National Water-Quality Assessment Program Research Articles

An ecological study regarding the association between paraquat exposure and end stage renal disease

BackgroundPersons who experience paraquat poisoning rapidly develop damage to a variety of organ systems including acute kidney injury (AKI), the occurrence of which is associated with an increased risk of death. However, little is known about the effects of chronic paraquat exposure on renal function and the onset of chronic renal disease. The objective of the current study is to assess the association between paraquat exposure and the incidence of end stage renal disease (ESRD) in the United States.MethodsData on the incidence of ESRD for the period 2010 through 2017 and kilograms of paraquat use per square mile for each county in the conterminous United States was obtained from the United States Renal Data System (USRDS) and the National Water Quality Assessment (NAWQA) Program, respectively. Negative binomial regression was used to estimate rate ratios (RRs) and 95% confidence intervals (CIs) for the association between quartiles of paraquat exposure and the incidence of ESRD.ResultsThe incidence of ESRD increased with increasing paraquat density. Based on a 20-year exposure lag, those in the highest paraquat density quartile had a 21% higher rate of ESRD compared to the lowest quartile whereas for a 15-year lag the increase was 26%. Adjusted associations were attenuated though still followed an increasing linear trend across quintiles.ConclusionsThe results of this study are consistent with a large number of studies documenting a high incidence of AKI and a small number of studies chronic renal disease following acute and chronic paraquat exposure, respectively. While the pathophysiological mechanisms underlying kidney injury following paraquat poisoning are well understood, more research is necessary to understand the natural history of chronic kidney disease due to chronic paraquat exposure.

Taxonomy and nomenclature of some members within the Obtusae section of Nitzschia Hassall (Bacillariophyceae) including descriptions of two new species

Light and scanning electron microscope observations are made on some members of the section Obtusae in the genus Nitzschia (Bacillariales, Bacillariophyceae). We use type and other material derived from both the National Water Quality Assessment (NAWQA) program in the USA and from Egypt to assess five different taxa, two of which are described as new to science (Nitzschia lowei, sp. nov., Nitzschia potapovae, sp. nov.) and another is made as a new nomenclatural combination (Nitzschia kurzeana var. multistriata (Hohn) comb. nov.). Other names used in the literature for some of these taxa are considered and discussed. Based on the features of sigmoid valves, eccentric raphe not in an elevated keel, lack of a conopeum, strongly bent, elongated external proximal raphe ends extending onto the valve face, and presence of a differentiated central 'nodule', members of this group are clearly differentiated from the generitype of Nitzschia and most other recognized sections of the genus.

Implications of climate change on nutrient pollution: a look into the nitrogen and phosphorus loadings in the Great Miami and Little Miami watersheds in Ohio

The Great Miami (GM) and Little Miami (LM) watersheds in Ohio are included in the 51 major river systems in the United States for long-term assessment of conditions of water quality under the USGS National Water-Quality Assessment (NAWQA) Program. Nitrogen (N) and phosphorus (P) loadings have decreased over the years in the GM and LM basins, however, they still remain among the highest concentrations detected in the nation. The latest nutrient loading analysis from Ohio Environmental Protection Agency (OEPA) showed slightly above period of record averages for N and P. Significant amounts of mobilized agricultural N and P from fertilizers in watersheds are transported to the Ohio River and other coastal marine systems such as the Gulf of Mexico, leading to increased growth of harmful algal blooms. In the GM and LM watersheds, changes in flood frequency and intensity are projected to occur in the future as heavy precipitation events are likely to increase as a result of climate change. As climate plays a role for nutrient transformation and transport, studies have shown that N and P inputs to surface waters from agriculture and other sources are projected to continue to increase over the next several decades. This paper reviewed the means of how the GM and LM watersheds are ecologically affected by the N and P nutrient pollution and how climate change impacts the quantity of N and P loadings that go into the river systems. Without better understanding of the nutrient loading processes and their association with various agricultural practices and interactions with climate, there could be additional threats to water quality in both the GM and LM watersheds in decades to come. This literature review has made references to many pertinent new publications that have become available in recent years, as well as to the classic literature.

Holding-time and method comparisons for the analysis of fecal-indicator bacteria in groundwater.

As part of the US Geological Survey National Water-Quality Assessment Program, groundwater samples from domestic- and public-supply wells were collected and analyzed for fecal-indicator bacteria. A holding time comparison for total coliforms, Escherichia coli, and enterococci was done by analyzing samples within 8h using presence/absence methods and within 18-30h using quantitative methods. The data indicate that results obtained within 18-30h were not significantly different from those obtained within 8h for total coliforms and enterococci, by Colilert® and Enterolert® methods (IDEXX Laboratories Inc., Westbrook, ME), respectively. Quantitative laboratory methods for samples analyzed within 18-30h showed a statistically significant higher detection frequency when compared to presence/absence methods done within 8h for the following methods, E. coli by Colilert and enterococci by membrane filtration on mEI agar. Additionally, a comparison of methods for the enumeration of enterococci was done. Using non-parametric statistical analyses, results from the two methods were statistically different. In this study, the membrane filtration method on mEI agar was more sensitive, resulted in more detections of enterococci, and results were easier to interpret than with the quantitative Enterolert method. The quantitative Enterolert method produced varying levels of fluorescence, which required additional verification steps to eliminate false-positive results. It may be more advantageous to analyze untreated groundwater for enterococci using the membrane filtration method on mEI agar.

Analyzing Correlations between Stream Benthic Macroinvertebrate Community Structure, Water Quality, and Watershed Land Use in the Continental United States

The objective of this project was to explore national-scale data relationships between land use, primarily variables pertaining to agricultural production, and in-stream biotic conditions in the mainland U.S. Using the Causal Analysis/Diagnosis Decision Information System (CADDIS) and data from the USGS National Water Quality Assessment Program (NAWQA), benthic macroinvertebrate observed versus predicted (O/E) index values were analyzed against a number of land use and habitat variables in 115 sites across the U.S. Constituent loads, or the mass amounts of a chemical constituent entering the waterway per year, were estimated for each site using the USGS LOADEST program and were evaluated with respect to macroinvertebrate O/E and land use values. Relationships between variables, defined causally through CADDIS, were analyzed using simple linear regression to avoid analytical bias. Regression analyses indicated that forest and urban land cover were significantly correlated to macroinvertebrate O/E values (p = 0.0012 and p

Iron supply constrains producer communities in stream ecosystems.

The current paradigm that stream producers are under exclusive macronutrient control was recently challenged by continental studies, demonstrating that iron supply constrained diatom biodiversity and energy flows. Using algal abundance and water chemistry data from the National Water-Quality Assessment Program, we determined for the first time community thresholds along iron gradients in non-acidic running waters, i.e. 30-79.5 μg L(-1) and 70-120 μg L(-1) in oligotrophic and eutrophic streams, respectively. Given that Fe concentrations fell below both thresholds in 50% of US streams, and below the eutrophic threshold in 75% of US streams, we suggest that Fe limitation is potentially widespread and attribute it to the restricted distribution of wetlands. We also report results from the first laboratory experiments on algal-iron interactions in streams, revealing that iron supplementation leads to significant biovolume and biodiversity increase in both nitrogen fixing and non-nitrogen fixing algae. Therefore, the progressive brownification of freshwaters due to rising dissolved organic carbon and iron levels can have a stimulating influence on microbial producers with cascading effects along the trophic hierarchy. Future research in running waters should focus on the role of iron in algal physiology and biofilm functions, including accumulation of biomass, fixing atmospheric nitrogen and improving water quality.

Elucidation of Contaminant-Induced Toxic Responses in the Biota of Lake Vembanad, Kerala, India

ABSTRACTThis study investigates the spatial distribution of various organochlorine insecticides (OCIs) and heavy metals in both sediment and a bivalve, Villorita cyprinoides var. Cochinensis, collected from three pristine areas of Lake Vembanad. The detected concentrations of OCIs in the bivalve and sediment are in the order DDT > Heptachlor > BHC and BHC > DDD > heptachlor, respectively. However the DDT concentration (28.5 ng/g) in the bivalve at Chambakkara exceeded the limit suggested by the U.S. Geological Survey's National Water-Quality Assessment Program (NWQAP) and indicated an anthropogenic input source for the parent DDT. The maximum potential contamination indices for Zn and Ni in the bivalve were 267.8 mg/kg and 22.3 mg/kg, respectively. The Enrichment Factor (EF) and geoaccumulation index (Igeo) values of sediment indicated that Cd showed high pollution status and the Biota Sediment Accumulation Factor value of heavy metals suggested that Zn, Cu, and Cd were bioaccumulated in the bivalve's tissue. The concentrations of Zn and Cd were above the threshold levels and regarded as harmful according to the international standard for metals in mollusc compiled by the Food and Agriculture Organization. Therefore, they create great health risk for mussel consumers.

Peak centiles of chlorpyrifos surface-water concentrations in the NAWQA and NASQAN programs

We provide upper bound estimates for peak centiles of surface water chlorpyrifos concentration readings within spatial, temporal, and land-use domains of the United States Geological Survey (USGS) National Water-Quality Assessment (NAWQA) and National Stream Quality Accounting Network (NASQAN) programs. These datasets have large overall sample sizes but variable sampling frequencies and, for chlorpyrifos, extremely high levels of non-detections. Point and interval estimates are provided for the 90th, 95th, 99th, and the 99.9th centiles, given sufficient sample size. Overall upper bound estimates for the NAWQA program over the period 1992–2011 for the 90th, 95th, 99th, and 99.9th centiles are <0.005, 0.0066, 0.0214, and 0.0852 ug/L, respectively. The estimation method is based on a survey sampling approach, finding centiles of pooled data across aggregates of site-years. Although the population quantity estimated by a pooled data centile is not the easily interpretable average of population site-year centiles, we provide strong support that it bounds this average by a combination of theory, comparison of NAWQA aggregate and direct estimates, and using modeled populations.

Use of algae to assess the trophic state of a stream in Middle Tennessee

Knowledge of the effects of nutrient concentration on the composition and structure of photoautotrophic periphyton is essential to understand the impact of eutrophication on shallow lotic systems. Reaches of Sulphur Fork Creek upstream and downstream of effluent from Springfield Wastewater Treatment Plant in Middle Tennessee were sampled to assess the effects of trophic state on characteristics of photoautotrophic periphyton, including composition of diatom and soft-algae assemblages. Pearson's correlation coefficient (r) for log10-transformed concentrations of soluble reactive phosphorus (log10 [SRP]) to percent composition was significant for 4 of 63 soft-algae taxa sampled from cobbles. Five algae trophic indices (ATIs) to assess the effects of trophic state on soft-algae assemblages were developed using different taxon-trophic indicators which included: (1) r values for log10 [SRP] to percent composition (ATIr), (2) abundance-weighted averages of [SRP] (ATIA-WA [SRP]), (3) abundance-weighted averages of log10 [SRP] (ATIA-WA log [SRP]), (4) weighted averages of log10 [SRP] where the taxa occur, and (5) abundance-weighted ranks to phosphorus tolerance listed by the National Water-Quality Assessment Program. Eutrophication-induced impairment of Sulphur Fork Creek downstream of the effluent from the wastewater treatment plant was indicated by: (1) high concentrations of photoautotrophic periphyton, (2) low values for the pollution tolerance index of diatom assemblages, (3) positive values for the ATIr, and (4) high values for both the ATIA-WA [SRP] and ATIA-WA log [SRP]. Of the indices using soft-algae taxa evaluated, the ATIr exhibits the strongest and significant correlations to [SRP], [NO2 + NO3 nitrogen], and the pollution tolerance index of diatom assemblages. The ATIr accurately reflects the trophic state of the sites studied and provides a novel additional tool to evaluate the effects of nutrient concentration on the structure of photoautotrophic-periphyton assemblages.

Elevated Dissolved Phosphorus in Riparian Groundwater along Gaining Urban Streams

Findings of low concentrations of dissolved phosphorus in groundwater in large surveys [e.g., United States Geological Survey's National Water-Quality Assessment (NAWQA) Program ( Dubrovsky, N. M.; et al. The Quality of Our Nation's Water: Nutrients in the Nation's Streams and Groundwater, 1992-2004 . U.S. Geological Survey Circular 1350; USGS : Reston, VA , 2010 . ); >5000 wells] support the common perception that groundwater is generally of little importance for transporting phosphorus. Here, we address whether this applies to urban riparian settings, where discharging groundwater may potentially contribute to urban stream syndrome and downstream eutrophication problems. This survey study includes 665 samples of groundwater collected along gaining stream reaches at six urban sites. Considering the combined sample set, 27% had soluble reactive phosphorus (SRP) concentrations >0.1 mg L(-1), which is more than double that determined in the NAWQA Program (12%), while for individual sites the range was 12-52%, excluding one site with consistently low SRP (0%). None of the sites showed significant correlation between SRP and the artificial sweetener acesulfame, a promising wastewater indicator, including two with known wastewater contamination (but the lowest SRP). Rather, high SRP concentrations were associated with geochemically reducing conditions. This could mean that natural aquifer or stream sediment materials were a primary contributor of the elevated SRP observed in this study.

Factors affecting the movement and persistence of nitrate and pesticides in the surficial and upper Floridan aquifers in two agricultural areas in the southeastern United States

Differences in the degree of confinement, redox conditions, and dissolved organic carbon (DOC) are the main factors that control the persistence of nitrate and pesticides in the Upper Floridan aquifer (UFA) and overlying surficial aquifer beneath two agricultural areas in the southeastern US. Groundwater samples were collected multiple times from 66 wells during 1993–2007 in a study area in southwestern Georgia (ACFB) and from 48 wells in 1997–98 and 2007–08 in a study area in South Carolina (SANT) as part of the US Geological Survey National Water-Quality Assessment Program. In the ACFB study area, where karst features are prevalent, elevated nitrate-N concentrations in the oxic unconfined UFA (median 2.5 mg/L) were significantly (p = 0.03) higher than those in the overlying oxic surficial aquifer (median 1.5 mg/L). Concentrations of atrazine and deethylatrazine (DEA; the most frequently detected pesticide and degradate) were higher in more recent groundwater samples from the ACFB study area than in samples collected prior to 2000. Conversely, in the SANT study area, nitrate-N concentrations in the UFA were mostly <0.06 mg/L, resulting from anoxic conditions and elevated DOC concentrations that favored denitrification. Although most parts of the partially confined UFA in the SANT study area were anoxic or had mixed redox conditions, water from 28 % of the sampled wells was oxic and had low DOC concentrations. Based on the groundwater age information, nitrate concentrations reflect historic fertilizer N usage in both the study areas, but with a lag time of about 15–20 years. Simulated responses to future management scenarios of fertilizer N inputs indicated that elevated nitrate-N concentrations would likely persist in oxic parts of the surficial aquifer and UFA for decades even with substantial decreases in fertilizer N inputs over the next 40 years.

Regional species richness, hydrological characteristics and the local species richness of assemblages of North American stream fishes

Summary1. Local assemblage structure, from a deterministic perspective, is presumably dictated by the regional species pool as well as regulated by local factors. We examined the relationships of the regional species pool and local hydrological characteristics to local species richness of North American freshwater fishes using data sets collected during the National Water Quality Assessment program conducted by the United States Geological Survey.2. We predicted that local species richness is ultimately constrained by the composition of the regional species pool and further associated with local hydrological factors. Moreover, we predicted that variation in local species richness within major families can be explained by different combinations of hydrological characteristics that represent lineage‐specific responses to the environment.3. Daily discharge and regional and local species richness data were assembled from 41 stream localities across the United States. Multiple stepwise regressions were conducted to predict local species richness, based on regional species richness, mean discharge and hydrological characteristics quantified by nine variables characterising flow variability. Species richness at each site was calculated for the entire assemblage as well as within the four most species‐rich families in the data set (Catostomidae, Centrarchidae, Cyprinidae and Percidae).4. Local species richness was best predicted by a combination of regional species richness and discharge magnitude when all species were considered. Regional species richness was a significant explanatory variable of local species richness for three of four families (Catostomidae, Centrarchidae, Cyprinidae), but not for Percidae. Local richness in Centrarchidae and Cyprinidae was positively correlated with temporal flow variability as well as high and low flow duration, respectively, while richness in Catostomidae and Percidae tended to be associated with discharge volume. In addition, local species richness for three of the four major families was positively correlated with species richness of the other families in the assemblage, potentially suggesting the influence of local habitat quality and heterogeneity.5. Results suggest the importance of the combined influences of the regional species pool and local hydrological characteristics on local richness in freshwater fishes, with variation in richness within each family predicted by different characteristics of flow regimes.

Vulnerability of Recently Recharged Groundwater in Principle Aquifers of the United States To Nitrate Contamination

Recently recharged water (defined here as <60 years old) is generally the most vulnerable part of a groundwater resource to nonpoint-source nitrate contamination. Understanding at the appropriate scale the interactions of natural and anthropogenic controlling factors that influence nitrate occurrence in recently recharged groundwater is critical to support best management and policy decisions that are often made at the aquifer to subaquifer scale. New logistic regression models were developed using data from the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) program and National Water Information System for 17 principal aquifers of the U.S. to identify important source, transport, and attenuation factors that control nonpoint source nitrate concentrations greater than relative background levels in recently recharged groundwater and were used to predict the probability of detecting elevated nitrate in areas beyond the sampling network. Results indicate that dissolved oxygen, crops and irrigated cropland, fertilizer application, seasonally high water table, and soil properties that affect infiltration and denitrification are among the most important factors in predicting elevated nitrate concentrations. Important differences in controlling factors and spatial predictions were identified in the principal aquifer and national-scale models and support the conclusion that similar spatial scales are needed between informed groundwater management and model development.

Higher concentrations of chlorides and other contaminants found in many U.S. well networks

Higher concentrations of chlorides and other contaminants found in many U.S. well networks A new analysis of 56 well networks that represent many major U.S. aquifers and urban and agricultural land use areas has found that in 66% of the networks there was a statistically significant increase in concentration of at least one of three indicators of water quality: chlorides, dissolved solids, and nitrates. The analysis, by the U.S. Geological Survey's (USGS) National Water Quality Assessment Program ( NAWQA), which was released on 30 April, compared samples of groundwater collected from wells during 1988–2000 with corresponding samples collected from 2001–2010. The analysis found statistically significant increases of chloride concentrations in 43% of the networks, with the largest median decadal increases in well networks in the upper Illinois River Basin and in the New England coastal basins, while the largest median decadal decrease was in the upper Snake River Basin in the Pacific Northwest. The analysis also found statistically significant increases of dissolved solids concentrations in 41% of the well networks and increases of nitrate concentrations in 23%.

Short‐term macroinvertebrate recruitment and sediment accumulation: A novel field chamber approach

Stream-deposited sediment is one of the major stressors affecting stream biota. Several methods exist to quantify stream sediment embeddedness, but they are relatively qualitative and operationally defined. The authors developed a short-term in situ embeddedness chamber method to measure aquatic insect recruitment and associated sediment accumulation in a more quantitative, better replicated manner. With sediment accumulation and aquatic insect recruitment as endpoints, three exposure periods were evaluated (4, 7, and 14 d) on a low-order stream (Honey Creek, New Carlisle, Ohio, USA) and a medium-order stream (Stillwater River, Covington, Ohio, USA). Chamber results show significant positive correlations between newly deposited fine sediment and insect recruitment. Embeddedness was also measured using the more conventional techniques of the Burns method and the U.S. Geological Survey National Water Quality Assessment Program method. This in situ chamber method allows for increased experimental options for assessing the stress of embeddedness and siltation on benthic communities and may prove useful for investigating the resilience of benthic communities after disturbances.

Atmospheric Deposition and Inorganic Nitrogen Flux

Flux of dissolved inorganic nitrogen (DIN—primarily nitrate) from terrestrial ecosystems has been considered an important contributor to acidification of linked aquatic systems. The basis of this concern is the nitrogen (N) saturation hypothesis, positing that additions of N to terrestrial ecosystems in excess of biological requirements will result in DIN leaching. There is a consensus (implicit hypothesis) in the literature that atmospheric deposition of DIN in excess of a threshold of approximately 10 kg ha−1 year−1 leads to significant flux. Diverse data from USA indicate that DIN flux is highly variable both in space and time; the spatial uncertainty as measured by the pooled coefficient of variation is about 0.95, and the temporal (inter-year) uncertainty is about 0.75. The relationship between atmospheric deposition of DIN and annual flux is near-linear within the range of current deposition for US sites (≤8 kg ha−1 year−1 wet deposition). If wet and dry depositions are approximately equal, over 85 % of total DIN deposition is retained. This is nearly equal to the retention reported by the US Geological Survey National Water-Quality Assessment Program, which considered all nonpoint sources of N as inputs and both DIN and organic N as fluxes. Although input–output data have high uncertainty, the 85 % retention of atmospheric DIN by terrestrial watersheds casts doubt on its importance as a contributor to aquatic acidification. There is no obvious threshold of deposition leading to DIN leaching. The nitrogen saturation hypothesis may not fully explain N behavior in terrestrial ecosystems.

Variance partitioning of stream diatom, fish, and invertebrate indicators of biological condition

Stream indicators used to make assessments of biological condition are influenced by many possible sources of variability. To examine this issue, we used multiple-year and multiple-reach diatom, fish, and invertebrate data collected from 20 least-disturbed and 46 developed stream segments between 1993 and 2004 as part of the US Geological Survey National Water Quality Assessment Program. We used a variance-component model to summarize the relative and absolute magnitude of 4 variance components (among-site, among-year, site × year interaction, and residual) in indicator values (observed/expected ratio [O/E] and regional multimetric indices [MMI]) among assemblages and between basin types (least-disturbed and developed). We used multiple-reach samples to evaluate discordance in site assessments of biological condition caused by sampling variability. Overall, patterns in variance partitioning were similar among assemblages and basin types with one exception. Among-site variance dominated the relative contribution to the total variance (64–80% of total variance), residual variance (sampling variance) accounted for more variability (8–26%) than interaction variance (5–12%), and among-year variance was always negligible (0–0.2%). The exception to this general pattern was for invertebrates at least-disturbed sites where variability in O/E indicators was partitioned between among-site and residual (sampling) variance (among-site = 36%, residual = 64%). This pattern was not observed for fish and diatom indicators (O/E and regional MMI). We suspect that unexplained sampling variability is what largely remained after the invertebrate indicators (O/E predictive models) had accounted for environmental differences among least-disturbed sites. The influence of sampling variability on discordance of within-site assessments was assemblage or basin-type specific. Discordance among assessments was nearly 2× greater in developed basins (29–31%) than in least-disturbed sites (15–16%) for invertebrates and diatoms, whereas discordance among assessments based on fish did not differ between basin types (least-disturbed = 16%, developed = 17%). Assessments made using invertebrate and diatom indicators from a single reach disagreed with other samples collected within the same stream segment nearly ⅓ of the time in developed basins, compared to ⅙ for all other cases.

Impacts of agricultural land use on biological integrity: a causal analysis

Agricultural land use has often been linked to nutrient enrichment, habitat degradation, hydrologic alteration, and loss of biotic integrity in streams. The U.S. Geological Survey's National Water Quality Assessment Program sampled 226 stream sites located in eight agriculture-dominated study units across the United States to investigate the geographic variability and causes of agricultural impacts on stream biotic integrity. In this analysis we used structural equation modeling (SEM) to develop a national and set of regional causal models linking agricultural land use to measured instream conditions. We then examined the direct, indirect, and total effects of agriculture on biotic integrity as it acted through multiple water quality and habitat pathways. In our nation-wide model, cropland affected benthic communities by both altering structural habitats and by imposing water quality-related stresses. Region-specific modeling demonstrated that geographic context altered the relative importance of causal p...

A Web-Based Decision Support System for Assessing Regional Water-Quality Conditions and Management Actions1

The U.S. Geological Survey National Water Quality Assessment Program has completed a number of water-quality prediction models for nitrogen and phosphorus for the conterminous United States as well as for regional areas of the nation. In addition to estimating water-quality conditions at unmonitored streams, the calibrated SPAtially Referenced Regressions On Watershed attributes (SPARROW) models can be used to produce estimates of yield, flow-weighted concentration, or load of constituents in water under various land-use condition, change, or resource management scenarios. A web-based decision support infrastructure has been developed to provide access to SPARROW simulation results on stream water-quality conditions and to offer sophisticated scenario testing capabilities for research and water-quality planning via a graphical user interface with familiar controls. The SPARROW decision support system (DSS) is delivered through a web browser over an Internet connection, making it widely accessible to the public in a format that allows users to easily display water-quality conditions and to describe, test, and share modeled scenarios of future conditions. SPARROW models currently supported by the DSS are based on the modified digital versions of the 1:500,000-scale River Reach File (RF1) and 1:100,000-scale National Hydrography Dataset (medium-resolution, NHDPlus) stream networks.

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.