderate, and low groundwater recharge potential. Within till-covered areas, statistically significant differences existed in nitrate, dissolved oxygen, manganese, and iron concentrations sampled from wells mapped in high, moderate, and low groundwater recharge potential areas. Boxplots indicate that till-covered areas mapped as low groundwater recharge potential generally have lower concentrations of nitrate and dissolved oxygen, but higher concentrations of manganese and iron. The Eastern Nebraska Water Resources Assessment project sponsors have identified five focus areas as locations facing unique and important groundwater resource management issues. These focus areas are experiencing the emergence of groundwater resource issues that include increased groundwater development, nitrate contamination, and the potential expansion of rural water districts. Hydrogeologic assessments were completed for the Dorchester-Sterling paleovalley aquifer, Dodge County, Knox County, Burt County, and the Fremont-Arlington-Leshara focus areas. Potential future studies to address data gaps were identified and discussed.

Nebraska↗

USGS2001

Aquifer-characteristics data for West Virginia

Specific-capacity, storage-coefficient, and specific-yield data for wells in West Virginia were compiled to provide a data set from which transmissivity could be estimated. This data can be used for analytical and mathematical groundwater flow modeling. Analysis of available storage-coefficient and (or) specific-yield data indicates the Ohio River alluvial aquifer has a median specific yield of 0.20, which is characteristic of an unconfined aquifer. The Kanawha River alluvial aquifer has a median specific yield of 0.003, which is characteristic of a semi-confined aquifer. The median storage coefficient of fractured-bedrock aquifers is only 0.007, which is characteristic of confined aquifers. The highest median transmissivity of a specific aquifer in West Virginia occurs in Ohio River alluvium (4,800 ft 2 /d); the second highest occurs in Kanawha River alluvium (1,600 ft 2 /d). The lowest median transmissivity (23 ft 2 /d) is for the McKenzie-Rose Hill-Tuscarora aquifer. Rocks of Cambrian age within the Waynesboro-Tomstown-Harpers-Weverton-Loudon aquifer had a low median transmissivity of only 67 ft 2 /d. Other aquifers with low transmissivities include the Hampshire Formation, Brallier-Harrell Formations, Mahantango Formations, Oriskany Sandstone, and the Conococheague Formation with median transmissivities of 74, 72, 92, 82, and 92 ft 2 /d, respectively. All other aquifers within the State had intermediate values of transmissivity (130-920 ft 2 /d). The highest median transmissivities among bedrock aquifers were those for aquifers within the Pennsylvanian age Pocahontas Formation (1,200 ft 2 /d) and Pottsville Group (1,300 ft 2 /d), and the Mississippian age Mauch Chunk Group (1,300 ft 2 /d). These rocks crop out primarily in the southern part of the State and to a lesser extent within the Valley and Ridge Physiographic Province in West Virginia's Eastern Panhandle. The highest mean annual ground-water recharge rates within West Virginia (24.6 in.) occur within a band that extends through the central part of the State within the eastern part of the Kanawha River Basin. This area of relatively high relief has peaks higher than 4,000 ft and precipitation greater than 50 in./yr. The band of high recharge rates extends northward towards Pennsylvania and includes the Monongahela River Basin, which has a mean annual recharge of 21.4 inches. To the west of this central band lies a region of lower relief with much lower mean annual precipitation rates. Mean annual recharge for the Tug Fork, Twelvepole Creek, and Guyandotte River Basins is only 12.6 inches. For the western part of the Kanawha River Basin, mean recharge is 11.9 inches. The lowest mean annual recharge rates (8.4 in.) within the State occur in the Little Kanawha River Basin and the tributary streams in the region that discharge directly to the Ohio River. West Virginia's Eastern Panhandle is an area characterized by long linear northeast to southwest trending ridges and valleys. The mean annual ground-water recharge rate for this region, which is drained almost entirely by the Potomac River and its tributaries, is 9.4 inches. This area, which is located within a rain shadow resulting from orographic lifting in the higher altitude area to the west, receives less precipitation (approximately 30 in.) than the region to the west.

West Virginia↗

USGS2012

Role of surface-water and groundwater interactions on projected summertime streamflow in snow dominated regions : An integrated modeling approach

Previous studies indicate predominantly increasing trends in precipitation across the Western United States, while at the same time, historical streamflow records indicate decreasing summertime streamflow and 25th percentile annual flows. These opposing trends could be viewed as paradoxical, given that several studies suggest that increased annual precipitation will equate to increased annual groundwater recharge, and therefore increased summertime flow. To gain insight on mechanisms behind these potential changes, we rely on a calibrated, integrated surface and groundwater model to simulate climate impacts on surface water/groundwater interactions using 12 general circulation model projections of temperature and precipitation from 2010 to 2100, and evaluate the interplay between snowmelt timing and other hydrologic variables, including streamflow, groundwater recharge, storage, groundwater discharge, and evapotranspiration. Hydrologic simulations show that the timing of peak groundwater discharge to the stream is inversely correlated to snowmelt runoff and groundwater recharge due to the bank storage effect and reversal of hydraulic gradients between the stream and underlying groundwater. That is, groundwater flow to streams peaks following the decrease in stream depth caused by snowmelt recession, and the shift in snowmelt causes a corresponding shift in groundwater discharge to streams. Our results show that groundwater discharge to streams is depleted during the summer due to earlier drainage of shallow aquifers adjacent to streams even if projected annual precipitation and groundwater recharge increases. These projected changes in surface water/groundwater interactions result in more than a 30% decrease in the projected ensemble summertime streamflow. Our findings clarify causality of observed decreasing summertime flow, highlight important aspects of potential climate change impacts on groundwater resources, and underscore the need for integrated hydrologic models in climate change studies.

California;Nevada↗

USGS2009

Assessing the vulnerability of public-supply wells to contamination: Floridan aquifer system near Tampa, Florida

This fact sheet highlights findings from the vulnerability study of a public-supply well in Temple Terrace, Florida, northeast of Tampa. The well selected for study typically produces water at the rate of 700 gallons per minute from the Upper Floridan aquifer. Water samples were collected at the public-supply well and at monitoring wells installed in or near the simulated zone of contribution to the supply well. Samples of untreated water from the public-supply wellhead contained the undesirable constituents nitrate, arsenic, uranium, radon-222, volatile organic compounds (VOCs), and pesticides, although all were detected at concentrations less than established drinking-water standards, where such standards exist. Overall, study findings point to four primary factors that affect the movement and fate of contaminants and the vulnerability of the public-supply well in Temple Terrace: (1) groundwater age (how long ago water entered, or recharged, the aquifer); (2) short-circuiting of contaminated water through sinkholes; (3) natural geochemical processes within the aquifer; and (4) pumping stress. Although the public-supply well is completed in the Upper Floridan aquifer, it produces water with concentrations of nitrate, VOCs, and the natural contaminant radon that are intermediate between the typical composition of water from the Upper Floridan aquifer and that of the overlying surficial aquifer system. Mixing calculations show that the water produced by the public-supply well could consist of upwards of 50 percent water from the surficial aquifer system mixed with water from the Upper Floridan aquifer. Anthropogenically affected water from the surficial aquifer system travels rapidly to depth through sinkholes that must be directly connected to the cavernous zone intersected by the public-supply well (and several other production wells in the region). Such solution features serve as fast pathways to the well and circumvent the natural attenuation of nitrate and radon that occurs when water from the surficial aquifer flows downward through the confining unit and then through the Upper Floridan aquifer matrix. Roughly 50 percent of the simulated flow to the public-supply well consists of water less than about 10 years old, thus making the well vulnerable to contamination from human activities. Sampling at various depths in the public-supply well during pumping and nonpumping conditions showed that water entering the well from the cavernous zone had much higher arsenic concentrations during pumping conditions (18.9 ug/L) than during nonpumping conditions (4.2 ug/L). This implies that movement of arsenic to the public-supply well from the cavernous zone is enhanced by pumping. One possible explanation is that pumping increases the movement of water with elevated dissolved oxygen content through the cavernous zone, which causes dissolution of arsenic associated with pyrite. All public-supply wells in the area may not have the same level of vulnerability as the well studied - many of the public-supply wells in the region have lower pumping rates and longer open intervals that may draw in a larger proportion of old water that predates anthropogenic influences. Determining the similarity of water produced by various public-supply wells in the region to that of the surficial aquifer system is one measure of well vulnerability that could be used to prioritize monitoring and land-use planning efforts to protect the most vulnerable wells.

Florida↗

USGS2000

Water levels and ground-water discharge, regional aquifer system of the midwestern Basins and Arches Region, in parts of Indiana, Ohio, Illinois, and Michigan

Aquifers in Quaternary glacial deposits and the underlying Silurian and Devonian carbonate bedrock in parts of Indiana, Ohio, Illinois, and Michigan compose the regional aquifer system under investigation as part of the Midwestern Basins and Arches Regional Aquifer System Analysis (Midwestern Basins and Arches—RASA) project of the U.S. Geological Survey (USGS). The Midwestern Basins and Arches—RASA is part of a USGS program to assess the regional hydrology, geology, and water quality of the Nation's most important aquifers (Sun, 1986). An objective specific to the Midwestern Basins and Arches—RASA project is to conceptualize and describe regional ground-water flow in the glacial-deposit and carbonate-bedrock aquifer system, including regional recharge and discharge areas and regional relations between surface and ground water (Bugliosi, 1990). Water-level and ground-water discharge data were collected and (or) analyzed to help meet the above objective. Specifically, data from the USGS Ground-Water Site Inventory (GWSI) data base were used to determine relations between land-surface altitude and water levels in glacial-deposit aquifers. Water levels in the carbonate-bedrock aquifer were synoptically measured during July 1990, and the data were used to construct a potentiometric surface map of the aquifer. Regional hydraulic gradients and general directions of regional flow in the carbonate-bedrock aquifer can be inferred from this map. Steady-state groundwater discharge to streams that drain the area underlain by the glacial-deposit and carbonate bedrock aquifer system was estimated from base-flow daily values computed from streamflow records. Water-level and ground-water-discharge data collectively form the sample information necessary to develop calibration targets for calibration of a ground-water-flow model (Anderson and Woessner, 1992). Such a ground-water-flow model of the glacial-deposit and carbonate-bedrock aquifer system was constructed to help conceptualize and describe regional ground-water flow in the aquifer system. The model was calibrated to the water-level and ground-water-discharge data presented in this atlas.

Hydrologic Atlas↗

USGS2018

Geologic framework and hydrostratigraphy of the Edwards and Trinity aquifers within Hays County, Texas

The Edwards and Trinity aquifers are classified as major aquifers by the Texas Water Development Board and are major sources of water in south-central Texas, where Hays County is located. Detailed maps and descriptions of the geologic framework and hydrostratigraphic units (HSUs) of these karstic aquifers in Hays County are needed for water managers to effectively manage groundwater resources in the area. During 2016–18, the U.S. Geological Survey, in cooperation with the Edwards Aquifer Authority, documented the geologic framework and hydrostratigraphy of the Edwards and Trinity aquifers for a large part of Hays County, characterizing approximately 560 square miles of the county. The report includes a 1:24,000-scale hydrostratigraphic map and descriptions of the geology and HSUs in the study area. In addition, parts of the adjacent upper confining unit to the Edwards aquifer are described. The rocks exposed within the study area are within outcrops of the Trinity and Edwards Groups and the overlying Washita, Eagle Ford, Austin, and Taylor Groups. The rocks are sedimentary and formed during the Cretaceous age. The principal structural feature in Hays County is the Balcones fault zone, which is the result of late Oligocene and early Miocene age high-angle normal faulting and fracturing. Hydrostratigraphically, the exposed rocks represent a section of the upper confining unit to the Edwards aquifer, the Edwards aquifer, the upper zone of the Trinity aquifer, the middle zone of the Trinity aquifer, and the upper part of the lower zone of the Trinity aquifer. Complexity in the aquifer system results from a combination of the original depositional history, bioturbation, primary and secondary porosity, diagenesis, fracturing, and faulting.

Texas↗

USGS2014

Water movement through the unsaturated zone of the High Plains Aquifer in the Central Platte Natural Resources District, Nebraska, 2008-12

Uncertainty about the effects of land use and climate on water movement in the unsaturated zone and on groundwater recharge rates can lead to uncertainty in water budgets used for groundwater-flow models. To better understand these effects, a cooperative study between the U.S. Geological Survey and the Central Platte Natural Resources District was initiated in 2007 to determine field-based estimates of recharge rates in selected land-use areas of the Central Platte Natural Resources District in Nebraska. Measured total water potential and unsaturated-zone profiles of tritium, chloride, nitrate as nitrogen, and bromide, along with groundwater-age dates, were used to evaluate water movement in the unsaturated zone and groundwater recharge rates in the central Platte River study area. Eight study sites represented an east-west precipitation contrast across the study area—four beneath groundwater-irrigated cropland (sites 2, 5, and 6 were irrigated corn and site 7 was irrigated alfalfa/corn rotation), three beneath rangeland (sites 1, 4, and 8), and one beneath nonirrigated cropland, or dryland (site 3). Measurements of transient vertical gradients in total water potential indicated that periodic wetting fronts reached greater mean maximum depths beneath the irrigated sites than beneath the rangeland sites, in part, because of the presence of greater and constant antecedent moisture. Beneath the rangeland sites, greater temporal variation in antecedent moisture and total water potential existed and was, in part, likely a result of local precipitation and evapotranspiration. Moreover, greater variability was noticed in the total water potential profiles beneath the western sites than the corresponding eastern sites, which was attributed to less mean annual precipitation in the west. The depth of the peak post-bomb tritium concentration or the interface between the pre-bomb/post-bomb tritium, along with a tritium mass balance, within sampled soil profiles were used to estimate water fluxes in the unsaturated zone at three of the eight study sites: site 2 (irrigated), site 3 (dryland), and site 8 (rangeland). Estimates for recharge were about 68 millimeters per year [(mm/yr), post-bomb peak], 133 to 159 mm/yr (tritium interface), and 137 mm/yr (mass balance) at site 2 (irrigated); about 63 mm/yr (tritium interface) and 12 mm/yr (mass balance) at site 3 (dryland); and about 53 mm/yr (tritium interface) and 10 mm/yr (mass balance) at site 8 (rangeland). Recharge values from the mass balance at site 2 were more than an order of magnitude greater than recharge values at site 3, suggesting irrigation is an important control on water movement through the unsaturated zone. For the remaining five sites, the post-bomb tritium had flushed through the system and recharge was considered modern (within 10 years of sampling). The chloride mass-balance method was used to determine water fluxes below the root zone (less than 2 meters below land surface) at the rangeland sites: sites 1, 4, and 8. At these rangeland sites, water fluxes ranged from 1.8 to 96 mm/yr at site 1, 1.1 to 9.6 mm/yr at site 4, and 1.1 to 68 mm/yr at site 8, with mean rates of 21, 4.3, and 13 mm/yr, respectively. Site 1 had a greater mean water flux, which was consistent with the greater precipitation in the east than at site 8 in the west. Chloride mass balance was not calculated at the irrigated and dryland sites because of uncertainty about additional sources of chloride. Concentrations of nitrate as nitrogen in pore water in the unsaturated zone were larger beneath the irrigated and dryland (agricultural) sites compared with the rangeland sites. The larger concentrations at the agricultural sites are consistent with the application of nitrogen fertilizer at the agricultural sites and no substantial accumulation at the rangeland sites. The shape of the nitrate as nitrogen and chloride concentration profiles at site 1 (rangeland) indicate a reasonably larger and more consistent water flux in the UZ than beneath the other two rangeland sites (sites 4 or 8). Excluding site 7, the general shape of the nitrate as nitrogen profiles was similar beneath the agricultural sites and supports the estimates of water movement and recharge rates determined from the tritium and chloride methods. Movement of bromide through the unsaturated zone indicated greater water fluxes are found beneath irrigated lands than beneath rangeland. Bromide profiles in the unsaturated zone, determined from center of mass and peak displacement methods, document water fluxes ranged from 58 to 394 mm/yr beneath irrigated sites and 9 to 201 mm/yr beneath rangeland sites. Water-flux estimates from the potassium bromide tests at most sites did not represent overall recharge rates because the bromide remained primarily in the root zone. Apparent groundwater age was used to determine the groundwater residence time at the eight sites and to estimate recharge rates. Groundwater ages in the study area ranged from old water (defined here as groundwater that was recharged more than 50 years ago) to modern (defined here as groundwater that has recharged within the past 10 years). Groundwater ages indicated that the shallow monitoring wells generally had younger residence times, whereas the deeper monitoring wells generally had the older residence times. Groundwater dates from the shallowest monitoring wells were used to determine recharge rates at the water table. These rates generally were similar to recharge rates determined from tritium and chloride mass-balance methods. Groundwater recharge rates generally increased with well depth, and the deeper monitoring wells likely do not represent local recharge conditions but recharge from a regional flow system that receives recharge from distant sources. Overall, these data generally indicate that water movement within the unsaturated zone primarily is affected by spatial contrasts in mean annual precipitation and by the land use or land cover. The eight unsaturated-zone sites each generated unique, valuable datasets that likely will improve the understanding of water movement and recharge rates in the central Platte River valley.

Nebraska↗

USGS2015

Historical and projected climate (1901–2050) and hydrologic response of karst aquifers, and species vulnerability in south-central Texas and western South Dakota

Two karst aquifers, the Edwards aquifer in the Balcones Escarpment region of south-central Texas and the Madison aquifer in the Black Hills of western South Dakota, were evaluated for hydrologic response to projected climate change through 2050. Edwards aquifer sites include Barton Springs, the Bexar County Index Well, and Comal Springs. Madison aquifer sites include Spearfish Creek and Rhoads Fork Spring. Climate projections at sites were based on output from the Community Climate System Model of global climate, linked to the Weather Research and Forecasting (WRF) model of regional climate. The WRF model output was bias adjusted to match means for 1981–2010 from records at weather stations near Madison and Edwards aquifer sites, including Boerne, Texas, and Custer and Lead, South Dakota. Hydrologic response at spring and well sites was based on the Rainfall-Response Aquifer and Watershed Flow (RRAWFLOW) model. The WRF model climate projections for 2011–50 indicate a significant upward trend in annual air temperature for all three weather stations and a significant downward trend in annual precipitation for the Boerne weather station. Annual springflow simulated by the RRAWFLOW model had a significant downward trend for Edwards aquifer sites and no trend for Madison aquifer sites. Flora and fauna that rely on springflow from Edwards and Madison aquifer sites were assessed for vulnerability to projected climate change on the basis of the Climate Change Vulnerability Index (CCVI). The CCVI is determined by the exposure of a species to climate, the sensitivity of the species, and the ability of the species to cope with climate change. Sixteen species associated with springs and groundwater were assessed in the Balcones Escarpment region. The Barton Springs salamander ( Eurycea sosorum ) was scored as highly vulnerable with moderate confidence. Nine species—three salamanders, a fountain darter ( Etheostoma fonticola ), three insects, and two amphipods—were scored as moderately vulnerable. The remaining six species—four vascular plants, the Barton cavesnail ( Stygopyrgus bartonensis ), and a cave shrimp—were scored as not vulnerable/presumed stable (not vulnerable and evidence does not support change in abundance or range of the species). Vulnerability of eight species associated with streams that receive springflow from the Madison aquifer in the Black Hills was assessed. Of these, the American dipper ( Cinclus mexicanus ) and the lesser yellow lady’s slipper ( Cypripedium parviflorum ) were scored as moderately vulernable with high confidence. The dwarf scouringrush ( Equisetum scirpoides ) and autumn willow ( Salix serissima ) were also scored as moderately vulnerable with moderate to low confidence, respectively. Other species were assessed as not vulnerable/presumed stable or not vulnerable/increase likely (not vulnerable and evidence supporting an increase in abundance or range of the species). Lower vulnerability scores for the Black Hills species in comparison to the Balcones Escarpment species reflect lower endemicity, higher projected springflow than in the historical period, and high thermal tolerance of many of the species for the Black Hills. Importantly, climate change vulnerability scores differed substantially for Edwards aquifer species when RRAWFLOW model projections were included, resulting in increased vulnerability scores for 12 of the 16 species.

South Dakota, Texas↗

USGS2015

Areas contributing recharge to production wells and effects of climate change on the groundwater system in the Chipuxet River and Chickasheen Brook Basins, Rhode Island

The Chipuxet River and Chickasheen Brook Basins in southern Rhode Island are an important water resource for public and domestic supply, irrigation, recreation, and aquatic habitat. The U.S. Geological Survey, in cooperation with the Rhode Island Department of Health, began a study in 2012 as part of an effort to protect the source of water to six large-capacity production wells that supply drinking water and to increase understanding of how climate change might affect the water resources in the basins. Soil-water-balance and groundwater-flow models were developed to delineate the areas contributing recharge to the wells and to quantify the hydrologic response to climate change. Surficial deposits of glacial origin ranging from a few feet to more than 200 feet thick overlie bedrock in the 24.4-square mile study area. These deposits comprise a complex and productive aquifer system. Simulated areas contributing recharge to the production wells covered a total area of 0.63 square miles for average well withdrawal rates from 2007 through 2011 (total rate of 583 gallons per minute). Simulated areas contributing recharge for the maximum well pumping capacities (total rate of 3,700 gallons per minute) covered a total area of 2.55 square miles. Most simulated areas contributing recharge extend upgradient of the wells to morainal and upland till deposits and to groundwater divides. Some simulated areas contributing recharge include small, isolated areas remote from the wells. Relatively short groundwater traveltimes from recharging locations to discharging wells indicated that the wells are vulnerable to contamination from land-surface activities; median traveltimes ranged from 3.5 to 8.6 years for the production wells examined, and 57 to 91 percent of the traveltimes were 10 years or less. Land cover in the areas contributing recharge includes a substantial amount of urban and agriculture land use for five wells adjacent to the Chipuxet River; for one well adjacent to a tributary stream, land use is less developed. The calibrated groundwater-flow model provided a single, best representation of the areas contributing recharge to a production well. The parameter variance-covariance matrix from model calibration was used to create parameter sets that reflect the uncertainty of the parameter estimates and the correlation among parameters to evaluate the uncertainty associated with the predicted contributing areas to the wells. A Monte Carlo analysis led to contributing areas expressed as a probability distribution that differed from a single deterministic contributing area. Because of the effects of parameter uncertainty, the size of the probabilistic contributing areas for both average and maximum pumping rates was larger than the size of the deterministic contributing areas for the wells. Thus, some areas not in the deterministic contributing area might actually be in the contributing area, including additional areas of urban and agricultural land use that has the potential to contaminate groundwater. Additional areas that might be in the contributing area included recharge originating near the pumping wells that have relatively short groundwater-flow paths and traveltimes. At the maximum pumping rates, areas associated with low probabilities extended long distances along groundwater divides in the uplands remote from the wells. Climate projections for the Chipuxet River and Chickasheen Brook Basins from downscaled output from general circulation models indicate that mean annual temperature might increase by 4.7 degrees Fahrenheit and 8.0 degrees Fahrenheit by the late 21st century (2070–99) compared with the late 20th century (1970–99) under scenarios of lower and higher emissions of greenhouse gases, respectively. By the late 21st century, winter and spring precipitation is projected to increase by 12 to 17 percent, summer precipitation to increase by about the same as mean annual precipitation (8 percent), and fall precipitation to decrease by 5 percent for both emission scenarios compared with the late 20th century. Soil-water-balance simulations indicate that, although precipitation is expected to increase in three seasons, only in winter do precipitation increases exceed actual evapotranspiration increases. Recharge is projected to decrease in fall and generally change little in spring and summer. By the late 21st century, winter recharge is expected to increase by 13 percent for the lower emissions scenario and by 15 percent for the higher emissions scenario. In fall, recharge is projected to diminish by 13 percent for the lower emissions scenario and by 24 percent for the higher emissions scenario. Although recharge is projected to change seasonally in the 21st century, mean annual recharge changes minimally. Soil moisture is projected to decrease in the 21st century from spring through fall because of increases in potential evapotranspiration, and in fall because of decreases in precipitation in addition to increases in potential evapotranspiration. By the late 21st century, soil moisture for the lower emissions scenario is expected to decrease by 11 percent in summer and 15 percent in fall, and for the higher emissions scenario, decrease by 23 percent for both seasons. These decreases in soil moisture during the growing season might have implications for agriculture in the study area. Predicted changes in the magnitude and seasonal distribution of recharge in the 21st century increase simulated base flows and groundwater levels in the winter months for both emission scenarios, but because of less recharge in the fall and less or about the same recharge in the preceding months of spring and summer, base flows and groundwater levels in the fall months decrease for both emission scenarios. October has the largest base flow and groundwater level decreases. By the late 21st century, base flows at the Chipuxet River in October are projected to decrease by 9 percent for the lower emissions scenario and 18 percent for the higher emissions scenario. For a headwater stream in the upland till with shorter groundwater-flow paths and lower storage properties in its drainage area, base flows in October are projected to diminish by 28 percent and 42 percent for the lower and higher emissions scenarios by the late 21st century. Groundwater level changes in the uplands show substantial decreases in fall, but because of the large storage capacity of stratified deposits, water levels change minimally in the valley. By the late 21st century, water levels in large areas of upland till deposits in October are projected to decrease by up to 2 feet for the lower emissions scenario, whereas large areas decrease by up to 5 feet, with small areas with decreases of as much as 10 feet, for the higher emissions scenario. For both emission scenarios, additional areas of till go dry in fall compared with the late 20th century. Thus projected changes in recharge in the 21st century might extend low flows and low water levels for the year later in fall and there might be more intermittent headwater streams compared with the late 20th century with corresponding implications to aquatic habitat. Finally, the size and location of the simulated areas contributing recharge to the production wells are minimally affected by climate change because mean annual recharge, which is used to determine the contributing areas to the production wells, is projected to change little in the 21st century.

Rhode Island↗

USGS2018

Simulation of groundwater storage changes in the Quincy Basin, Washington

The Miocene Columbia River Basalt Group and younger sedimentary deposits of lacustrine, fluvial, eolian, and cataclysmic-flood origins compose the aquifer system of the Quincy Basin in eastern Washington. Irrigation return flow and canal leakage from the Columbia Basin Project have caused groundwater levels to rise substantially in some areas. Water resource managers are considering extraction of additional stored groundwater to supply increasing demand. To help address these concerns, the transient groundwater model of the Quincy Basin documented in this report was developed to quantify the changes in groundwater flow and storage. The model based on the U.S. Geological Survey modular three-dimensional finite-difference numerical code MODFLOW uses a 1-kilometer finite-difference grid and is constrained by logs from 698 wells in the study area. Five model layers represent two sedimentary hydrogeologic units and underlying basalt formations. Head-dependent flux boundaries represent the Columbia River and other streams, lakes and reservoirs, underflow to and (or) from adjacent areas, and discharge to agricultural drains and springs. Specified flux boundaries represent recharge from precipitation and anthropogenic sources, including irrigation return flow and leakage from water-distribution canals and discharge through groundwater withdrawal wells. Transient conditions were simulated from 1920 to 2013 using annual stress periods. The model was calibrated with the parameter-estimation code PEST to a total of 4,064 water levels measured in 710 wells. Increased recharge since predevelopment resulted in an 11.5 million acre-feet increase in storage in the Quincy Groundwater Management Subarea of the Quincy Basin. Four groundwater-management scenarios were formulated with input from project stakeholders and were simulated using the calibrated model to provide representative examples of how the model could be used to evaluate the effect on groundwater levels as a result of potential changes in recharge, groundwater withdrawals, or increased flow in Crab Creek. Decreased recharge and increased groundwater withdrawals both resulted in declines in groundwater levels over 2013 conditions, whereas increasing the flow in Crab Creek resulted in increased groundwater levels over 2013 conditions.

Washington↗

USGS2020

Simulated effects of pumping in the Death Valley Regional Groundwater Flow System, Nevada and California—Selected management scenarios projected to 2120

Declining water levels and reduced natural discharge at springs, seeps, and phreatophyte areas primarily are the result of decades of groundwater development in the Death Valley regional flow system, in Nevada and California. A calibrated groundwater-flow model was used to simulate potential future effects of groundwater pumping on water levels and natural groundwater discharge in the study area. Effects of climate change on future groundwater pumping were not considered and were beyond the scope of the study. Four groundwater-pumping scenarios were developed by stakeholders to predict and compare (1) the extent of regional water-level declines; (2) drawdown at Devils Hole; and (3) reductions in natural discharge at select discharge areas, including the Amargosa Wild and Scenic River, the Ash Meadows discharge area, the Furnace Creek area, and Stump Spring. Scenarios were simulated from 1913 to 2120, with historical pumping occurring from 1913 to 2010, historical 2010 pumping rates projected from 2010 to 2020, and scenario pumping beginning in 2020. Pumping scenarios included a base case and scenarios A, B, and C. The base case projected 2010 pumping rates from 2010 to 2120, and scenarios A, B, and C projected base case pumping plus additional pumping at various locations from 2020 to 2120. By 2020, historical (1913–2020) pumping resulted in the propagation of simulated drawdown of 1 foot (ft) or more westward from Pahrump Valley to areas north of Shoshone in the Pahrump to Death Valley South (PDVS) groundwater basin and the merging of simulated 1-ft drawdown contours between the Alkali Flat–Furnace Creek Ranch (AFFCR) and Ash Meadows groundwater basins. In the base case scenario, extent and magnitude of simulated drawdown continued to increase in the Ash Meadows and AFFCR groundwater basins from 2020 to 2120. In the base case, the magnitude of simulated drawdown continued to increase in western Pahrump Valley from 2020 to 2120, whereas simulated water levels rose in eastern Pahrump Valley from 2020 to 2070 and then stabilized from 2070 to 2120. Scenarios A and B primarily affected the PDVS and AFFCR groundwater basins by increasing the magnitude of drawdown in 2120, compared to the base case. In scenario C, drawdown propagated throughout a high-transmissivity part of the carbonate aquifer known as the megachannel, greatly affecting water levels in the Ash Meadows discharge area. Scenario C resulted in an additional 10–100 ft of drawdown (compared to the base case) throughout the southeastern part of the Ash Meadows groundwater basin by 2120. Simulated drawdowns in Devils Hole in 2120 were 3.2, 3.4, 3.8, and 25.4 ft for the base case and scenarios A, B, and C, respectively. The federally mandated minimum water level for Devils Hole is 2.7 ft below a reference point. In 2020, the simulated water level in Devils Hole was above the minimum water level, at 1.7 ft below the reference. Simulated water levels in Devils Hole fell below the federally mandated water level by 2078, 2073, 2058, and 2025 for the base case and scenarios A, B, and C, respectively, assuming a hypothetical recharge scenario of constant natural recharge. Simulated reductions in predevelopment (natural) discharge at select discharge areas ranged from 3 to 38 percent by 2120 for all scenarios. Amargosa Wild and Scenic River was the least affected discharge area with simulated capture rates ranging from 3 to 4 percent of predevelopment discharge by 2120. Ash Meadows discharge area was greatly affected by groundwater pumping in scenario C with a simulated capture rate of 38 percent, compared to simulated capture rates of 8, 8, and 9 percent for the base case, scenario A, and scenario B, respectively, in 2120. Simulated capture rates in the Furnace Creek area ranged from 10 to 11 percent for all scenarios in 2120. Simulated capture rates at Stump Spring ranged from 32 to 36 percent for all scenarios in 2120.

California, Nevada↗

USGS2021

Characterization of factors affecting groundwater levels in and near the former Lake Traverse Indian Reservation, South Dakota, water years 1956–2017

The U.S. Geological Survey (USGS), in cooperation with the Sisseton Wahpeton Oyate, completed a study to characterize water-level fluctuations in observation wells relative to driving factors that affect water levels in and near the historical 1867 boundary of the Lake Traverse Indian Reservation. The study investigated concerns regarding potential effects of groundwater withdrawals and climate conditions on groundwater levels within an area that includes the historical boundary of the reservation and a surrounding area that extends 10 miles in all directions within South Dakota. Characterization of water-level fluctuations in observation wells and relative driving factors was accomplished by statistical trend analysis. Monthly data from the Parameter-elevation Regressions on Independent Slopes Model (PRISM) were aggregated to obtain annual and seasonal datasets for total precipitation, minimum air temperature ( T min ), and maximum air temperature ( T max ) for the study area and a surrounding buffer area. Trend tests for gridded data for total precipitation, T min , and T max were completed for annual and seasonal time series for water years 1956–2017, which is about 2 years before the earliest available water-level measurements. A 2-year offset was arbitrarily selected because scrutiny of water-level and precipitation data indicated that responses of groundwater levels for many of the observation wells lagged major changes in precipitation patterns by about 2 years. Statistically significant upward trends were detected for annual precipitation and annual T min for most of the study area and the surrounding buffer area. Statistically significant downward trends in T max were detected for only a few 2.5 arc-minute grid cells; however, the sparsity of the spatial coverage reduces confidence that these are true trends, in contrast to the near completeness of the spatial coverage in upward trends for T min . Spatial distributions of statistically significant trends in seasonal climate data were generally similar to the annual trends, but with substantial differences in the spatial density of the trends. Potential interactions among water levels in observation wells and streamflow were examined through correlation analyses of the annual median water level for each of 76 observation wells versus the annual mean streamflow for each of four area streamgages. Potential interactions among water levels in observation wells and lake levels were examined through correlation analyses involving 25 area lakes. Resulting correlation coefficients were used as part of an approach for selecting a lake to be plotted in conjunction with water-level and precipitation data for each observation well. Groundwater trends for 76 observation wells were analyzed for three separate water-level parameters (minimum, median, and maximum) because wells are measured sporadically, and data are biased towards more frequent measurements during periods of heaviest irrigation demand. Trends in the time series of annual precipitation (from PRISM) starting 2 years earlier than the associated water-level trend also were analyzed for the location of each individual observation well. Sen’s slope and Mann-Kendall p -values were computed for the three water-level parameters and for the annual precipitation time series. Graphs showing results of trend analyses for each observation well also showed changes with time in the sum of licensed groundwater withdrawals within six specified radii (0.5, 1.0, 2.0, 3.0, 4.0, and 5.0 miles) of each well as a qualitative indicator of proximal groundwater demand. Trends in groundwater levels in observation wells in the study area are predominantly upward, with 43 of 76 wells having significant upward trends for at least one of the three water-level parameters and only 8 wells having significant downward trends for at least one water-level parameter. The upward groundwater trends are driven by predominantly upward precipitation trends, with 43 wells (not all the same wells) also having significant upward trends and no wells having significant downward trends. Significant upward precipitation trends were detected for only two of the eight wells with significant downward groundwater trends. Groundwater levels in some observation wells likely are also substantially affected by interactions with surface water, especially with lakes. Water levels in many area lakes increased in response to wet conditions of the early 1990s and have maintained high water levels ever since. It is recognized that in many cases lakes that were selected for plotting with groundwater hydrographs likely are not hydraulically connected with a groundwater system or aquifer associated with an individual well; however, interactions also are plausible for numerous other lakes for which water-level records are not available.

South Dakota↗