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At least 1,009 records · Page 56Linked to original sources
USGS2001

Aquifer/aquitard interfaces: Mixing zones that enhance biogeochemical reactions

Several important biogeochemical reactions are known to occur near the interface between aquifer and aquitard sediments. These reactions include O2 reduction; denitrification; and Fe3+, SO42-, and CO2 (methanogenesis) reduction. In some settings, these reactions occur on the aquitard side of the interface as electron acceptors move from the aquifer into the electron-donor-enriched aquitard. In other settings, these reactions occur on the aquifer side of the interface as electron donors move from the aquitard into the electron-acceptor-enriched, or microorganism-enriched, aquifer. Thus, the aquifer/aquitard interface represents a mixing zone capable of supporting greater microbial activity than either hydrogeologic unit alone. The extent to which biogeochemical reactions proceed in the mixing zone and the width of the mixing zone depend on several factors, including the abundance and solubility of electron acceptors and donors on either side of the interface and the rate at which electron acceptors and donors react and move across the interface. Biogeochemical reactions near the aquifer/aquitard interface can have a substantial influence on the chemistry of water in aquifers and on the chemistry of sediments near the interface.

Hydrogeology Journal↗
USGS2011

Carbon and nitrogen biogeochemistry of a Prairie Pothole Wetland, Stutsman County, North Dakota, USA

The concentration and form of dissolved organic C (DOC) and N species (NH 4 + and NO 3 - ) were investigated as part of a larger hydrogeochemical study of the Cottonwood Lake Study Area within the Prairie Potholes region. Groundwater, pore water and surface wetland water data were used to help characterize the relationships between surface and groundwater with respect to nutrient dynamics. Photosynthesis and subsequent decomposition of vegetation in these hydrologically dynamic wetlands generates a large amount of dissolved C and N, although the subsurface till, derived in part from organic matter rich Pierre Shale, is a likely secondary source of nutrients in deeper groundwater. While surface water DOC concentrations ranged from 2.2 to 4.6 mM, groundwater values were 0.15 mM to 3.7 mM. Greater specific UV absorbance (SUVA 254 ) in the wetland water column and in soil pore waters relative to groundwater indicate more reactive DOC in the surface to near-surface waters. Circumneutral wetlands had greater SUVA254, possibly because of variations in vegetation communities. The dominant inorganic nitrogen species was NH 4 + in both wetland water and most ground water samples. The exceptions were 3 wells with NO 3 - ranging from 38 to 115 μM. Shallow groundwater wells (Well 28 and Well 13S) with greater connection to wetland surface water had greater NH 4 + concentrations (1.1 mM and 120 μM) than other well samples (3–90 μM). Pore water nutrient chemistry was more similar to surface water than ground water. Nitrogen results suggest reducing conditions in both groundwater and surface water, possibly due to the microbial uptake of O 2 by decaying vegetation in the wetland water column, labile organic C available in shallow groundwater, or the oxidation of pyrite associated with the subsurface.

North Dakota↗
USGS1974

The calculation of aquifer chemistry in hot-water geothermal systems

The temperature and chemical conditions (pH, gas pressure, and ion activities) in a geothermal aquifer supplying a producing bore can be calculated from the enthalpy of the total fluid (liquid + vapor) produced and chemical analyses of water and steam separated and collected at known pressures. Alternatively, if a single water phase exists in the aquifer, the complete analysis (including gases) of a sample collected from the aquifer by a downhole sampler is sufficient to determine the aquifer chemistry without a measured value of the enthalpy. The assumptions made are that the fluid is produced from a single aquifer and is homogeneous in enthalpy and chemical composition. These calculations of aquifer chemistry involving large amounts of ancillary information and many iterations require computer methods. A computer program in PL-1 to perform these calculations is available from the National Technical Information Service as document PB-219 376.

Journal of Research of the U.S. Geological Survey↗
USGS2005

Experimental evaluation of factors affecting temporal variability of water samples obtained from long-screened wells

As a well is pumped through time, concentrations of specific constituents in the water discharging from the well may change as a result of their transport within the well and the aquifer. A series of experiments conducted at a research site on Cape Cod, Massachusetts, examined the effects of transport on the chemistry of water samples obtained from a long-screened well. Analyses of time series of constituent concentrations in water pumped from the long-screened well showed persistent temporal trends during the first experiment. Iron concentrations decreased over a five-hour test (15 casing volumes), whereas the calcium and magnesium concentrations increased. In contrast, the time series of constituent concentrations of water discharging from the same well showed less change with time during a later experiment. Numerical simulations were undertaken to test the relative importance of several possible factors affecting the temporal variations of these constituents. During the process of quantitatively explaining the changes in concentrations over time observed in the two experiments, different system conceptualizations were used, including (1) flow and transport in the aquifer without wellbore transport, (2) flow and transport in the aquifer with advective flow and transport in the wellbore, and (3) flow and transport in the aquifer with advective flow and transport in the wellbore and a thin layer (skin) of water surrounding the well with constituent concentrations that had been altered by the presence of the well. The conjectured skin of wellbore water, which could have invaded the aquifer because of nearby sampling or dispersion and diffusion near the wellbore, in conjunction with flow and transport in the aquifer and advective transport within the wellbore, produced a reasonable match between the simulated and observed concentrations. The data analysis confirms the known fact that long-screened wells provide mechanisms for the redistribution of chemical constituents in the aquifer. The analysis provides new insight into the mechanisms responsible for the observed changes in concentrations during the sampling of long-screened wells. The analysis quantifies the effects of the various conceptual models of flow and transport in and around wells on the interpretation and meaning of chemical analyses of water withdrawn from long-screened wells.A series of experiments was conducted at a research site on Cape Cod, Massachusetts, to examine the effects of transport on the chemistry of water samples obtained from a long-screened well. A time-series analysis of the constituents concentration in pumped water revealed a persistent temporal trend during the first experiment. Iron concentrations decreased over a five-hour test, while calcium and magnesium concentrations increased. Numerical simulations were undertaken to test the relative importance of several possible factors affecting the temporal variations of the constituents to quantify the effects of the various conceptual models of flow and transport in and around the wells.

Ground Water↗
USGS2020

The response of stream ecosystems in the Adirondack region of New York to historical and future changes in atmospheric deposition of sulfur and nitrogen

The present-day acid-base chemistry of surface waters can be directly linked to contemporary observations of acid deposition; however, pre-industrial conditions are key to predicting the potential future recovery of stream ecosystems under decreasing loads of atmospheric sulfur (S) and nitrogen (N) deposition. The integrated biogeochemical model PnET-BGC was applied to 25 forest watersheds that represent a range of acid sensitivity in the Adirondack region of New York, USA to simulate the response of streams to past and future changes in atmospheric S and N deposition, and calculate the target loads of acidity for protecting and restoring stream water quality and ecosystem health. Using measured data, the model was calibrated and applied to simulate soil and stream chemistry at all study sites. Model hindcasts indicate that historically stream water chemistry in the Adirondacks was variable, but inherently sensitive to acid deposition. The median model-simulated acid neutralizing capacity (ANC) of the streams was projected to be 55 μeq L −1 before the advent of anthropogenic acid deposition (~1850), decreasing to minimum values of 10 μeq L −1 around the year 2000. The median simulated ANC increased to 13 μeq L −1 by 2015 in response to decreases in acid deposition that have occurred over recent decades. Model projections suggest that simultaneous decreases in sulfate, nitrate and ammonium deposition are more effective in restoring stream ANC than individual decreases in sulfur or nitrogen deposition. However, the increases in stream ANC per unit equivalent decrease in S deposition is greater compared to decreases in N deposition. Using empirical algorithms, fish community density and biomass are projected to increase under several deposition-control scenarios that coincide with increases in stream ANC. Model projections suggest that even under the most aggressive deposition-reduction scenarios, stream chemistry and fisheries will not fully recover from historical acidification by 2200.

New York↗
USGS2024

The dominance and growth of shallow groundwater resources in continuous permafrost environments

Water is a limited resource in Arctic watersheds with continuous permafrost because freezing conditions in winter and the impermeability of permafrost limit storage and connectivity between surface water and deep groundwater. However, groundwater can still be an important source of surface water in such settings, feeding springs and large aufeis fields that are abundant in cold regions and generating runoff when precipitation is rare. Whether groundwater is sourced from suprapermafrost taliks or deeper regional aquifers will impact water availability as the Arctic continues to warm and thaw. Previous research is ambiguous about the role of deep groundwater, leading to uncertainty regarding Arctic water availability and changing water resources. We analyzed chemistry and residence times of spring, stream, and river waters in the continuous permafrost zone of Alaska, spanning the mountains to the coastal plain. Water chemistry and age tracers show that surface waters are predominately sourced from recent precipitation and have short (<50 y) subsurface residence times. Remote sensing indicates trends in the areal extent of aufeis over the last 37 y, and correlations between aufeis extent and previous year summer temperature. Together, these data indicate that surface waters in continuous permafrost regions may be impacted by short flow paths and shallow suprapermafrost aquifers that are highly sensitive to climatic and hydrologic change over annual timescales. Despite the lack of connection to regional aquifers, continued warming and permafrost thaw may promote deepening of the shallow subsurface aquifers and creation of shallow taliks, providing some resilience to Arctic freshwater ecosystems.

Proceedings of the National Academy of Sciences↗
USGS2007

Nutrient Enrichment in Estuaries from Discharge of Shallow Ground Water, Mt. Desert Island, Maine

Nutrient enrichment from atmospheric deposition, agricultural activities, wildlife, and domestic sources is a concern at Acadia National Park because of the potential problem of water-quality degradation and eutrophication in its estuaries. Water-quality degradation has been observed at the Park?s Bass Harbor Marsh estuary but not in Northeast Creek estuary. Previous studies at Acadia National Park have estimated nutrient inputs to estuaries from atmospheric deposition and surface-water runoff, but the importance of shallow ground water that may contain nutrients derived from domestic or other sources is unknown. Northeast Creek and Bass Harbor Marsh estuaries were studied to (1) identify shallow ground-water seeps, (2) assess the chemistry of the water discharged from selected seeps, and (3) assess the chemistry of ground water in shallow ground-water hyporheic zones. The hyporheic zone is defined here as the region beneath and lateral to a stream bed, where there is mixing of shallow ground water and surface water. This study also provides baseline chemical data for ground water in selected bedrock monitoring wells and domestic wells on Mt. Desert Island. Water samples were analyzed for concentrations of nutrients, wastewater compounds, dissolved organic carbon, pH, dissolved oxygen, temperature and specific conductance. Samples from bedrock monitoring wells also were analyzed for alkalinity, major cations and anions, and trace metals. Shallow ground-water seeps to Northeast Creek and Bass Harbor Marsh estuaries at Acadia National Park were identified and georeferenced using aerial infrared digital imagery. Monitoring included the deployment of continuously recording temperature and specific conductance sensors in the seep discharge zone to access marine or freshwater signatures related to tidal flooding, gradient-driven shallow ground-water flow, or shallow subsurface flow related to precipitation events. Many potential shallow ground-water discharge zones were identified from aerial thermal imagery during flights in May and December 2003 in both estuaries. The occurrence of ground-water seeps was confirmed using continuous and discrete measurements of temperature and specific conductance in selected seeps and in the adjacent estuaries that showed salinity anomalies reflecting the input of freshwater in these complex tidal systems. Analysis of water samples from shallow ground water in the hyporheic zone and from ground-water seeps indicated the presence of elevated concentrations of dissolved nitrogen, compared to concentrations in the adjacent estuaries and surface-water tributaries draining into the estuaries. These findings indicate that shallow ground water is a source of dissolved nitrogen to the estuaries. Orthophosphate levels were low in ground water in the hyporheic zone in Bass Harbor Marsh, but somewhat higher in one hyporheic-zone well in Northeast Creek compared with the concentrations in both estuaries that were at or below detection limits. Household wastewater-related compounds were not detected in ground water in the hyporheic zone. Analysis of water samples from domestic and bedrock monitoring wells developed in fractured bedrock indicated that concentrations of dissolved nitrogen, phosphorus, and household wastewater-related compounds were typically at or below detection, suggesting that the aquifers sampled had not been contaminated from septic sources.

Scientific Investigations Report↗
USGS2004

Hydrologic and geochemical controls on pesticide and nutrient transport to two streams on the Delmarva Peninsula

Pesticides and nutrients move from application areas through ground water and surface runoff to streams on the Delmarva Peninsula. The relative importance of different transport media to the movement of these compounds in different watersheds is related to locally variable hydrologic and geochemical conditions among areas of regionally similar land use, geology, and soils. Consideration of such local variability is important to land-management efforts or future environmental investigations on the Peninsula. Chemical analyses of samples collected over a multiyear period from two streams on the Delmarva Peninsula were analyzed along with similar available analyses of ground water to document the occurrence of pesticides and nutrients, and illustrate important processes controlling their movement through watersheds to streams. The upper Pocomoke River and Chesterville Branch drain predominantly agricultural watersheds typical of the Delmarva Peninsula. Chesterville Branch drains a watershed of moderate relief, good drainage, and a permeable surficial aquifer that ranges in thickness from about 15 to 25 meters. The upper Pocomoke River Watershed, however, is extremely flat with poorly drained soils and abundant artificial drainage. Influences on the chemistry of water in each stream were determined from seasonal patterns in the concentrations of selected constituents from 1996 through 2001, and relations with streamflow. Nutrients and pesticides are detectable throughout the year in the upper Pocomoke River and Chesterville Branch. Water in both streams is typically dilute, slightly acidic, and well oxygenated, and nitrate and phosphorus concentrations generally exceed estimated natural levels. Pesticide concentrations are generally low, although concentrations of selected metabolites commonly exceed 1 microgram per liter, particularly in Chesterville Branch. Nitrate and metabolites of pesticide compounds are apparently transported to Chesterville Branch preferentially through ground water in the surficial aquifer, although selected pesticide parent compounds and less soluble nutrients move primarily in surface runoff. Conversely, the relative proportion of discharge from surficial and partially confined aquifers is the most important factor controlling the chemistry of water in the upper Pocomoke River. Surface runoff in the larger and predominantly flat upper Pocomoke River Watershed is apparently limited to particularly significant precipitation events. Transport of pesticides in surface runoff becomes important in both watersheds during such events. Instantaneous loads of pesticides in streams typically stabilize or continue to increase with increasing flow even after runoff begins, although in-stream concentrations may decrease due to dilution.

Scientific Investigations Report↗
USGS2003

Chemistry and age of ground water in the southwestern Hueco Bolson, New Mexico and Texas

This report, prepared in cooperation with El Paso Water Utilities, presents the results of an investigation to determine the chemistry and age of ground water on the southwestern side of the Hueco Bolson. The radioactive isotope carbon-14 was used to estimate the length of time that water from wells has been isolated from the atmosphere, which is the modern carbon-14 reservoir. Nine wells on the southwestern side of the Hueco Bolson were sampled for analysis of common constituents, nutrients, total organic carbon, trace elements, stable isotopes, and radioactive isotopes. Dissolved-solids concentrations in water from the wells sampled ranged from 269 to 2,630 milligrams per liter. Sodium concentrations generally increased linearly with chloride concentrations, possibly indicating mixing of dilute recharge water with sodium chloride brine. Concentrations of nutrients and trace elements generally were small. The deuterium and oxygen-18 composition in all samples except those from wells adjacent to the Rio Grande indicates that infiltration of precipitation is the main source of water to these wells and that evaporation has not affected the isotopic composition of the water. The source of water from wells adjacent to the Rio Grande is probably not the same source as the water from wells adjacent to the Franklin Mountains. The calculated apparent carbon-14 ages ranged from 12,100 to 25,500 years.

Water-Resources Investigations Report↗
USGS2017

Matching watershed and otolith chemistry to establish natal origin of an endangered desert lake sucker

Stream habitat restoration and supplemental stocking of hatchery-reared fish have increasingly become key components of recovery plans for imperiled freshwater fish; however, determining when to discontinue stocking efforts, prioritizing restoration areas, and evaluating restoration success present a conservation challenge. In this study, we demonstrate that otolith microchemistry is an effective tool for establishing natal origin of the June Sucker Chasmistes liorus , an imperiled potamodromous fish. This approach allows us to determine whether a fish is of wild or hatchery origin in order to assess whether habitat restoration enhances recruitment and to further identify areas of critical habitat. Our specific objectives were to (1) quantify and characterize chemical variation among three main spawning tributaries; (2) understand the relationship between otolith microchemistry and tributary chemistry; and (3) develop and validate a classification model to identify stream origin using otolith microchemistry data. We quantified molar ratios of Sr:Ca, Ba:Ca, and Mg:Ca for water and otolith chemistry from three main tributaries to Utah Lake, Utah, during the summer of 2013. Water chemistry (log e transformed Sr:Ca, Ba:Ca, and Mg:Ca ratios) differed significantly across all three spawning tributaries. We determined that Ba:Ca and Sr:Ca ratios were the most important variables driving our classification models, and we observed a strong linear relationship between water and otolith values for Sr:Ca and Ba:Ca but not for Mg:Ca. Classification models derived from otolith element : Ca signatures accurately sorted individuals to their experimental tributary of origin (classification tree: 89% accuracy; random forest model: 91% accuracy) and determined wild versus hatchery origin with 100% accuracy. Overall, this study aids in evaluating the effectiveness of restoration, tracking progress toward recovery, and prioritizing future restoration plans for fishes of conservation concern. Our results have further application, such as identifying subpopulations that provide the greatest reproductive contribution to a metapopulation or finding the reproductive area and origin of invasive fishes.

Utah↗
USGS1990

Coupled variations in helium isotopes and fluid chemistry: Shoshone Geyser Basin, Yellowstone National Park

Early studies of 3 He/ 4 He variations in geothermal systems have generally attributed these fluctuations to either differences in the source of the magmatic 3 He-rich helium or to local differences in the deep flux of magmatic 3 He-rich helium. Kennedy et al. (1987), however, show that near-surface processes such as boiling and dilution may also drastically affect 3 He 4 "> 3 He 4 He ratios of geothermal vapors. Helium isotope ratios were determined for several hot springs at Shoshone Geyser Basin of Yellowstone National Park for this study, along with other noble gas data. Stable isotope data and water and gas chemistry data for each spring were also compiled. The water chemistry indicates that there is one deep, hot thermal water in the area which is mixing with dilute meteoric water that has entered the system at depth. Spring HCO 3 − concentrations correlate with 3 He 4 He "> 3 He 4 He values, as in nearby Lower Geyser Basin. This correlation is attributed to variable amounts of deep dilution of thermal waters with a relatively cool water that inhibits boiling at depth, thus preventing the loss of CO 2 (and therefore HCO 3 − ) and magmatic He in the most diluted samples. Oxygen and hydrogen isotope data also support a boiling and dilution model, but to produce the observed fractionations, the boiling event would have to be extensive, with steam loss at the surface, whereas the boiling that affected the helium isotope ratios was probably a small scale event with steam loss at depth. It is possible that deep boiling occurred in the basin and that small amounts of steam escaped along fractures at about 500 m below the surface while all subsequently produced steam was lost near or at the surface.

Geochimica et Cosmochimica Acta↗
USGS1994

Chemistry of spring and well waters on Kilauea Volcano, Hawaii, and vicinity

Published and new data for chemical and isotopic samples from wells and springs on Kilauea Volcano and vicinity are presented. These data are used to understand processes that determine the chemistry of dilute meteoric water, mixtures with sea water, and thermal water. Data for well and spring samples of non-thermal water indicate that mixing with sea water and dissolution of rock from weathering are the major processes that determine the composition of dissolved constituents in water. Data from coastal springs demonstrate that there is a large thermal system south of the lower east rift of Kilauea. Samples of thermal water from shallow wells in the lower east rift and vicinity have rather variable chemistry indicating that a number of processes operate in the near surface. Water sampled from the available deep wells is different in composition from the shallow thermal water, indicating that generally there is not a significant component of deep water in the shallow wells. Data for samples from available deep wells show significant gradients in chemistry and steam content of the reservoir fluid. These gradients are interpreted to indicate that the reservoir tapped by the existing wells is an evolving vapor-dominated system.

Open-File Report↗
USGS2005

An evaluation of effects of groundwater exchange on nearshore habitats and water quality of western Lake Erie

Historically, the high potentiometric surface of groundwater in the Silurian/Devonian carbonate aquifer in Monroe County, MI resulted in discharge of highly mineralized, SO 4 -rich groundwater to the Lake Erie shoreline near both Erie State Game Area (ESGA) and Pointe Mouillee State Game Area (PMSGA). Recently, regional groundwater levels near PMSGA have been drawn down as much as 45 m below lake level in apparent response to quarry dewatering. From August to November of 2003, we conducted preliminary studies of groundwater flow dynamics and chemistry, shallow lake water chemistry, and fish and invertebrate communities at both sites. Consistent with regional observations, groundwater flow direction in the nearshore at ESGA was upward, or toward Lake Erie, and shallow nearshore groundwater chemistry was influenced by regional groundwater chemistry. In contrast, at PMSGA, the groundwater flow potential was downward and lake water, influenced by quarry discharge seeping downward into nearshore sediments, produced a different lake and shallow groundwater chemistry than at ESGA. Although the invertebrate and young fish community was similar at the two sites, taxonomic groups tolerant of degraded water quality were more prevalent at PMSGA. Sensitive taxa were more prevalent at ESGA. We propose a conceptual model, based on well-described models of groundwater/seawater interaction along coastal margins, to describe the interconnection among geologic, hydrologic, chemical, and biological processes in the different nearshore habitats of Lake Erie, and we identify processes that warrant further detailed study in the Great Lakes.

Lake Erie↗
USGS2005

Surface water acidification responses and critical loads of sulfur and nitrogen deposition in Loch Vale watershed, Colorado

We evaluated the sensitivity of The Loch, a subalpine lake in Rocky Mountain National Park in Colorado, to acidification in response to increased atmospheric loading of sulfur (S) and nitrogen (N) using the Model of Acidification of Groundwater in Catchments (MAGIC). Lake water acid‐base chemistry was moderately sensitive to changes in both S and N deposition. However, the loads of S deposition that would drive chronic lake water acid neutralizing capacity (ANC) to below 0 or 20 μeq L −1 were estimated to be 11 and 8 kg S ha −1 yr −1 , respectively, assuming constant future N deposition at current levels. Comparable loads for N deposition, assuming constant future S deposition, were estimated to be 21 and 12 kg N ha −1 yr −1 , respectively. Modeling results for Andrews Creek, an alpine tributary to The Loch, suggested critical loads for surface water acidification that averaged about one third lower. Surface water ANC = 50 μeq L −1 was projected to occur in 50 years in The Loch if S or N deposition increased by a moderate amount (<40%) but could not be achieved in Andrews Creek by reducing either S or N deposition to zero. On the basis of the results of synoptic surveys of lake water chemistry, about one fifth of the wilderness lakes in the Colorado Front Range are more acid‐sensitive than The Loch. This modeling exercise suggests the need for a regional analysis of critical loads for the larger population of acid‐sensitive aquatic resources in order to provide part of the scientific foundation for federally mandated land management decisions.

Water Resources Research↗
USGS2011

Influence of changing water sources and mineral chemistry on the everglades ecosystem

Human influences during the previous century increased mineral inputs to the Florida Everglades by changing the sources and chemistry of surface inflows. Biogeochemical responses to this enrichment include changes in the availability of key limiting nutrients such as P, the potential for increased turnover of nutrient pools due to accelerated plant decomposition, and increased rates of mercury methylation associated with sulfate enrichment. Mineral enrichment has also been linked to the loss of sensitive macrophyte species, although dominant Everglades species appear tolerant of a broad range of mineral chemistry. Shifts in periphyton community composition and function provide an especially sensitive indicator of mineral enrichment. Understanding the influence of mineral chemistry on Everglades processes and biota may improve predictions of ecosystem responses to ongoing hydrologic restoration efforts and provide guidelines for protecting remaining mineral-poor areas of this peatland.

Florida↗
USGS2023

Interaction of a legacy groundwater contaminant plume with the Little Wind River from 2015 through 2017, Riverton Processing site, Wyoming

The Riverton Processing site was a uranium mill 4 kilometers southwest of Riverton, Wyoming, that prepared uranium ore for nuclear reactors and weapons from 1958 to 1963. The U.S. Department of Energy completed surface remediation of the uranium tailings in 1989; however, groundwater below and downgradient from the tailings site and nearby Little Wind River was not remediated. Beginning in 2010, a series of floods along the Little Wind River began to mobilize contaminants in the unsaturated zone, resulting in substantial increases of uranium and other contaminants of concern in monitoring wells completed inside the contaminant plume. In 2011, the U.S. Department of Energy started a series of university and Government agency retrospective and field investigations to understand the processes controlling contaminant increases in the groundwater plume. The goals of the field investigations described in this report were to (1) identify and quantify the contaminant flux and potential associated biological effects from groundwater associated with the legacy plume as it enters a perennial stream reach, and (2) assess chemical exposure and potential effects to biological receptors from the interaction of the contaminant plume and the river. Field investigations along the Little Wind River were completed by the U.S. Geological Survey during 2015–17 in cooperation with the U.S. Department of Energy Office of Legacy Management to characterize: (1) seepage areas and seepage rates; (2) pore-water and bed sediment chemistry and hyporheic exchange and reactive loss; and (3) exposure pathways and biological receptors. All data collected during the study are contained in two U.S. Geological Survey data releases, available at https://doi.org/10.5066/F7BR8QX4 and https://doi.org/10.5066/P9J9VJBR . A variety of tools and methods were used during the field characterizations. Streambed temperature mapping, electrical resistivity tomography, electromagnetic induction, fiber-optic distributed temperature sensing, tube seepage meters, vertical thermal sensor arrays, and an environmental tracer (radon) were used to identify areas of groundwater seepage and associated seepage rates along specific sections of the study reach of the river. Drive points, minipiezometers, diffusive equilibrium in thin-film/diffusive gradients in thin-film probes, bed-sediment samples, and equal discharge increment sampling methods were used to characterize pore-water chemistry, estimate hyporheic exchange and reactive loss of selected chemical constituents, and quantify contaminant loadings entering the study reach. Sampling and analysis of surface sediments, filamentous algae, periphytic algae, and macroinvertebrates were used to characterize biological exposure pathways, metal uptake, and receptors. Areas of focused groundwater discharge identified by the fiber-optic distributed temperature sensing surveys corresponded closely with areas of elevated electrical conductivity identified by the electromagnetic induction survey results in the top 5 meters of sediment. During three monitoring periods in 2016, the mean vertical seepage rate measured with tube seepage meters was 0.45 meter per day, ranging from −0.02 to 1.55 meters per day. Five of the 11 locations where vertical thermal profile data were collected along the study reach during August 2017 indicated mean upwelling values ranging from 0.11 to 0.23 meter per day. Radon data collected from the Little Wind River during June, July, and August 2016 indicated a consistent inflow of groundwater to the central part of the study reach, in the area congruous with the center of the previously mapped groundwater plume discharge zone. During August 2017, the greatest attenuation of uranium from reactive loss in pore-water samples was observed at three locations along the study reach, at depths between 6 and 15 centimeters, and similar trends in molybdenum attenuation were also observed. Bed-sediment concentration profiles collected during 2017 also indicated attenuation of uranium and molybdenum from groundwater during hyporheic mixing of surface water with the legacy plume during groundwater upwelling into the river. Streamflow measurements combined with equal discharge increment water sampling along the study reach indicated an increase in dissolved uranium concentrations in the downstream direction during 2016 and 2017. Net uranium load entering the Little Wind River study reach was about 290 and 435 grams per day during 2016 and 2017, respectively. Biological samples indicated that low levels of uranium and molybdenum exposure were confined to the benthos in the Little Wind River within and immediately downstream from the perimeter of the groundwater plume. Concentrations of molybdenum and uranium in filamentous algae were consistently low at all sites in the study reach with no indication of increased exposure of dissolved bioavailable molybdenum or uranium at sites next to or downstream from the groundwater plume. Comparison of the August 2017 results from electromagnetic induction, tube seepage meters, vertical thermal profiling, and pore-water chemistry surveys were in general agreement in identifying areas with upwelling groundwater conditions along the study reach. However, the electroconductivity values measured with electromagnetic induction in the top 100 centimeters of sediment did not agree with sodium concentrations measured in pore-water samples collected at similar streambed depths. Differences and similarities between multiple methods can result in additional insights into hydrologic and biogeochemical processes that may be occurring along a reach of a river system interacting with shallow groundwater inputs. It may be advantageous to apply a variety of geophysical, geochemical, hydrologic, and biological tools at other Uranium Mill Tailings Remedial Action ( https://www.energy.gov/sites/prod/files/2014/10/f19/UMTRCA.pdf ) sites during the investigation of legacy contaminant plume interactions with surface-water systems.

Wyoming↗