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At least 343 records · Page 19Linked to original sources
USGS2025

Mercury cycling in the Czech GEOMON network catchments recovering from acid deposition and facing climate change

We used the catchment mass balance approach to investigate mercury (Hg) cycling at the 14 forested GEOMON catchments of the Czech Geological Survey. The temperate forest catchments had variable exposure to historic high sulfur (S) and Hg emissions, and span a range of size and elevation. We monitored monthly Hg inputs (bulk precipitation, throughfall, litterfall) and outputs (stream runoff) during 2020–2022. The catchments spanned a large gradient of historic Hg deposition, but current Hg patterns more closely aligned with catchment factors like local climate, as influenced by elevation, dissolved organic carbon (DOC) concentrations, and geology. The dominant pathway of Hg input was litterfall (averaging 44.5 ± 15.7 µg m −2 yr −1 ; > 91% of total input). Two surprising findings were that GEOMON had low Hg concentrations and fluxes in general but had the highest litterfall Hg fluxes in Europe, and these increased even further in forested areas that had bark beetle infestations. Gaseous elemental mercury (GEM), measured using passive samplers, was consistently low (1.25 to 1.66 ng m −3 ) across the 14 catchments. Stream Hg output varied across catchments and averaged 1.5 ± 1.7 µg m −2 yr −1 . The average Hg retention rate at the 14 GEOMON catchments, calculated as the fraction of average Hg inputs (throughfall + litterfall) that remained in the catchment and did not run off in streamwater, was 97%. The high catchment Hg retention combined with its strong association with DOC suggests that with climate change intensification of carbon cycling, these catchments will be a Hg source for decades to come.

Biogeochemistry↗
USGS2007

Manganese biogeochemistry in a central Czech Republic catchment

Mn biogeochemistry was studied from 1994 to 2003 in a small forested catchment in the central Czech Republic using the watershed mass balance approach together with measurements of internal stores and fluxes. Mn inputs in bulk deposition were relatively constant during a period of sharply decreasing acidic deposition, suggesting that the Mn source was terrestrial, and not from fossil fuel combustion. Mn inputs in bulk deposition and Mn supplied by weathering each averaged 13 mg m-2 year-1 (26 mg m -2 year-1 total input), whereas Mn export in streamwater and groundwater averaged 43 mg m-2 year-1. Thus an additional Mn source is needed to account for 17 mg m-2 year -1. Internal fluxes and pools of Mn were significantly greater than annual inputs and outputs. Throughfall Mn flux was 70 mg m-2 year-1, litterfall Mn flux was 103 mg m-2 year -1, and Mn net uptake by vegetation was 62 mg m-2 year-1. Large pools of labile or potentially labile Mn were present in biomass and surficial soil horizons. Small leakages from these large pools likely supply the additional Mn needed to close the watershed mass balance. This leakage may reflect an adjustment of the ecosystem to recent changes in atmospheric acidity. ?? 2007 Springer Science+Business Media B.V.

Water, Air, & Soil Pollution↗
USGS1992

Interoceanic variation in the rare earth, major, and trace element depositional chemistry of chert: Perspectives gained from the DSDP and ODP record

Rare earth element (REE), major, and trace element abundances and relative fractionations in forty nodular cherts sampled by the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) indicate that the REE composition of chert records the interplay between terrigenous sources and scavenging from the local seawater. Major and (non-REE) trace element ratios indicate that the aluminosilicate fraction within the chert is similar to NASC (North American Shale Composite), with average Pacific chert including ~7% NASC-like particles, Indian chert ~ 11% NASC, Atlantic chert ~ 17% NASC, and southern high latitude (SHL) chert 53% NASC. Using La as a proxy for ∑REE, approximations of La ex (the amount of La in excess of that supplied by the detrital aluminosilicate fraction) indicate that Pacific chert contains the greatest La ex (85% of La total ) and SHL chert the least (38% of La total ). As shown by interelement associations, this La ex is most likely an adsorbed component onto aluminosilicate and phosphatic phases. Accordingly, chert from the large Pacific Ocean, where deposition occurs relatively removed from significant terrigenous input, records a depositional REE signal dominated by adsorption of dissolved REEs from seawater. Pacific chert CeCe * ⪡ 1 and La n Yb n ~ 0.8-1, resulting from adsorption of local Ce-depleted seawater and preferential adsorption of LREEs from seawater (e.g., La n Yb n ~ 0.4), which increases the La n Yb n ratio recorded in chert. Chert from the Atlantic basin, a moderately sized ocean basin lined by passive margins and with more terrigenous input than the Pacific, records a mix of adsorptive and terrigenous REE signals, with moderately negative Ce anomalies and La n Yb n "> La n Yb n ratios intermediate to those of the Pacific and those of terrigenous input. Chert from the SHL region is dominated by the large terrigenous input on the Antarctic passive margin, with inherited Ce Ce * ~1 "> CeCe * ~1 and inherited La n Yb n "> La n Yb n values of ~1.2–1.4. Ce Ce * "> ~1.2–1.4.CeCe * does not vary with age, either throughout the entire data base or within a particular basin. Overall, Ce Ce * "> CeCe * does not correlate with P 2 O 5 concentrations, even though phosphatic phases may be an important REE carrier. This and previous studies of the large-scale controlling parameters of sedimentary REEs across ocean basins collectively indicate that REE indices of depositional regime (e.g., Ce Ce * "> CeCe * , La n Yb n "> La n Yb n , La ex ) are reproducible in a variety of sediment and rock lithologies, ages, and ocean basins, and present a coherent tool for paleoceanographic and tectonic basin reconstructions.

Geochimica et Cosmochimica Acta↗
USGS1990

Using 222Rn to examine groundwater/surface discharge interaction in the Rio Grande de Manati, Puerto Rico

222 Rn was used in the karst drainage basin of the Rio Grande de Manati in Puerto Rico to study groundwater/surface flow relationships. Locations of groundwater influx along two sections of the Rio Grande de Manati were identified. The 222 Rn measurements were used together with stream discharge data in a mass balance equation to quantify the groundwater inputs. The investigation established that both of the sections of the Rio Manati surveyed not only gained groundwater, but lost surface flow. It was calculated that the river gained about 1.2m 3 s −1 and lost 0.5m 3 s −1 to the aquifer between Ciales and United States Geological Survey gauging station 5. Between United States Geological Survey gauging stations 6 and 7, groundwater influx and stream flow loss occurred simultaneously with groundwater inputs equalling surface discharge losses of 4m 3 s −1 . The study successfully demonstrated the innovative application of 222 Rn as a geochemical tracer in examining groundwater/surface flow relationships in a karst system.

Puerto Rico↗
USGS1996

Spatial uncertainty analysis: Propagation of interpolation errors in spatially distributed models

In simulation modelling, it is desirable to quantify model uncertainties and provide not only point estimates for output variables but confidence intervals as well. Spatially distributed physical and ecological process models are becoming widely used, with runs being made over a grid of points that represent the landscape. This requires input values at each grid point, which often have to be interpolated from irregularly scattered measurement sites, e.g., weather stations. Interpolation introduces spatially varying errors which propagate through the model We extended established uncertainty analysis methods to a spatial domain for quantifying spatial patterns of input variable interpolation errors and how they propagate through a model to affect the uncertainty of the model output. We applied this to a model of potential evapotranspiration (PET) as a demonstration. We modelled PET for three time periods in 1990 as a function of temperature, humidity, and wind on a 10-km grid across the U.S. portion of the Columbia River Basin. Temperature, humidity, and wind speed were interpolated using kriging from 700- 1000 supporting data points. Kriging standard deviations (SD) were used to quantify the spatially varying interpolation uncertainties. For each of 5693 grid points, 100 Monte Carlo simulations were done, using the kriged values of temperature, humidity, and wind, plus random error terms determined by the kriging SDs and the correlations of interpolation errors among the three variables. For the spring season example, kriging SDs averaged 2.6??C for temperature, 8.7% for relative humidity, and 0.38 m s-1 for wind. The resultant PET estimates had coefficients of variation (CVs) ranging from 14% to 27% for the 10-km grid cells. Maps of PET means and CVs showed the spatial patterns of PET with a measure of its uncertainty due to interpolation of the input variables. This methodology should be applicable to a variety of spatially distributed models using interpolated inputs.

Ecological Modelling↗
USGS2011

Arsenic in sediments, groundwater, and streamwater of a glauconitic Coastal Plain terrain, New Jersey, USA-Chemical " fingerprints" for geogenic and anthropogenic sources

Glauconite-bearing deposits are found worldwide, but As levels have been determined for relatively few. The As content of glauconites in sediments of the Inner Coastal Plain of New Jersey can exceed 100 mg/kg, and total As concentrations (up to 5.95 μg/L) found historically and recently in streamwaters exceed the State standard. In a major watershed of the Inner Coastal Plain, chemical “fingerprints” were developed for streambed sediments and groundwater to identify contributions of As to the watershed from geologic and anthropogenic sources. The fingerprint for streambed sediments, which included Be, Cr, Fe and V, indicated that As was predominantly of geologic origin. High concentrations of dissolved organic C, nutrients (and Cl − ) in shallow groundwater indicated anthropogenic inputs that provided an environment where microbial activity released As from minerals to groundwater discharging to the stream. Particulates in streamwater during high flow constituted most of the As load; the chemical patterns for these particulates resembled the geologic fingerprint of the streambed sediments. The As/Cr ratio of these suspended particles likely indicates they derived not only from runoff, but from groundwater inputs, because As contributed by groundwater is sequestered on streambed sediments. Agricultural inputs of As were not clearly identified, although chemical characteristics of some sediments indicated vehicle-related inputs of metals. Sediment sampling during dry and wet years showed that, under differing hydrologic conditions, local anthropogenic fingerprints could be obscured but the geologic fingerprint, indicating glauconitic sediments as an As source, was robust.

New Jersey↗
USGS2011

Prediction and assimilation of surf-zone processes using a Bayesian network: Part II: Inverse models

A Bayesian network model has been developed to simulate a relatively simple problem of wave propagation in the surf zone (detailed in Part I). Here, we demonstrate that this Bayesian model can provide both inverse modeling and data-assimilation solutions for predicting offshore wave heights and depth estimates given limited wave-height and depth information from an onshore location. The inverse method is extended to allow data assimilation using observational inputs that are not compatible with deterministic solutions of the problem. These inputs include sand bar positions (instead of bathymetry) and estimates of the intensity of wave breaking (instead of wave-height observations). Our results indicate that wave breaking information is essential to reduce prediction errors. In many practical situations, this information could be provided from a shore-based observer or from remote-sensing systems. We show that various combinations of the assimilated inputs significantly reduce the uncertainty in the estimates of water depths and wave heights in the model domain. Application of the Bayesian network model to new field data demonstrated significant predictive skill (R 2 = 0.7) for the inverse estimate of a month-long time series of offshore wave heights. The Bayesian inverse results include uncertainty estimates that were shown to be most accurate when given uncertainty in the inputs (e.g., depth and tuning parameters). Furthermore, the inverse modeling was extended to directly estimate tuning parameters associated with the underlying wave-process model. The inverse estimates of the model parameters not only showed an offshore wave height dependence consistent with results of previous studies but the uncertainty estimates of the tuning parameters also explain previously reported variations in the model parameters.

Coastal Engineering↗
USGS2019

The effects of restored hydrologic connectivity on floodplain trapping vs. release of phosphorus, nitrogen, and sediment along the Pocomoke River, Maryland USA

River channelization and artificial levees have decreased the hydrologic connectivity of river-floodplain systems around the world. In response, restoration through enhancing connectivity has been advocated to improve the functions of floodplains, but uncertain benefits and the possibility of phosphate release from re-flooded soils has limited implementation. In this study, we measured change in floodplain P, N, and sediment mass balances after restoration along channelized reaches in the lowland Pocomoke River, Maryland USA. Two floodplains (one headwater, one mainstem) restored through partial levee breaches were compared to two additional mainstem floodplains (one natural unchannelized, one unrestored channelized). Potential soluble reactive P (SRP) release from soil cores during experimental laboratory floods; soil P, Fe, and Al fractionation; and deposition and P and N content of sediment were measured before and after the restoration period, as well as in situ inputs and release of SRP and dissolved inorganic N from soils after restorations. Potential SRP release, during both the before and after restoration period, was greatest at the channelized mainstem and restored mainstem sites, lower at the restored headwater site, and small at the natural mainstem site. Both restored sites had smaller potential SRP release after restoration compared to before restoration. In situ SRP release slightly exceeded inputs to soils at connected sites during the post-restoration period, with less net release at the restored sites compared to the natural mainstem site. The magnitude of gross and net SRP release from soils in the field was smaller than, and uncorrelated with, potential SRP release estimated from laboratory experimental floods. Gross soil SRP release rates in the field were predictable using the ratio of soil oxalate-extractable P/Al. Sedimentation inputs of P and N increased at all sites during the post-restoration period, with rates at restored sites intermediate compared to the much higher rates at the natural mainstem site and somewhat lower rates at the channelized mainstem site. These sediment inputs of nutrients were much larger than rates of inorganic P and N release from soils, indicating net trapping of P and N after restoration. Restoring floodplain hydrologic connectivity showed moderate success at increasing the trapping of P, N, and sediment, with relatively little phosphate release, and therefore improving water quality.

Maryland↗
USGS2004

New constraints on the sources and behavior of neodymium and hafnium in seawater from Pacific Ocean ferromanganese crusts

The behavior of dissolved Hf in the marine environment is not well understood due to the lack of direct seawater measurements of Hf isotopes and the limited number of Hf isotope time-series obtained from ferromanganese crusts. In order to place better constraints on input sources and develop further applications, a combined Nd-Hf isotope time-series study of five Pacific ferromanganese crusts was carried out. The samples cover the past 38 Myr and their locations range from sites at the margin of the ocean to remote areas, sites from previously unstudied North and South Pacific areas, and water depths corresponding to deep and bottom waters. For most of the samples a broad coupling of Nd and Hf isotopes is observed. In the Equatorial Pacific ENd and EHf both decrease with water depth. Similarly, ENd and EHf both increase from the South to the North Pacific. These data indicate that the Hf isotopic composition is, in general terms, a suitable tracer for ocean circulation, since inflow and progressive admixture of bottom water is clearly identifiable. The time-series data indicate that inputs and outputs have been balanced throughout much of the late Cenozoic. A simple box model can constrain the relative importance of potential input sources to the North Pacific. Assuming steady state, the model implies significant contributions of radiogenic Nd and Hf from young circum-Pacific arcs and a subordinate role of dust inputs from the Asian continent for the dissolved Nd and Hf budget of the North Pacific. Some changes in ocean circulation that are clearly recognizable in Nd isotopes do not appear to be reflected by Hf isotopic compositions. At two locations within the Pacific Ocean a decoupling of Nd and Hf isotopes is found, indicating limited potential for Hf isotopes as a stand-alone oceanographic tracer and providing evidence of additional local processes that govern the Hf isotopic composition of deep water masses. In the case of the Southwest Pacific there is evidence that decoupling may have been the result of changes in weathering style related to the buildup of Antarctic glaciation. Copyright ?? 2004 Elsevier Ltd.

Geochimica et Cosmochimica Acta↗
USGS2018

Geochemical characterization and modeling of regional groundwater contributing to the Verde River, Arizona between Mormon Pocket and the USGS Clarkdale gage

We use synoptic surveys of stream discharge, stable isotopes, and dissolved noble gases to identify the source of groundwater discharge to the Verde River in central Arizona. The Verde River more than doubles in discharge in Mormon Pocket over a 1.4 km distance that includes three discrete locations of visible spring input to the river and other diffuse groundwater inputs. A detailed study of the Verde River between Mormon Pocket and the USGS Clarkdale Gage was conducted to better constrain the location of groundwater inputs, the geochemical signature and constrain the source of groundwater input. Discharge, water quality parameters (temperature, pH, specific conductance, and dissolved oxygen), stable isotopes (δ18O and δ2H), noble gases (He, Ne, Ar, Kr and Xe), and radon (222Rn) from river water were collected. Groundwater samples from springs and wells in the area were collected and analyzed for tracers measured in the stream along with some additional analytes (major ions, strontium isotopes (87Sr/86Sr), carbon-14, δ13C, and tritium). Groundwater isotopic signature is consistent with a regional groundwater source. Groundwater springs discharging to the river have a depleted stable isotopic signature indicating recharge source up to 1000 m higher than the discharge location in the Verde River and are significantly fresher than stream water. Spring water has a radiocarbon age of several thousand years and some areas have tritium less than the laboratory reporting level or low concentrations of tritium (1.5 TU). The strontium isotopes indicate groundwater interaction with tertiary volcanic rock and Paleozoic sedimentary rocks. Along the study reach with distance downstream, Verde stream water chemistry shows increased 222Rn, freshening, increased 4He, and isotopic depletion with distance downstream. We estimated total groundwater discharge by inverting a stream transport model against 222Rn and discharge measured in the stream. The salinity, 4He, and stable isotope composition of discharging groundwater was then estimated by fitting modeled values to observed in-stream values. Estimated groundwater inflow to the stream was well within the ranges observed in springs, indicating that the main source of streamflow is deep, regional groundwater. These results show that synoptic surveys of environmental tracers in streams can be used to estimate the isotopic composition and constrain the source of groundwater discharging to streams. Our data provide direct field evidence that deep, regional groundwater discharge can be a significant source of streamflow generation in arid, topographically complex watersheds.

Arizona↗
USGS2022

Stable isotopes provide insight into sources and cycling of N compounds in the Sacramento-San Joaquin Delta, California, USA

River deltas and their diverse array of aquatic environments are increasingly impacted by anthropogenic inputs of nitrogen (N). These inputs can alter the N biogeochemistry of these systems and promote undesirable phenomena including harmful algae blooms and invasive aquatic macrophytes. To examine N sources and biogeochemical processes in the Sacramento-San Joaquin Delta, a river delta located in central California, USA, that is fed primarily by the Sacramento River, we utilized a multi-tracer approach that measured N species concentrations and stable isotope values monthly from April 2011 to November 2012 in samples collected from the channelized mainstem of the Sacramento River, two channelized distributaries of the Sacramento River, and the Cache Slough Complex, a network of Sacramento River tributaries and shallow water wetland habitat. We found that the Sacramento River and its channelized distributaries received N primarily in the form of NH 4 + from treated wastewater effluent and that NH 4 + was lost rapidly while NO 3 − was gained more slowly during subsequent downstream transit, driven by an array of biogeochemical processes whose identities could be constrained via examination of stable isotope values. The Cache Slough Complex, which was characterized by lower net flows and higher water residence times than the Sacramento River and its distributaries, received variable inputs of low conductivity water elevated in NH 4 + from the Sacramento River and higher conductivity water elevated in NO 3 − from landward tributaries. Deviations from expected conservative mixing of these sources were spatially variable but broadly indicative of local inputs of treated wastewater effluent NO 3 − , conversion of Sacramento River NH 4 + to NO 3 − via nitrification, uptake of NH 4 + and NO 3 − by phytoplankton, and remineralization of organic N. These findings highlight both the diversity in N dynamics in anthropogenically impacted river delta environments and the utility of a multi-tracer approach in constraining these processes in such complex systems.

California↗
USGS2023

Long short-term memory models to quantify long-term evolution of streamflow discharge and groundwater depth in Alabama

Long short-term memory (LSTM) models have been shown to be efficient for rainfall-runoff modeling, and to a lesser extent, for groundwater depth forecasting. In this study, LSTMs were applied to quantify the spatiotemporal evolution of surface and subsurface hydrographs in Alabama in the Southeastern United States, where water sustainability has not been fully quantified across spatiotemporal scales. First, the surface water LSTM model with extensive dynamic (precipitation and other weather variables) and static (basin characteristics) inputs predicted the main characteristics of streamflow for six years at 19 gauged basins in Alabama. The model tended to underestimate extremely high streamflow but adding drainage density as an input feature slightly improved the predictions of extreme events. Second, to predict the groundwater depth evolution, a groundwater LSTM (GW-LSTM) model was proposed and applied using static inputs capturing the aquifers' hydrogeological properties and dynamic inputs of meteorological information. Three precipitation scenarios were also explored to evaluate the groundwater hydrograph evolution in the next two decades. The GW-LSTM model predicted the general trend of daily groundwater depth fluctuations (at 21 wells distributed across Alabama from 1990 to 2021) including most extremely high groundwater levels, and recovered groundwater depth for locations withheld from model training and validation. This study, therefore, extended the application of LSTMs in quantifying the spatiotemporal evolution of surface water and groundwater, two manifestations of a single integrated resource.

Alabama↗
USGS2001

Processes regulating watershed chemical export during snowmelt, fraser experimental forest, Colorado

In the Central Rocky Mountains, snowfall dominates precipitation. Airborne contaminants retained in the snowpack can affect high elevation surface water chemistry during snowmelt. At the Fraser Experimental Forest (FEF), located west of the Continental Divide in Central Colorado, snowmelt dominates the annual hydrograph, and accounts for >95% of annual stream water discharge. During the winters of 1989-1993, we measured precipitation inputs, snowpack water equivalent (SWE) and ion content, and stream water chemistry every 7-10 days along a 3150-3500 m elevation gradient in the subalpine and alpine Lexen Creek watershed. The study objectives were to (1) quantify the distribution of SWE and snowpack chemical content with elevation and aspect, (2) quantify snowmelt rates, temperature of soil, snowpack, and air with elevation and aspect, and (3) use change in upstream-downstream water chemistry during snowmelt to better define alpine and subalpine flowpaths. The SWE increased with elevation (P < 0.001) and was greater on NE aspects (P < 0.05). Subalpine soils were unfrozen. Snowmelt occurred throughout winter at low elevation (3150 m), SE aspect stations. By snowpack peak water equivalent (PWE), the snowpack lost about a third (0.24 m) of its moisture. The subalpine snowpack had warm (> - 3??C) temperatures throughout winter which resulted in significant snowpack ion loss. By snowpack PWE in mid May, the snowpack had lost almost half the cumulative precipitation H+, NH4+, and SO42- inputs and a third of the NO3- input. Windborne soil particulate inputs late in winter increased snowpack base cation content. Variation in subalpine SWE and snowpack ion content with elevation and aspect, and wind redistribution of snowfall in the alpine resulted in large year-to-year differences in the timing and magnitude of SWE, PWE, and snowpack ion content. The alpine stream water ion concentrations changed little during snowmelt indicating meltwater passed quickly through surface porous soils and was well mixed before entering the stream. Conversely, subalpine stream water chemistry was diluted during snowmelt suggesting much melt water moved to the stream as shallow subsurface lateral flow. Published by Elsevier Science B.V.

Journal of Hydrology↗
USGS2002

Spatial and temporal variability of trace element concentrations in an urban subtropical watershed, Honolulu, Hawaii

Trace metal concentrations in soils and in stream and estuarine sediments from a subtropical urban watershed in Hawaii are presented. The results are placed in the context of historical studies of environmental quality (water, soils, and sediment) in Hawaii to elucidate sources of trace elements and the processes responsible for their distribution. This work builds on earlier studies on sediments of Ala Wai Canal of urban Honolulu by examining spatial and temporal variations in the trace elements throughout the watershed. Natural processes and anthropogenic activity in urban Honolulu contribute to spatial and temporal variations of trace element concentrations throughout the watershed. Enrichment of trace elements in watershed soils result, in some cases, from contributions attributed to the weathering of volcanic rocks, as well as to a more variable anthropogenic input that reflects changes in land use in Honolulu. Varying concentrations of As, Cd, Cu, Pb and Zn in sediments reflect about 60 a of anthropogenic activity in Honolulu. Land use has a strong impact on the spatial distribution and abundance of selected trace elements in soils and stream sediments. As noted in continental US settings, the phasing out of Pb-alkyl fuel additives has decreased Pb inputs to recently deposited estuarine sediments. Yet, a substantial historical anthropogenic Pb inventory remains in soils of the watershed and erosion of surface soils continues to contribute to its enrichment in estuarine sediments. Concentrations of other elements (e.g., Cu, Zn, Cd), however, have not decreased with time, suggesting continued active inputs. Concentrations of Ba, Co, Cr, Ni, V and U, although elevated in some cases, typically reflect greater proportions attributed to natural sources rather than anthropogenic input. ?? 2002 Elsevier Science Ltd. All rights reserved.

Applied Geochemistry↗
USGS2010

Nitrate in groundwater of the United States, 1991-2003

An assessment of nitrate concentrations in groundwater in the United States indicates that concentrations are highest in shallow, oxic groundwater beneath areas with high N inputs. During 1991-2003, 5101 wells were sampled in 51 study areas throughout the U.S. as part of the U.S. Geological Survey National Water-Quality Assessment (NAWQA) program. The well networks reflect the existing used resource represented by domestic wells in major aquifers (major aquifer studies), and recently recharged groundwater beneath dominant land-surface activities (land-use studies). Nitrate concentrations were highest in shallow groundwater beneath agricultural land use in areas with well-drained soils and oxic geochemical conditions. Nitrate concentrations were lowest in deep groundwater where groundwater is reduced, or where groundwater is older and hence concentrations reflect historically low N application rates. Classification and regression tree analysis was used to identify the relative importance of N inputs, biogeochemical processes, and physical aquifer properties in explaining nitrate concentrations in groundwater. Factors ranked by reduction in sum of squares indicate that dissolved iron concentrations explained most of the variation in groundwater nitrate concentration, followed by manganese, calcium, farm N fertilizer inputs, percent well-drained soils, and dissolved oxygen. Overall, nitrate concentrations in groundwater are most significantly affected by redox conditions, followed by nonpoint-source N inputs. Other water-quality indicators and physical variables had a secondary influence on nitrate concentrations.

Environmental Science & Technology↗
USGS1997

Risk of nitrate in groundwaters of the United States - A national perspective

Nitrate contamination of groundwater occurs in predictable patterns, based on findings of the U.S. Geological Survey's (USGS) National Water Quality Assessment (NAWQA) Program. The NAWQA Program was begun in 1991 to describe the quality of the Nation's water resources, using nationally consistent methods. Variables affecting nitrate concentration in groundwater were grouped as 'input' factors (population density end the amount of nitrogen contributed by fertilizer, manure, and atmospheric sources) and 'aquifer vulnerability' factors (soil drainage characteristic and the ratio of woodland acres to cropland acres in agricultural areas) and compiled in a national map that shows patterns of risk for nitrate contamination of groundwater. Areas with high nitrogen input, well-drained soils, and low woodland to cropland ratio have the highest potential for contamination of shallow groundwater by nitrate. Groundwater nitrate data collected through 1992 from wells less than 100 ft deep generally verified the risk patterns shown on the national map. Median nitrate concentration was 0.2 mg/L in wells representing the low-risk group, and the maximum contaminant level (MCL) was exceeded in 3% of the wells. In contrast, median nitrate concentration was 4.8 mg/L in wells representing the high-risk group, and the MCL was exceeded in 25% of the wells.Nitrate contamination of groundwater occurs in predictable patterns, based on findings of the U.S. Geological Survey's (USGS) National Water Quality Assessment (NAWQA) Program. The NAWQA Program was begun in 1991 to describe the quality of the Nation's water resources, using nationally consistent methods. Variables affecting nitrate concentration in groundwater were grouped as `input' factors (population density and the amount of nitrogen contributed by fertilizer, manure, and atmospheric sources) and `aquifer vulnerability' factors (soil drainage characteristic and the ratio of woodland acres to cropland acres in agricultural areas) and compiled in a national map that shows patterns of risk for nitrate contamination of groundwater. Areas with high nitrogen input, well-drained soils, and low woodland to cropland ratio have the highest potential for contamination of shallow groundwater by nitrate. Groundwater nitrate data collected through 1992 from wells less than 100 ft deep generally verified the risk patterns shown on the national map. Median nitrate concentration was 0.2 mg/L in wells representing the low-risk group, and the maximum contaminant level (MCL) was exceeded in 3% of the wells. In contrast, median nitrate concentration was 4.8 mg/L in wells representing the high-risk group, and the MCL was exceeded in 25% of the wells.

Environmental Science & Technology↗
USGS1998

Rapid oxidation of geothermal arsenic(III) in streamwaters of the eastern Sierra Nevada

Arsenic redox cycling was examined in source waters of the Los Angeles Aqueduct, specifically at Hot Creek, a tributary of the Owens River. Elevated arsenic concentrations in Hot Creek result from geothermal inputs. Total arsenic and As(III) concentrations were determined in the creek and in hot spring pools along its banks. Samples were processed in the field using anion-exchange columns to separate inorganic As(III) and As(V) species. Downstream of the geothermal inputs, decreasing contributions of As(III) to total arsenic concentrations indicated rapid in-stream oxidation of As(III) to As(V) with almost complete oxidation occurring within 1200 m. Based on assumed plug flow transport and a flow velocity of about 0.4 m/s, the pseudo- first-order half-life calculated for this reaction was approximately 0.3 h. Conservative transport of total dissolved arsenic was observed over the reach. Pseudo-first-order reaction rates determined for As(III) oxidation in batch studies conducted in the field with aquatic macrophytes and/or macrophyte surface matter were comparable to the in-stream oxidation rate observed along Hot Creek. In batch kinetic studies, oxidation was not observed after sterile filtration or after the addition of antibiotics, which indicates that bacteria attached to submerged macrophytes are mediating the rapid As(III) oxidation reaction.Arsenic redox cycling was examined in source waters of the Los Angeles Aqueduct, specifically at Hot Creek, a tributary of the Owens River. Elevated arsenic concentrations in Hot Creek result from geothermal inputs. Total arsenic and As(III) concentrations were determined in the creek and in hot spring pools along its banks. Samples were processed in the field using anion-exchange columns to separate inorganic As(III) and As(V) species. Downstream of the geothermal inputs, decreasing contributions of As(III) to total arsenic concentrations indicated rapid in-stream oxidation of As(III) to As(V) with almost complete oxidation occurring within 1200 m. Based on assumed plug flow transport and a flow velocity of about 0.4 m/s, the pseudo-first-order half-life calculated for this reaction was approximately 0.3 h. Conservative transport of total dissolved arsenic was observed over the reach. Pseudo-first-order reaction rates determined for As(III) oxidation in batch studies conducted in the field with aquatic macrophytes and/or macrophyte surface matter were comparable to the in-stream oxidation rate observed along Hot Creek. In batch kinetic studies, oxidation was not observed after sterile filtration or after the addition of antibiotics, which indicates that bacteria attached to submerged macrophytes are mediating the rapid As(III) oxidation reaction.

Environmental Science & Technology↗
USGS2000

Selenium stable isotope ratios as indicators of sources and cycling of selenium: Results from the northern reach of San Francisco Bay

Selenium stable isotope ratios can serve as indicators of Se sources and reduction of Se oxyanions, much as sulfur and nitrogen isotope ratios do in sulfur and nitrogen biogeochemical studies. A new analytical method, which allows precise Se isotope ratio measurements on 500 ng of Se, greatly enhances analysis of environmental samples. This paper presents the first environmental study to use Se stable isotopes. 80Se/76Se ratios, relative to a provisional standard, were measured in water, oil refinery wastewater, total sediment digest, and sediment extracts from the Carquinez area in the San Francisco Bay Estuary, Se isotope ratios in total sediment Se and in extracts designed to recover Se0 are slightly (about 2???) enriched in the lighter isotope relative to local bay water Se. This difference is smaller than the isotopic fractionations expected upon reduction of Se(VI) or Se(IV) to Se(0) and suggests that reduction of soluble selenium from the overlying waters is not the dominant process by which Se is incorporated into the sediments. Consistent isotopic differences between riverine and refinery inputs were not observed, and thus tracing of refinery inputs with Se isotopes is not possible in this system.Selenium stable isotope ratios can serve as indicators of Se sources and reduction of Se oxyanions, much as sulfur and nitrogen isotope ratios do in sulfur and nitrogen biogeochemical studies. A new analytical method, which allows precise Se isotope ratio measurements on 500 ng of Se, greatly enhances analysis of environmental samples. This paper presents the first environmental study to use Se stable isotopes. 80Se/76Se ratios, relative to a provisional standard, were measured in water, oil refinery wastewater, total sediment digests, and sediment extracts from the Carquinez area in the San Francisco Bay Estuary. Se isotope ratios in total sediment Se and in extracts designed to recover Se0 are slightly (about 2 per mil) enriched in the lighter isotope relative to local bay water Se. This difference is smaller than the isotopic fractionations expected upon reduction of Se(VI) or Se(IV) to Se(0) and suggests that reduction of soluble selenium from the overlying waters is not the dominant process by which Se is incorporated into the sediments. Consistent isotopic differences between riverine and refinery inputs were not observed, and thus tracing of refinery inputs with Se isotopes is not possible in this system.

Environmental Science & Technology↗