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Groundwater & aquifers

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At least 1,477 records · Page 82Linked to original sources

USGS2013

Groundwater quality in western New York, 2011

Water samples collected from 16 production wells and 15 private residential wells in western New York from July through November 2011 were analyzed to characterize the groundwater quality. Fifteen of the wells were finished in sand and gravel aquifers, and 16 were finished in bedrock aquifers. Six of the 31 wells were sampled in a previous western New York study, which was conducted in 2006. Water samples from the 2011 study were analyzed for 147 physiochemical properties and constituents that included major ions, nutrients, trace elements, radionuclides, pesticides, volatile organic compounds (VOCs), and indicator bacteria. Results of the water-quality analyses are presented in tabular form for individual wells, and summary statistics for specific constituents are presented by aquifer type. The results are compared with Federal and New York State drinking-water standards, which typically are identical. The results indicate that groundwater generally is of acceptable quality, although at 30 of the 31 wells sampled, at least one of the following constituents was detected at a concentration that exceeded current or proposed Federal or New York State drinking-water standards: pH (two samples), sodium (eight samples), sulfate (three samples), total dissolved solids (nine samples), aluminum (two samples), arsenic (one sample), iron (ten samples), manganese (twelve samples), radon-222 (sixteen samples), benzene (one sample), and total coliform bacteria (nine samples). Existing drinking-water standards for color, chloride, fluoride, nitrate, nitrite, antimony, barium, beryllium, cadmium, chromium, copper, lead, mercury, selenium, silver, thallium, zinc, gross alpha radioactivity, uranium, fecal coliform, Escherichia coli, and heterotrophic bacteria were not exceeded in any of the samples collected. None of the pesticides analyzed exceeded existing drinking-water standards.

New York↗

USGS2018

Simulation of zones of groundwater contribution to wells south of the Naval Weapons Industrial Reserve Plant in Bethpage, New York

A steady-state three-dimensional groundwater-flow model that simulates present conditions was coupled with the particle-tracking program MODPATH to delineate zones of contribution to wells pumping from the Magothy aquifer near a chlorinated volatile organic compound (VOC) plume. This modeling was part of a study by the U.S. Geological Survey in cooperation with the Naval Facilities Engineering Command to delineate groundwater near the Naval Weapons Industrial Reserve Plant in Bethpage, New York. Because rates of advection within the coarse-grained sediments typically exceed 0.1 foot per day, transport by dispersion and (or) diffusion was assumed to be negligible. Resulting zones of contribution are complex shapes, influenced by hydrogeologic features including confining beds and a basal gravel zone, and the interplay of nearby hydrologic stresses. The use of two particle tracking techniques identified zones of contribution to wells. Particles are backtracked from pumping well screens, and particles are forward tracked from the location of a VOC plume, as defined by surfaces of equal total VOC concentration. During any period of 5 years or less, about 1 to 3 percent of particles backtracked from pumping wells within a focus area intersected the 5-part per billion (ppb) VOC plume shell, indicating that the vast majority of particles were not sourced from the plume. During 5 years or less, none of the particles backtracked from pumping wells intersected the 50-ppb VOC plume shell. Forward-tracking techniques identified the fate of water within the VOC plume after 5 years as it moves toward ultimate well capture and (or) discharge to model constant head and drain boundaries. Out of 4,813 forward tracked particles started within the 50-ppb VOC plume shell, 1 forward-tracked particle was captured by well ANY8480. Out of 22,958 forward tracked particles started within the 5-ppb VOC plume shell, 100 were captured by production wells (less than 1 percent). The subset of forward pathlines that represent well plume capture are similar in number and shape to those of backtracked pathlines. Model simulations were conducted to assess uncertainties and improve understanding of how variability in hydraulic properties, pumpage rates, and maximum particle traveltime affect delineation of zones of contribution. By use of driller’s’ logs, a transitional probability approach generated nine alternative realizations of heterogeneity within the Magothy aquifer to assess uncertainty in model representation. Fine-grained sediments with low hydraulic conductivity were realized as laterally discontinuous, thickening towards the south, and comprising about 27 percent of the total aquifer volume within the transitional probability subgrid. Model simulations with alternative pumpage rates, porosity terms, and alternative maximum particle traveltime were also used to demonstrate how the size and shape of zones of contribution may vary.

New York↗

USGS2023

Selected anthropogenic contaminants in groundwater, Papio-Missouri River Natural Resources District, eastern Nebraska, 1992–2020

A study in cooperation with the Papio-Missouri River Natural Resources District was completed in 2019 to determine the concentration of contaminants of emerging concern (CEC) in groundwater in the Papio-Missouri River Natural Resources District, eastern Nebraska. Each well was sampled twice (in June and October or November) in 2019, totaling 34 samples. Samples were analyzed for 132 CECs, which include pharmaceutical, steroid hormone, and other organic chemicals. Seven of the 132 CEC analytes were detected in samples collected during this study. The most commonly detected CEC in this study was the antibiotic sulfamethoxazole. Other CECs detected in this study were nicotine, methyl-1 H -benzotiazole (industrial product), acetaminophen (analgesic), caffeine, and metformin (diabetes medicine). None of the detected CECs have health-based water-quality standards. The agricultural herbicide atrazine was also sampled for and was detected in 15 of 26 samples from 8 wells, but all samples were below the established water-quality standard. Nitrate, dissolved oxygen, and iron sampling results for 2010–19 and 1992–2020 were also assessed to determine the extent and trend of anthropogenic contamination in the Papio-Missouri River Natural Resources District. Nitrate as nitrogen was detected at a concentration greater than 4 milligrams per liter in 92 samples (19 percent), and detections in 36 samples (7.6 percent) exceeded 10 milligrams per liter, which is the U.S. Environmental Protection Agency’s maximum contaminant level for drinking water and Nebraska’s Title 118 maximum contaminant level for groundwater. Time series analysis showed that nitrate concentrations are not increasing or decreasing in any of the aquifers except for in three specific well nests, which are in phase 2 management areas. Dissolved oxygen results indicate potential denitrification throughout the Elkhorn alluvial aquifer; iron concentrations indicate potential denitrification in parts of the Missouri River alluvial aquifer.

Nebraska↗

USGS1982

Hydrologic maps of the High Plains aquifer, January 1981, southwestern Kansas

The U.S. Geological Survey furnishes maps depicting saturated thickness of the High Plains aquifer to the Southwest Kansas Groundwater Management District No. 3 through a continuing cooperative agreement. The Groundwater Management District uses the maps to calculated a planned depletion rate of 40 percent of the saturated thickness of the aquifer in 25 years for a 13-county area. Because the information contained on the maps is used in management decisions, the Groundwater Management District is concerned with the accuracy, reliability, and consistency of the hydrologic information that the maps show. A mathematical technique, called kriging, has been used in the form of a computer program to interpolate hydrologic data based on a network of measured values (Karlinger and Skrivan, 1980). The technique has been applied to geohydrologic data in west-central Kansas with good results (Dunlap and Spinazola, 1981). The program calculates the values at the center of each 1-mile section in a predefined region and facilitates contouring of selected values. The kriging technique was used to produce the hydrologic maps in this report.

Kansas↗

USGS1988

Simulation of ground-water flow in aquifers along the Susquehanna River in Columbia County, Pennsylvania

A numerical model of groundwater flow was developed for a 10.3 sq mi area along the Susquehanna River in Columbia County, east central Pennsylvania. Groundwater in the model area primarily is in secondary openings in the carbonate--and clastic-rock aquifers and primary openings in the glacial-outwash aquifer that discontinuously overlies bedrock. The groundwater flow model was calibrated under average steady-state conditions for 1981. The simulated 1981 water budget indicates an average inflow rate of 7.24 cu ft/sec. Of this, 93% is recharge from precipitation and 6.6% is boundary flow. 62% of the outflow is leakage to streams, 21% to pumpage, and 17% to evapotranspiration. The model was calibrated under transient conditions for December 22, 1980 through April 21, 1982. Water level fluctuations caused by natural stresses were more successfully simulated than those caused by pumping stresses. Three 10-year, hypothetical stress periods were simulated with the calibrated, transient model. The general impact of three pumping schemes under hypothetical drought and drought-recovery conditions were simulated. (USGS)

Pennyslvania↗

USGS1988

Ethylene dibromide (EDB) trends in the upper Floridan Aquifer, Seminole County, Georgia, October 1981 to November 1987

Results of an investigation to assess the persistence and trends of ethylene dibromide (EDB) in the Upper Floridan aquifer, in Georgia indicate that in November 1987, EDB was present in the groundwater 4 years after it was last applied as a soil fumigant in the intensively farmed area of central Seminole County. At that time, EDB was detected in water samples from one domestic well and four irrigation wells. Concentrations ranged from less than 0.01 micrograms/L to 3.3 micrograms/L. The investigation in November 1987 revealed the groundwater contamination was limited to the 4-sq-mi area in the vicinity of Buck Hole, a sinkhole in a swampy depression in the central part of the County. In three of the five wells in which EDB was detected, concentrations have not changed significantly since August 1983. However, in one irrigation well near Buck Hole, the concentration decreased from 110 to 0.7 microgram/L since October 1981. In another irrigation well near Buck Hole, the concentration decreased from 26 to about 2.5 microgram/L during that period. The groundwater level data indicate that from Buck Hole, the hydraulic gradient in the Upper Floridan aquifer is about 2.8 ft/mi toward the east. However, the movement of EDB from the area of relatively high concentration near Buck Hole in the direction of inferred flow lines is not apparent. Although concentration gradients were not detected along flow lines, the movement of water induced by pumping wells W4 and W5 produced short-term decreasing and increasing trends in EDB Concentrations in wells W4 and W5, respectively. (USGS)

Georgia↗

USGS1989

Ground-water levels in the alluvial aquifer at Louisville, Kentucky, 1987-88

Water-level data have been collected in the alluvial aquifer at Louisville, Kentucky by the U.S. Geological Survey since 1943. Summaries and analysis of these data are published periodically to update the record and to help public officials who must manage this groundwater supply. Maps and hydrographs are presented in this report to update the record for 1987-88 and to interpret recent trends in water levels. The data show that groundwater levels were stable for the period 1980-86 after rising rapidly in the 1970's. There has been a slight decline in the water levels during 1987-88 because of below normal precipitation for the period 1985-88. The decline has been about 1 to 3 ft in wells along the Ohio River and about 3 to 6 ft in wells away from the influence of the river. Groundwater withdrawals have also caused slight declines in the water table in three areas in Louisville. Declines in downtown Louisville due to pumpage seem to be stabilizing at or slightly below the normal pool stage of the Ohio River which indicates that water is probably being induced from the river into the aquifer. (USGS)

Water-Resources Investigations Report↗

USGS2024

Basin-scale responses of groundwater-resource quality to drought and recovery, San Joaquin Valley, California

Groundwater-resource quality is assumed to be less responsive to drought compared to that of surface water due to relatively long transit times of recharge to drinking-supply wells. Here, we evidence dynamic perturbations in aquifer pressure dynamics during drought and subsequent recovery periods cause dramatic shifts in groundwater quality on a basin scale. We used a novel application of time-series clustering on annual nitrate anomalies at >450 public-supply wells (PSWs) across California's San Joaquin Valley during 2000–22 to group sub-populations of wells with similar water-quality responses to drought. Additionally, we statistically evaluated the direction and magnitude of multi-constituent water-quality changes across the San Joaquin Valley using a broader dataset of >3000 PSWs with data during two select hydrologic stress periods representing an extreme drought (2012–16) and subsequent recovery (2016–19). Results of time-series clustering and stress-period change analyses corroborate a predominant regional response to pumping stress characterized by increased concentrations of anthropogenic constituents (nitrate, total dissolved solids) and decreased concentrations of geogenic constituents (arsenic, fluoride), which largely reversed during recovery. Cluster analysis also identified a secondary, less commonly occurring group of PSWs where nitrate decreased during drought, but explanatory factor analysis was not able to discern hydrogeologic drivers for these two divergent response patterns. Long-term tracer data support the hypothesis that the predominant regional signal of nitrate increase during drought is caused by enhanced capture of modern-aged groundwater by PSWs during periods of pumping stress, which can drive rapid changes in water quality on seasonal and multiannual timescales. Pumping-induced migration of modern, oxic groundwater to depth during drought may affect geochemical conditions in deeper portions of regional aquifers controlling the mobility of geogenic contaminants over the long term.

California↗

USGS2017

Using enteric pathogens to assess sources of fecal contamination in the Silurian Dolomite Aquifer: Preliminary results

The fractured Silurian dolomite aquifer is an important, but vulnerable, source of drinking water in northeast Wisconsin (Sherrill in Geology and ground water in Door County, Wisconsin, with emphasis on contamination potential in the Silurian dolomite, 1978; Bradbury and Muldoon in Hydrogeology and groundwater monitoring of fractured dolomite in the Upper Door Priority Watershed, Door County, Wisconsin, 1992; Muldoon and Bradbury in Assessing seasonal variations in recharge and water quality in the Silurian aquifer in areas with thicker soil cover. p 45, 2010). Areas underlain by the Silurian dolomite aquifer are extremely vulnerable to groundwater contamination from various land-use activities, especially the disposal of human wastewater and dairy manure. Currently there is no consensus as to which source of wastewater generates the greater impact to the aquifer.

Conference Paper↗

USGS1998

Spatial and temporal study of nitrate concentration in groundwater by means of coregionalization

Spatial and temporal behavior of hydrochemical parameters in groundwater can be studied using tools provided by geostatistics. The cross-variogram can be used to measure the spatial increments between observations at two given times as a function of distance (spatial structure). Taking into account the existence of such a spatial structure, two different data sets (sampled at two different times), representing concentrations of the same hydrochemical parameter, can be analyzed by cokriging in order to reduce the uncertainty of the estimation. In particular, if one of the two data sets is a subset of the other (that is, an undersampled set), cokriging allows us to study the spatial distribution of the hydrochemical parameter at that time, while also considering the statistical characteristics of the full data set established at a different time. This paper presents an application of cokriging by using temporal subsets to study the spatial distribution of nitrate concentration in the aquifer of the Lucca Plain, central Italy. Three data sets of nitrate concentration in groundwater were collected during three different periods in 1991. The first set was from 47 wells, but the second and the third are undersampled and represent 28 and 27 wells, respectively. Comparing the result of cokriging with ordinary kriging showed an improvement of the uncertainty in terms of reducing the estimation variance. The application of cokriging to the undersampled data sets reduced the uncertainty in estimating nitrate concentration and at the same time decreased the cost of the field sampling and laboratory analysis.Spatial and temporal behavior of hydrochemical parameters in groundwater can be studied using tools provided by geostatistics. The cross-variogram can be used to measure the spatial increments between observations at two given times as a function of distance (spatial structure). Taking into account the existence of such a spatial structure, two different data sets (sampled at two different times), representing concentrations of the same hydrochemical parameter, can be analyzed by cokriging in order to reduce the uncertainty of the estimation. In particular, if one of the two data sets is a subset of the other (that is, an undersampled set), cokriging allows us to study the spatial distribution of the hydrochemical parameter at that time, while also considering the statistical characteristics of the full data set established at a different time. This paper presents an application of cokriging by using temporal subsets to study the spatial distribution of nitrate concentration in the aquifer of the Lucca Plain, central Italy. Three data sets of nitrate concentration in groundwater were collected during three different periods in 1991. The first set was from 47 wells, but the second and the third are undersampled and represent 28 and 27 wells, respectively. Comparing the result of cokriging with ordinary kriging showed an improvement of the uncertainty in terms of reducing the estimation variance. The application of cokriging to the undersampled data sets reduced the uncertainty in estimating nitrate concentration and at the same time decreased the cost of the field sampling and laboratory analysis.

Environmental Geology↗

USGS2000

Microbial control of mineral–groundwater equilibria: Macroscale to microscale

Using field and laboratory experiments, the geomicrobiology of a petroleum-contaminated aquifer and the relationship between mineral alteration, groundwater chemistry, and microbial colonization were examined. Results indicate that indigenous microorganisms influence mineral weathering at two scales of interaction: macroscale processes that perturb general groundwater chemistry and therefore mineralwater equilibria; and microscale interactions, where attached organisms locally perturb mineral±water equilibria, potentially releasing limiting trace nutrients from the dissolving mineral. In the contaminated unconfined glacio-fluvial aquifer near Bemidji, Minnesota, USA, carbonate chemistry is influenced primarily at the macroscale. Under oxic conditions, respiration by native aerobic heterotrophs produces excess carbon dioxide that promotes calcite and dolomite dissolution. Aerobic microorganisms do not colonize dolomite surfaces and few occur on calcite. Within the anoxic groundwater, calcite overgrowths form on uncolonized calcite cleavage surfaces, possibly due to the consumption of acidity by dissimilatory iron-reducing bacteria. As molecular oxygen concentration increases downgradient of the oil pool, aerobes again dominate and residual hydrocarbons and ferrous iron are oxidized, resulting in macroscale carbonate-mineral dissolution and iron precipitation. Feldspars, in contrast, weather exclusively at the microscale near attached microorganisms, principally in the anoxic region of the plume. Native organisms preferentially colonize feldspars that contain trace phosphorus as apatite inclusions, apparently as a consequence of the low P concentration in the groundwater. These feldspars weather rapidly, whereas nearby feldspars without trace P are uncolonized and unweathered. Feldspar dissolution is accompanied by the precipitation of secondary minerals, sometimes on the bacterial cell wall itself. These observations suggest a tightly linked biogeochemical system whereby microbial processes control mineral diagenesis at many scales of interaction, and the mineralogy and mineral chemistry influence microbial ecology. Only the macroscale interaction, however, is easily observable by standard geochemical methods, and documentation of the microscale interactions requires microscopic examination of microorganisms on mineral surfaces and the locally intense diagenetic reactions that result.

Hydrogeology Journal↗

USGS2006

Kinetics of sorption and abiotic oxidation of arsenic(III) by aquifer materials

The fate of arsenic in groundwater depends largely on its interaction with mineral surfaces. We investigated the kinetics of As(III) oxidation by aquifer materials collected from the USGS research site at Cape Cod, MA, USA, by conducting laboratory experiments. Five different solid samples with similar specific surface areas (0.6–0.9 m 2 g −1 ) and reductively extractable iron contents (18–26 μmol m −2 ), but with varying total manganese contents (0.5–3.5 μmol m −2 ) were used. Both dissolved and adsorbed As(III) and As(V) concentrations were measured with time up to 250 h. The As(III) removal rate from solution increased with increasing solid manganese content, suggesting that manganese oxide is responsible for the oxidation of As(III). Under all conditions, dissolved As(V) concentrations were very low. A quantitative model was developed to simulate the extent and kinetics of arsenic transformation by aquifer materials. The model included: (1) reversible rate-limited adsorption of As(III) onto both oxidative and non-oxidative (adsorptive) sites, (2) irreversible rate-limited oxidation of As(III), and (3) equilibrium adsorption of As(V) onto adsorptive sites. Rate constants for these processes, as well as the total oxidative site densities were used as the fitting parameters. The total adsorptive site densities were estimated based on the measured specific surface area of each material. The best fit was provided by considering one fast and one slow site for each adsorptive and oxidative site. The fitting parameters were obtained using the kinetic data for the most reactive aquifer material at different initial As(III) concentrations. Using the same parameters to simulate As(III) and As(V) surface reactions, the model predictions were compared to observations for aquifer materials with different manganese contents. The model simulated the experimental data very well for all materials at all initial As(III) concentrations. The As(V) production rate was related to the concentrations of the free oxidative surface sites and dissolved As(III), as r As ( V ) = k ox ′ [ Mn ( IV ) OH ] [ H 3 AsO 3 ] "> rAs(V)=kox′[Mn(IV)OH][H3AsO3] with apparent second-order rate constants of k ox f ′ = 6.28 × 10 - 1 "> koxf′=6.28×10-1 and k ox s ′ = 1.25 × 10 - 2 M - 1 s - 1 "> koxs′=1.25×10-2M-1s-1 for the fast and the slow oxidative sites, respectively. The As(III) removal rate decreased approximately by half for a pH increase from 4 to 7. The pH dependence was explained using the acid–base behavior of the surface oxidative sites by considering a surface p K a = 6.2 ( I = 0). In the presence of excess surface adsorptive and oxidative sites, phosphate diminished the rate of As(III) removal and As(V) production only slightly due to its interaction with the oxidative sites. The observed As(III) oxidation rate here is consistent with previous observations of As(III) oxidation over short transport distances during field-scale transport experiments. The model developed here may be incorporated into groundwater transport models to predict arsenic speciation and transport in chemically heterogeneous systems.

Geochimica et Cosmochimica Acta↗

USGS2011

Characterization of the Cretaceous aquifer structure of the Meskala region of the Essaouira Basin, Morocco

The aquifer of early Cretaceous age in the Meskala region of the Essaouira Basin is defined by interpretation of geological drilling data of oil and hydrogeological wells, field measurement and analysis of in situ fracture orientations, and the application of a morphostructural method to identify lineaments. These analyzes are used to develop a stratigraphic–structural model of the aquifer delimited by fault zones of two principal orientations: NNE and WNW. These fault zones define fault blocks that range in area from 4 to 150 km 2 . These blocks correspond either to elevated zones (horsts) or depressed zones (grabens). This structural setting with faults blocks of Meskala region is in accordance with the structure of the whole Essaouira Basin. Fault zones disrupt the continuity of the aquifer throughout the study area, create recharge and discharge zones, and create dip to the units from approximately 10° to near vertical in various orientations. Fracture measurements and morphometric-lineament analyzes help to identify unmapped faults, and represent features important to groundwater hydraulics and water quality within fault blocks. The above geologic features will enable a better understanding of the behaviour and hydro-geo-chemical and hydrodynamics of groundwater in the Meskala aquifer.

Essaouira Basin↗

USGS2017

Prediction and visualization of redox conditions in the groundwater of Central Valley, California

Regional-scale, three-dimensional continuous probability models, were constructed for aspects of redox conditions in the groundwater system of the Central Valley, California. These models yield grids depicting the probability that groundwater in a particular location will have dissolved oxygen (DO) concentrations less than selected threshold values representing anoxic groundwater conditions, or will have dissolved manganese (Mn) concentrations greater than selected threshold values representing secondary drinking water-quality contaminant levels (SMCL) and health-based screening levels (HBSL). The probability models were constrained by the alluvial boundary of the Central Valley to a depth of approximately 300 m. Probability distribution grids can be extracted from the 3-D models at any desired depth, and are of interest to water-resource managers, water-quality researchers, and groundwater modelers concerned with the occurrence of natural and anthropogenic contaminants related to anoxic conditions. Models were constructed using a Boosted Regression Trees (BRT) machine learning technique that produces many trees as part of an additive model and has the ability to handle many variables, automatically incorporate interactions, and is resistant to collinearity. Machine learning methods for statistical prediction are becoming increasing popular in that they do not require assumptions associated with traditional hypothesis testing. Models were constructed using measured dissolved oxygen and manganese concentrations sampled from 2767 wells within the alluvial boundary of the Central Valley, and over 60 explanatory variables representing regional-scale soil properties, soil chemistry, land use, aquifer textures, and aquifer hydrologic properties. Models were trained on a USGS dataset of 932 wells, and evaluated on an independent hold-out dataset of 1835 wells from the California Division of Drinking Water. We used cross-validation to assess the predictive performance of models of varying complexity, as a basis for selecting final models. Trained models were applied to cross-validation testing data and a separate hold-out dataset to evaluate model predictive performance by emphasizing three model metrics of fit: Kappa; accuracy; and the area under the receiver operator characteristic curve (ROC). The final trained models were used for mapping predictions at discrete depths to a depth of 304.8 m. Trained DO and Mn models had accuracies of 86–100%, Kappa values of 0.69–0.99, and ROC values of 0.92–1.0. Model accuracies for cross-validation testing datasets were 82–95% and ROC values were 0.87–0.91, indicating good predictive performance. Kappas for the cross-validation testing dataset were 0.30–0.69, indicating fair to substantial agreement between testing observations and model predictions. Hold-out data were available for the manganese model only and indicated accuracies of 89–97%, ROC values of 0.73–0.75, and Kappa values of 0.06–0.30. The predictive performance of both the DO and Mn models was reasonable, considering all three of these fit metrics and the low percentages of low-DO and high-Mn events in the data.

California↗

USGS2020

Fluoride occurrence in United States groundwater

Data from 38,105 wells were used to characterize fluoride (F) occurrence in untreated United States (U.S.) groundwater. For domestic wells (n = 11,032), water from which is generally not purposely fluoridated or monitored for quality, 10.9% of the samples have F concentrations >0.7 mg/L (U.S. Public Health Service recommended optimal F concentration in drinking water for preventing tooth decay) (87% are <0.7 mg/L); 2.6% have F > 2 mg/L (EPA Secondary Maximum Contaminant Level, SMCL); and 0.6% have F > 4 mg/L (EPA MCL). The data indicate the biggest concern with F in domestic wells at the national scale could be one of under consumption of F with respect to the oral-health benchmark (0.7 mg/L). Elevated F concentrations relative to the SMCL and MCL are regionally important, particularly in the western U.S. Statistical comparisons of potentially important controlling factors in four F-concentration categories (<0.1–0.7 mg/L; >0.7–2 mg/L; >2–4 mg/L; >4 mg/L) at the national scale indicate the highest F-concentration category is associated with groundwater that has significantly greater pH values, TDS and alkalinity concentrations, and well depths, and lower Ca/Na ratios and mean annual precipitation, than the lowest F-concentration category. The relative importance of the controlling factors appears to be regionally variable. Three case studies illustrate the spatial variability in controlling factors using groundwater-age (groundwater residence time), water-isotope (evaporative concentration), and water-temperature (geothermal processes) data. Populations potentially served by domestic wells with F concentrations <0.7, >0.7, >2, and >4 mg/L are estimated to be ~28,200,000, ~3,110,000; ~522,000; and ~172,000 people, respectively, in 40 principal aquifers with at least 25 F analyses per aquifer.

Science of the Total Environment↗

USGS2025

Assessing nonpoint-source uranium pollution in an irrigated stream-aquifer system

Uranium (U) in rocks and soils of arid and semi-arid environments can be mobilized by irrigation and fertilization, posing environmental and health risks. Elevated U, along with selenium (Se) and nitrate (NO 3 ) co-constituents, necessitates careful monitoring and management. We developed a distributed-parameter numerical model to assess U pollution in an irrigated stream-aquifer system, applying it to a 552 km 2 region in Colorado's Lower Arkansas River Valley (LARV) over 14 years. A MODFLOW model, describing groundwater and stream flow, was coupled with an RT3D-OTIS model to portray reactive U transport. Calibration using the PESTPP-iES iterative ensemble smoother (iES) software indicated good agreement with observed U concentrations. The model revealed substantial and variable U levels across the LARV, highlighting potential hotspots and possible contributing factors, such as geological composition of the bedrock and near-surface shale and aquifer sediments derived from them, irrigation practices, and riparian landscape. U levels exceed the chronic standard (85th percentile = 30 μg/L, set by the US Environmental Protection Agency), which is the permissible regulatory threshold, in groundwater across 44 % of the region and along the river by an average factor of 2.9. Simulated average U concentrations in the non-riparian aquifer and river are 124 μg/L and 60 μg/L, respectively, compared with 112 μg/L and 62 μg/L for measured values. The average 85th percentile U concentration is 222 μg/L in the aquifer and 82 μg/L in the river. Average simulated U mass loading to the river is 0.17 kg/day per km, compared to an estimated 0.23 kg/day per km. Findings provide a baseline for comparing future simulated outcomes of alternative best management practices (BMPs) for U pollution mitigation and offer a methodology applicable to other irrigated regions.

Colorado↗

USGS2000

Geochemical modeling of iron, sulfur, oxygen and carbon in a coastal plain aquifer

Fe(III) reduction in the Magothy aquifer of Long Island, NY, results in high dissolved-iron concentrations that degrade water quality. Geochemical modeling was used to constrain iron-related geochemical processes and redox zonation along a flow path. The observed increase in dissolved inorganic carbon is consistent with the oxidation of sedimentary organic matter coupled to the reduction of O2 and SO4/2- in the aerobic zone, and to the reduction of SO4/2- in the anaerobic zone; estimated rates of CO2 production through reduction of Fe(III) were relatively minor by comparison. The rates of CO2 production calculated from dissolved inorganic carbon mass transfer (2.55 x 10-4 to 48.6 x 10-4 mmol 1-1 yr-1) generally were comparable to the calculated rates of CO2 production by the combined reduction of O2, Fe(III) and SO4/2- (1.31 x 10-4 to 15 x 10-4 mmol 1-1 yr-1). The overall increase in SO4/2- concentrations along the flow path, together with the results of mass-balance calculations, and variations in ??34S values along the flow path indicate that SO4/2- loss through microbial reduction is exceeded by SO4/2- gain through diffusion from sediments and through the oxidation of FeS2. Geochemichal and microbial data on cores indicate that Fe(III) oxyhydroxide coatings on sediment grains in local, organic carbon- and SO4/2- -rich zones have localized SO4/2- -reducing zones in which the formation of iron disulfides been depleted by microbial reduction and resulted in decreases dissolved iron concentrations. These localized zones of SO4/2- reduction, which are important for assessing zones of low dissolved iron for water-supply development, could be overlooked by aquifer studies that rely only on groundwater data from well-water samples for geochemical modeling. (C) 2000 Elsevier Science B.V.Fe(III) reduction in the Magothy aquifer of Long Island, NY, results in high dissolved-iron concentrations that degrade water quality. Geochemical modeling was used to constrain iron-related geochemical processes and redox zonation along a flow path. The observed increase in dissolved inorganic carbon is consistent with the oxidation of sedimentary organic matter coupled to the reduction of O2 and SO42- in the aerobic zone, and to the reduction of SO42- in the anaerobic zone; estimated rates of CO2 production through reduction of Fe(III) were relatively minor by comparison. The rates of CO2 production calculated from dissolved inorganic carbon mass transfer (2.55??10-4 to 48.6??10-4mmol l-1yr-1) generally were comparable to the calculated rates of CO2 production by the combined reduction of O2, Fe(III) and SO42- (1.31??10-4 to 15??10-4mmol l-1yr-1). The overall increase in SO42- concentrations along the flow path, together with the results of mass-balance calculations, and variations in ??34S values along the flow path indicate that SO42- loss through microbial reduction is exceeded by SO42- gain through diffusion from sediments and through the oxidation of FeS2. Geochemical and microbial data on cores indicate that Fe(III) oxyhydroxide coatings on sediment grains in local, organic carbon- and SO42--rich zones have been depleted by microbial reduction and resulted in localized SO42--reducing zones in which the formation of iron disulfides decreases dissolved iron concentrations. These localized zones of SO42- reduction, which are important for assessing zones of low dissolved iron for water-supply development, could be overlooked by aquifer studies that rely only on groundwater data from well-water samples for geochemical modeling.

Journal of Hydrology↗

USGS1997

Modeling saltwater upconing in a freshwater aquifer in south-central Kansas

The Great Bend Prairie freshwater alluvial aquifer in south-central Kansas overlies a bedrock brine aquifer of Permian age. The continuous extraction of freshwater mainly for irrigation in this area has accelerated the upward movement of the saltwater, resulting in the deterioration of water quality. Predicting saltwater upconing is critical for maintaining a long-term supply of water of good quality to the Groundwater Management District No. 5. This paper uses a numerical model to predict the effect of saltwater upconing on the salinity of pumped water. A sensitivity analysis was conducted for estimating the effects of uncertainties in model parameters on the numerical results. The most important factors affecting the salinity of discharged water are found to be the location and nature of clay layers in the aquifer, the pumping rate, the location of the well screen, and the hydraulic conductivities of the medium. The effect of uncertainties in aquifer porosity and dispersivity on salinity is appreciable but not substantial, whereas that of recharge from precipitation in the study area is relatively insignificant. In addition, a numerical model based on the field conditions observed at the Siefkes site was constructed and calibrated to reproduce and project the variation of measured water levels and discharged groundwater concentration. Finally, a number of practical management recommendations based on this study are presented.

Journal of Hydrology↗