Transport and fate of nitrate at the ground-water/surface-water interface

Linking denitrification and pesticide transformation potentials with community ecology and groundwater discharge in hyporheic sediments in a lowland stream

Contamination of rivers by nitrate and pesticides poses a risk for aquatic ecosystems in lowland catchments that are often intensively used for agriculture. Here, the hyporheic zone, the streambed underneath the stream, plays a vital role due to its efficient self-purification capacity. The present study aims to evaluate the denitrification and transformation potential of 14 pesticides and three transformation products in the hyporheic sediment from a lowland stream with a high N load and by comparing an agricultural straightened section to a natural meandering part of the stream influenced by different groundwater discharges. Batch experiments were set up to evaluate the denitrification and pesticide transformation potentials in hyporheic sediment from two depths (5-15 cm (a) and 15-25 cm (b)). Our results revealed that (i) differences between the agricultural and natural sections of the river did not influence pollutant attenuation, (ii) both the nitrate and pesticide attenuation processes were more rapid in the upper "a" layer compared to the "b" layer due to higher microbial abundance, (iii) high groundwater discharge reduced the denitrification potential while pesticide transformation was unaffected, (iv) denitrification correlated with denitrifier abundance (nirK) in the "b" layer, while this correlation was not seen in the "a" layer, and (v) a microbial community with low diversity can explain limited transformation for the majority of tested pesticides. Overall, our results suggest that high groundwater discharge zones with reduced residence time in the hyporheic zone can be an important source of pesticides and nitrate to surface water.

Predicting algal blooms: Are we overlooking groundwater?

Significant advances in understanding and predicting freshwater algal bloom dynamics have emerged in response to both increased occurrence and financial burden of nuisance and harmful blooms. Several factors have been highlighted as key controls of bloom occurrence, including nutrient dynamics, local hydrology, climatic perturbations, watershed geomorphology, biogeochemistry, food-web control, and algal competition. However, a major research gap continues to be the degree to which groundwater inputs modulate microbial biomass production and food-web dynamics at the terrestrial-aquatic interface. We present a synthesis of groundwater related algal bloom literature, upon which we derive a foundational hypothesis: long residence times cause groundwater to be geochemically and biologically distinct from surface water, allowing groundwater inputs to modulate algal bloom dynamics (growth, decline, toxicity) through its control over in-stream water chemistry. Distinct groundwater chemistry can support or prevent algal blooms, depending on specific local conditions. We highlight three mechanisms that influence the impact of groundwater discharge on algal growth: 1) redox state of the subsurface, 2) extent of water-rock interactions, and 3) stability of groundwater discharge. We underscore that in testing hypotheses related to groundwater control over algal blooms, it is critical to understand how changes in land use, water management, and climate will influence groundwater dynamics and, thus, algal bloom probabilities. Given this challenge, we argue that advances in both modeling and data integration, including genomics data and integrated process-based models that capture groundwater dynamics, are needed to illuminate mechanistic controls and improve predictions of algal blooms.

Species and potential sources of phosphorus in groundwater in and around Mataram City, Lombok Island, Indonesia

Geochemical evaluation of the species and potential sources of P in groundwater in and around Mataram City, Lombok Island, Indonesia can aid in the initial research on evaluating the fate of P when detailed geological information is unavailable. The results of ion chromatography and inductively coupled plasma-optical emission spectroscopy show that the concentrations of PO43– and total dissolved P (TDP) in groundwater range from approximately 0.1 to 8.5 mg l–1 and from 0.04 to 2.95 mg l–1, respectively. Dissolved inorganic P accounted for 86%, on average, of the TDP concentration, and PO43– represented the predominant P species in this groundwater. The potential sources for most of this dissolved PO43–, according to indices developed based on the PHREEQC software and groundwater quality data, could be the dissolution of hydroxyapatite and/or vivianite minerals. However, the potential sources of dissolved PO43– in groundwater with a TDP concentration of ≥ 1 mg l–1 is likely to be the reduction of Fe(III)–(hydro)oxides, the initial decomposition of organic matter, or the dissolution of carbonate-rich fluorapatite considering the Fe2+, dissolved organic carbon, Ca2+ and F− concentrations measured. In addition, as several groundwater samples had a TDP concentration of ≤ 1 mg l–1 and comparatively high concentration of NO3–, other potential sources of dissolved PO43– in this groundwater could be anthropogenic.

Hydrogeological controls on ammonium enrichment in shallow groundwater in the central Yangtze River Basin

The controlling processes of excessive ammonium in surface water and groundwater in the central Yangtze River Basin remain unclear. In this study, monitoring of water levels and temporal-spatial distributions of major N compounds were implemented at the large Jiangshan plain and at the local site scale in the central Yangtze River Basin. The results indicate that the recharge, movement and transformation of ammonium were controlled by hydrogeological conditions. Manure and sewage from anthropogenic activities were identified as the main source of nitrogen compounds. The nitrogen loading into aquifers were governed by water table and groundwater flow. After entering subsurface soils, nitrification and dissimilatory nitrate reduction to ammonium (DNRA) were proposed as the ammonium consumption and production mechanisms, respectively, by combining the concentrations of ammonium‑nitrogen and nitrate‑nitrogen with the corresponding isotopic compositions. These microbially mediated processes controlling transport and transformation of nitrogen compounds were influenced by the seasonally varying groundwater flow regime that changed the redox conditions in the aquifers. In the subsurface environments, ammonium was converted to nitrate when sufficient oxygen supply was available, and this process was reversed under anoxic conditions along the groundwater flow path. A conceptual model for the reactive transport of nitrogen compounds jointly controlled by the vertical groundwater flows and biogeochemical processes was proposed, which provides new insights into the genesis of high ammonium groundwater.

Quantifying background nitrate removal mechanisms in an agricultural watershed with contrasting subcatchment baseflow concentrations

Numerous studies have documented the linkages between agricultural nitrogen loads and surface water degradation. In contrast, potential water quality improvements due to agricultural best management practices are difficult to detect because of the confounding effect of background nitrate removal rates, as well as the groundwater-driven delay between land surface action and stream response. To characterize background controls on nitrate removal in two agricultural catchments, we calibrated groundwater travel time distributions with subsurface environmental tracer data to quantify the lag time between historic agricultural inputs and measured baseflow nitrate. We then estimated spatially distributed loading to the water table from nitrate measurements at monitoring wells, using machine learning techniques to extrapolate the loading to unmonitored portions of the catchment to subsequently estimate catchment removal controls. Multiple models agree that in-stream processes remove as much as 75% of incoming loads for one subcatchment while removing <20% of incoming loads for the other. The use of a spatially variable loading field did not result in meaningfully different optimized parameter estimates or model performance when compared with spatially constant loading derived directly from a county-scale agricultural nitrogen budget. Although previous studies using individual well measurements have shown that subsurface denitrification due to contact with a reducing argillaceous confining unit plays an important role in nitrate removal, the catchment-scale contribution of this process is difficult to quantify given the available data. Nonetheless, the study provides a baseline characterization of nitrate transport timescales and removal mechanisms that will support future efforts to detect water quality benefits from ongoing best management practice implementation.

Excess N2 and denitrification in hyporheic porewaters and groundwaters of the San Joaquin River, California

The San Joaquin River (SJR) in California is purported to receive high nitrate loadings from surrounding agricultural lands through both surface and groundwater inputs. To investigate the potential removal of nitrate (NO₃^-) from surface and ground water sources, the spatial variations in dinitrogen (N₂) gas concentrations and direct measurements of sediment denitrification potential (DNP), with amended NO₃^- and carbon (C) treatments, were investigated in the summer along a 95-km reach of the San Joaquin River. Excess N₂ in hyporheic porewaters ranged from <0.1 to 8.65 mg L^-1 and was significantly higher in porewaters from the 1.3 m (ground water source) versus 0.3 m (mixed surface and ground water) depths. In deep groundwater wells (3-7 m), median excess N₂ concentration was 5.39 mg L^-1 (range = <0.1-14.6 mg L^-1). Excess N₂ concentrations were inversely correlated with dissolved oxygen and NO₃^- concentrations suggesting denitrification as an important process in the dominantly anaerobic sediments. Hyporheic porewater NO₃^- concentrations exceeded the detection limit of 0.01 mg L^-1 in only 20% of the hyporheic porewaters, in spite of high NO₃^- concentrations measured in both surface waters (mean = 2.25 mg N L^-1) and surrounding groundwaters. Sediment DNP rates averaged 253 and 297 μg N kg^-1 hr^-1 for NO₃^- amended, and NO₃^- + C amended sediments, respectively, supporting the prevalence of denitrification in hyporheic sediments. Our results indicate that the hyporheic/riparian zones act as an anoxic barrier to nitrate transport from regional groundwater and as a location to remove NO₃^- from surface waters exchanging with the hyporheic zone.

Streambed Flux Measurement Informed by Distributed Temperature Sensing Leads to a Significantly Different Characterization of Groundwater Discharge

Groundwater discharge though streambeds is often focused toward discrete zones, indicating that preliminary reconnaissance may be useful for capturing the full spectrum of groundwater discharge rates using point-scale quantitative methods. However, many direct-contact reconnaissance techniques can be time-consuming, and remote sensing (e.g., thermal infrared) typically does not penetrate the water column to locate submerged seepages. In this study, we tested whether dozens of groundwater discharge measurements made at “uninformed” (i.e., selected without knowledge on high-resolution temperature variations at the streambed) point locations along a reach would yield significantly different Darcy-based groundwater discharge rates when compared with “informed” measurements, focused at streambed thermal anomalies that were identified a priori using fiber-optic distributed temperature sensing (FO-DTS). A non-parametric U-test showed a significant difference between median discharge rates for uninformed (0.05 m·day−1; n = 30) and informed (0.17 m·day−1; n = 20) measurement locations. Mean values followed a similar pattern (0.12 versus 0.27 m·day−1), and frequency distributions for uninformed and informed measurements were also significantly different based on a Kolmogorov–Smirnov test. Results suggest that even using a quick “snapshot-in-time” field analysis of FO-DTS data can be useful in streambeds with groundwater discharge rates <0.2 m·day−1, a lower threshold than proposed in a previous study. Collectively, study results highlight that FO-DTS is a powerful technique for identifying higher-discharge zones in streambeds, but the pros and cons of informed and uninformed sampling depend in part on groundwater/surface water exchange study goals. For example, studies focused on measuring representative groundwater and solute fluxes may be biased if high-discharge locations are preferentially sampled. However, identification of high-discharge locations may complement more randomized sampling plans and lead to improvements in interpolating streambed fluxes and upscaling point measurements to the stream reach scale.

Laboratory study on nitrate removal and nitrous oxide emission in intact soil columns collected from nitrogenous loaded riparian wetland, Northeast China

Nitrate (NO3−) pollution of surface and groundwater systems is a major problem globally. For some time now wetlands have been considered potential systems for improving water quality. Nitrate dissolved in water moving through wetlands can be removed through different processes, such as the denitrification process, where heterotrophic facultative anaerobic bacteria use NO3− for respiration, leading to the production of nitrogen (N₂) and nitrous oxide (N₂O) gases. Nitrate removal and emission of N₂O in wetlands can vary spatially, depending on factors such as vegetation, hydrology and soil structure. This study intended to provide a better understanding of the spatial variability and processes involved in NO3− removal and emission of N₂O in riparian wetland soils. We designed a laboratory experiment simulating surface water flow through soil columns collected from different sites dominated by different plant species within a wetland. Water and gas samples for NO3−,NH4+ and N₂O analyses were collected every 5 days for a period of 30 days. The results revealed significant removal of NO3− in all the soil columns, supporting the role of riparian wetland soils in removing nitrogen from surface runoff. Nitrate removal at 0 and 10cm depths in sites dominated by Phragmites australis and Carex schnimdtii was significantly higher than in the site dominated by Calamagrostis epigeio. Nitrous oxide emissions varied spatially and temporally with negative flux observed in sites dominated by P. australis and C. schnimdtii. These results reveal that in addition to the ability of wetlands to remove NO3−, some sites within wetlands are also capable of consuming N₂O, hence mitigating not only agricultural nitrate pollution but also climate change.

Transport and potential attenuation of nitrogen in shallow groundwaters in the lower Rangitikei catchment, New Zealand

Intensive agricultural activities are generally associated with nitrogen leaching from agricultural soils, and this nitrogen has the potential to percolate and contaminate groundwater and surface waters. We assessed surface water and groundwater interactions, and nitrogen leaching and its potential attenuation in shallow groundwater in the lower Rangitikei River catchment (832km²), New Zealand. We combined regional- and local-scale field surveys and experiments, nutrient budget modelling, and hydraulic and geochemical methods, to gain an insight into leaching, transformation and transport of nitrogen via groundwaters to the river in the study area. Concurrent river flow gaugings (in January 2015) and a piezometric map, developed from measured depths to groundwater in 110 bores (in October 2014), suggest groundwater discharges to the Rangitikei River in the upper parts of the study area, while there is groundwater recharge near the coast. The groundwater redox characterisation, based on sampling and analysis of 15 mostly shallow bores (<30m below ground level (bgl)), suggests groundwater across the lower Rangitikei catchment in general is under anoxic/reduced conditions. The groundwater typically has low dissolved oxygen (DO) concentrations (<1mg/L), suggesting the subsurface environment is conducive to potential attenuation by 'denitrification' of NO₃-N in groundwater. We further measured NO₃-N attenuation in shallow groundwater piezometers (3-6mbgl) using single-well push-pull tests. We found generally low levels (<0.5mg/L) of NO₃-N in shallow groundwater piezometers (>5mbgl), despite being installed under intensive land uses, such as dairying and cropping. Our in-field push-pull tests showed NO₃-N reduction at four shallow groundwater piezometers, with the rates of reduction varying from 0.04mgNL^-1h^-1 to 1.57mgNL^-1h^-1. This highlights the importance of a sound understanding of not only the sources, but also transport and transformation, or fate, of nutrients leached from farms, to mitigate the likely impacts of land use on water quality and ecosystem health in agricultural catchments.

Denitrification potential in the subsurface environment in the Manawatu River catchment, New Zealand: Indications from oxidation-reduction conditions, hydrogeological factors, and implications for nutrient management

A sound understanding of the effects of hydrogeological factors on loss, transport and transformation of farm nutrients is essential for predicting their impacts on ecosystem health of receiving waters. We assessed the potential of groundwater to attenuate nitrate through denitrification, and the distribution of this potential across the Tararua Groundwater Management Zone (GWMZ) in the Manawatu River catchment, New Zealand. We combined a number of methods in an unprecedented manner to confirm findings and obtain supporting evidence for the features that determine the subsurface denitrification characteristics. Our results showed that the denitrification characteristics of groundwater varied considerably in the Tararua GWMZ. The southern part of the Tararua GWMZ contained mainly oxic groundwater with low potential to denitrify, whereas the middle and northern parts of the Tararua GWMZ contained reduced groundwater with high denitrification potential. The hydrogeological features that influence denitrification potential in groundwater were identified as soil texture and drainage class, and the aquifer material or rock type. Low dissolved oxygen levels and nitrate concentrations were found in groundwater where the combinations of soil and rock types had poor drainage characteristics as opposed to higher concentrations in groundwater under well-drained soils and rocks (e.g. gravels). Intensive pastoral farming over well-drained soils and rocks showed high nitrate concentration in groundwater. This spatial variability in denitrification potential of groundwater offers a targeted management of nutrients runoff and leaching from pastoral lands to reduce their impacts on receiving surface waters.

Stream-aquifer and in-stream processes affecting nitrogen along a major river and contributing tributary

This study assesses the spatio-temporal patterns of water and nutrient mass exchange in a stream-riparian system of a major river and a contributing tributary in an irrigated semi-arid region. Field monitoring is performed along reaches of the Arkansas River (4.7km) and Timpas Creek (2.0km) in southeastern Colorado during the 2014 growing season, with water quantity and water quality data collected using a network of in-stream sampling sites and groundwater monitoring wells. Mass balance approaches were used to identify temporal and spatial trends in flow, nitrogen (N), and salinity in stream-aquifer exchange. In the Arkansas River, percent decrease of N concentration along the study reach averaged 36% over the period, with results from a stochastic mass balance simulation indicating a 90% probability that 44% to 50% of NO₃-N mass in the study reach (109-124kg/day/km) was removed by in-stream processes between 1 September and 8 November. Results suggest that contact with organic-rich river bed sediments has a strong impact on N removal. A greater decrease in concentrations of NO₃-N along the reach during the low flow period suggests the effect of both in-stream processes and dilution by inflowing groundwater that undergoes denitrification as it flows through the riparian and hyporheic zones into the river. In contrast, N concentration decreases in the smaller Timpas Creek were negligible. Results for the Arkansas River also are in contrast with other large agriculturally-influenced rivers, which have not exhibited capacity to remove N at significant rates. Results provide important insights across spatial and temporal scales and point to the need for investigating nutrient dynamics in large streams draining agriculturally-dominated watersheds.

Contrasting Nitrogen Fate in Watersheds Using Agricultural and Water Quality Information

Surplus nitrogen (N) estimates, principal component analysis (PCA), and end-member mixing analysis (EMMA) were used in a multisite comparison contrasting the fate of N in diverse agricultural watersheds. We applied PCA-EMMA in 10 watersheds located in Indiana, Iowa, Maryland, Nebraska, Mississippi, and Washington ranging in size from 5 to 1254 km with four nested watersheds. Watershed Surplus N was determined by subtracting estimates of crop uptake and volatilization from estimates of N input from atmospheric deposition, plant fixation, fertilizer, and manure for the period from 1987 to 2004. Watershed average Surplus N ranged from 11 to 52 kg N ha and from 9 to 32% of N input. Solute concentrations in streams, overland runoff, tile drainage, groundwater (GW), streambeds, and the unsaturated zone were used in the PCA-EMMA procedure to identify independent components contributing to observed stream concentration variability and the end-members contributing to streamflow and NO load. End-members included dilute runoff, agricultural runoff, benthic-processing, tile drainage, and oxic and anoxic GW. Surplus N was larger in watersheds with more permeable soils (Washington, Nebraska, and Maryland) that allowed greater infiltration, and oxic GW was the primary source of NO load. Subsurface transport of NO in these watersheds resulted in some removal of Surplus N by denitrification. In less permeable watersheds (Iowa, Indiana, and Mississippi), NO was rapidly transported to the stream by tile drainage and runoff with little removal. Evidence of streambed removal of NO by benthic diatoms was observed in the larger watersheds.

High resolution synoptic salinity mapping to identify groundwater--surface water discharges in lowland rivers

Quantifying distributed lateral groundwater contributions to surface water (GW-SW discharges) is a key aspect of tracking nonpoint-source pollution (NPSP) within a watershed. In this study, we characterized distributed GW-SW discharges and associated salt loading using elevated GW specific conductance (SC) as a tracer along a 38 km reach of the Lower Merced River in Central California. High resolution longitudinal surveys for multiple flows (1.3-150 m(3) s(-1)) revealed river SC gradients that mainly decreased with increasing flow, suggesting a dilution effect and/or reduced GW-SW discharges due to hydraulic gradient reductions. However, exceptions occurred (gradients increasing with increasing flow), pointing to complex spatiotemporal influences on GW-SW dynamics. The surveys revealed detailed variability in salinity gradients, from which we estimated distributed GW-SW discharge and salt loading using a simple mixing model. Modeled cumulative GW discharges for two surveys unaffected by ungauged SW discharges were comparable in magnitude to differential gauging-based discharge estimates and prior GW-SW studies along the same river reach. Ungauged lateral inlets and sparse GW data limited the study, and argue for enhancing monitoring efforts. Our approach provides a rapid and economical method for characterizing NPSP for gaining rivers in the context of integrated watershed modeling and management.

Low transient storage and uptake efficiencies in seven agricultural streams: implications for nutrient demand

We used mass load budgets, transient storage modeling, and nutrient spiraling metrics to characterize nitrate (NO), ammonium (NH), and inorganic phosphorus (SRP) demand in seven agricultural streams across the United States and to identify in-stream services that may control these conditions. Retention of one or all nutrients was observed in all but one stream, but demand for all nutrients was low relative to the mass in transport. Transient storage metrics (/, , , and ) correlated with NO retention but not NH or SRP retention, suggesting in-stream services associated with transient storage and stream water residence time could influence reach-scale NO demand. However, because the fraction of median reach-scale travel time due to transient storage () was ≤1.2% across the sites, only a relatively small demand for NO could be generated by transient storage. In contrast, net uptake of nutrients from the water column calculated from nutrient spiraling metrics were not significant at any site because uptake lengths calculated from background nutrient concentrations were statistically insignificant and therefore much longer than the study reaches. These results suggest that low transient storage coupled with high surface water NO inputs have resulted in uptake efficiencies that are not sufficient to offset groundwater inputs of N. Nutrient retention has been linked to physical and hydrogeologic elements that drive flow through transient storage areas where residence time and biotic contact are maximized; however, our findings indicate that similar mechanisms are unable to generate a significant nutrient demand in these streams relative to the loads.

An integrated pressure and pathway approach to the spatial analysis of groundwater nitrate: a case study from the southeast of Ireland

Excess nitrogen in soil, aquatic and atmospheric environments is an escalating global problem. Eutrophication is the principal threat to surface water quality in the Republic of Ireland. European Union Water Framework Directive (2000/60/EC) water quality status assessments found that 16% of Irish groundwater bodies were 'at risk' of poor status due to the potential deterioration of associated estuarine and coastal water quality by nitrate from groundwater. This paper presents a methodology for evaluating pressure and pathway parameters affecting the spatial distribution of groundwater nitrate, investigated at a regional scale using existing national spatial datasets. The potential for nitrate transfer to groundwater was rated based on the introduced concepts of Pressure Loading and Pathway Connectivity Rating, each based on a combination of selected pressure and pathway parameters respectively. In the region studied, the South Eastern River Basin District of Ireland, this methodology identified that pathway parameters were more important than pressure parameters in understanding the spatial distribution of groundwater nitrate. Statistical analyses supported these findings and further demonstrated that the proportion of poorly drained soils, arable land, karstic flow regimes, regionally important bedrock aquifers and high vulnerability groundwater within the zones of contribution of the monitoring points are statistically significantly related to groundwater nitrate concentrations. Soil type was found to be the most important parameter. Analysis of variance showed that a number of the pressure and pathway parameters are interrelated. The parameters identified by the presented methodology may provide useful insights into the best way to manage and mitigate the influence of nitrate contamination of groundwater in this region. It is suggested that the identification of critical source areas based on the identified parameters would be an appropriate management tool, enabling planning and enforcement resources to be focussed on areas which will yield most benefit.

Vulnerability of streams to legacy nitrate sources

The influence of hydrogeologic setting on the susceptibility of streams to legacy nitrate was examined at seven study sites having a wide range of base flow index (BFI) values. BFI is the ratio of base flow to total streamflow volume. The portion of annual stream nitrate loads from base flow was strongly correlated with BFI. Furthermore, dissolved oxygen concentrations in streambed pore water were significantly higher in high BFI watersheds than in low BFI watersheds suggesting that geochemical conditions favor nitrate transport through the bed when BFI is high. Results from a groundwater-surface water interaction study at a high BFI watershed indicate that decades old nitrate-laden water is discharging to this stream. These findings indicate that high nitrate levels in this stream may be sustained for decades to come regardless of current practices. It is hypothesized that a first approximation of stream vulnerability to legacy nutrients may be made by geospatial analysis of watersheds with high nitrogen inputs and a strong connection to groundwater (e.g., high BFI).

Groundwater and surface-water exchange and resulting nitrate dynamics in the Bogue Phalia basin in northwestern Mississippi

During April 2007 through September 2008, the USGS collected hydrogeologic and water-quality data from a site on the Bogue Phalia to evaluate the role of groundwater and surface-water interaction on the transport of nitrate to the shallow sand and gravel aquifer underlying the Mississippi Alluvial Plain in northwestern Mississippi. A two-dimensional groundwater/surface-water exchange model was developed using temperature and head data and VS2DH, a variably saturated flow and energy transport model. Results from this model showed that groundwater/surface-water exchange at the site occurred regularly and recharge was laterally extensive into the alluvial aquifer. Nitrate was consistently reported in surface-water samples (n = 52, median concentration = 39.8 μmol/L) although never detected in samples collected from in-stream piezometers or shallow monitoring wells adjacent to the stream (n = 46). These two facts, consistent detections of nitrate in surface water and no detections of nitrate in groundwater, coupled with model results that indicate large amounts of surface water moving through an anoxic streambed, support the case for denitrification and nitrate loss through the streambed.

A review of the impact of climate change on future nitrate concentrations in groundwater of the UK

This paper reviews the potential impacts of climate change on nitrate concentrations in groundwater of the UK using a Source-Pathway-Receptor framework. Changes in temperature, precipitation quantity and distribution, and atmospheric carbon dioxide concentrations will affect the agricultural nitrate source term through changes in both soil processes and agricultural productivity. Non-agricultural source terms, such as urban areas and atmospheric deposition, are also expected to be affected. The implications for the rate of nitrate leaching to groundwater as a result of these changes are not yet fully understood but predictions suggest that leaching rate may increase under future climate scenarios. Climate change will affect the hydrological cycle with changes to recharge, groundwater levels and resources and flow processes. These changes will impact on concentrations of nitrate in abstracted water and other receptors, such as surface water and groundwater-fed wetlands. The implications for nitrate leaching to groundwater as a result of climate changes are not yet well enough understood to be able to make useful predictions without more site-specific data. The few studies which address the whole cycle show likely changes in nitrate leaching ranging from limited increases to a possible doubling of aquifer concentrations by 2100. These changes may be masked by nitrate reductions from improved agricultural practices, but a range of adaption measures need to be identified. Future impact may also be driven by economic responses to climate change.

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.

Predicting unsaturated zone nitrogen mass balances in agricultural settings of the United States

Unsaturated zone N fate and transport were evaluated at four sites to identify the predominant pathways of N cycling: an almond [Prunus dulcis (Mill.) D.A. Webb] orchard and cornfield (Zea mays L.) in the lower Merced River study basin, California; and corn-soybean [Glycine max (L.) Merr.] rotations in study basins at Maple Creek, Nebraska, and at Morgan Creek, Maryland. We used inverse modeling with a new version of the Root Zone Water Quality Model (RZWQM2) to estimate soil hydraulic and nitrogen transformation parameters throughout the unsaturated zone; previous versions were limited to 3-m depth and relied on manual calibration. The overall goal of the modeling was to derive unsaturated zone N mass balances for the four sites. RZWQM2 showed promise for deeper simulation profiles. Relative root mean square error (RRMSE) values for predicted and observed nitrate concentrations in lysimeters were 0.40 and 0.52 for California (6.5 m depth) and Nebraska (10 m), respectively, and index of agreement (d) values were 0.60 and 0.71 (d varies between 0 and 1, with higher values indicating better agreement). For the shallow simulation profile (1 m) in Maryland, RRMSE and d for nitrate were 0.22 and 0.86, respectively. Except for Nebraska, predictions of average nitrate concentration at the bottom of the simulation profile agreed reasonably well with measured concentrations in monitoring wells. The largest additions of N were predicted to come from inorganic fertilizer (153-195 kg N ha(-1) yr(-1) in California) and N fixation (99 and 131 kg N ha(-1) yr(-1) in Maryland and Nebraska, respectively). Predicted N losses occurred primarily through plant uptake (144-237 kg N ha(-1) yr(-1)) and deep seepage out of the profile (56-102 kg N ha(-1) yr(-1)). Large reservoirs of organic N (up to 17,500 kg N ha(-1) m(-1) at Nebraska) were predicted to reside in the unsaturated zone, which has implications for potential future transfer of nitrate to groundwater.

Impact of point-source pollution on phosphorus and nitrogen cycling in stream-bed sediments

Diffusive equilibration in thin films was used to study the cycling of phosphorus and nitrogen at the sediment-water interface in situ and with minimal disturbance to redox conditions. Soluble reactive phosphate (SRP), nitrate, nitrite, ammonium, sulfate, iron, and manganese profiles were measured in a rural stream, 12 m upstream, adjacent to, and 8 m downstream of a septic tank discharge. Sewage fungus adjacent to the discharge resulted in anoxic conditions directly above the sediment. SRP and ammonium increased with depth through the fungus layer to environmentally significant concentrations (440 and 1800 microM, respectively) due to release at the sediment surface. This compared to only 0.8 microM of SRP and 2.0 microM of ammonium in the water column upstream of the discharge. Concomitant removal of ammonium, nitrite and nitrate within 0.5 cm below the fungus-water interface provided evidence for anaerobic ammonium oxidation (anammox). "Hotspots" of porewater SRP (up to 350 microM) at the downstream site demonstrated potential in-stream storage of the elevated P concentrations from the effluent. These results provide direct in situ evidence of phosphorus and nitrogen release from river-bed sediments under anoxic conditions created by sewage-fungus, and highlight the wider importance of redox conditions and rural point sources on in-stream nutrient cycling.

Hydrologic and biogeochemical controls of river subsurface solutes under agriculturally enhanced ground water flow

The relative influences of hydrologic processes and biogeochemistry on the transport and retention of minor solutes were compared in the riverbed of the lower Merced River (California, USA). The subsurface of this reach receives ground water discharge and surface water infiltration due to an altered hydraulic setting resulting from agricultural irrigation. Filtered ground water samples were collected from 30 drive point locations in March, June, and October 2004. Hydrologic processes, described previously, were verified by observations of bromine concentrations; manganese was used to indicate redox conditions. The separate responses of the minor solutes strontium, barium, uranium, and phosphorus to these influences were examined. Correlation and principal component analyses indicate that hydrologic processes dominate the distribution of trace elements in the ground water. Redox conditions appear to be independent of hydrologic processes and account for most of the remaining data variability. With some variability, major processes are consistent in two sampling transects separated by 100 m.

Identifying pathways and processes affecting nitrate and orthophosphate inputs to streams in agricultural watersheds

Understanding nutrient pathways to streams will improve nutrient management strategies and estimates of the time lag between when changes in land use practices occur and when water quality effects that result from these changes are observed. Nitrate and orthophosphate (OP) concentrations in several environmental compartments were examined in watersheds having a range of base flow index (BFI) values across the continental United States to determine the dominant pathways for water and nutrient inputs to streams. Estimates of the proportion of stream nitrate that was derived from groundwater increased as BFI increased. Nitrate concentration gradients between groundwater and surface water further supported the groundwater source of nitrate in these high BFI streams. However, nitrate concentrations in stream-bed pore water in all settings were typically lower than stream or upland groundwater concentrations, suggesting that nitrate discharge to streams was not uniform through the bed. Rather, preferential pathways (e.g., springs, seeps) may allow high nitrate groundwater to bypass sites of high biogeochemical transformation. Rapid pathway compartments (e.g., overland flow, tile drains) had OP concentrations that were typically higher than in streams and were important OP conveyers in most of these watersheds. In contrast to nitrate, the proportion of stream OP that is derived from ground water did not systematically increase as BFI increased. While typically not the dominant source of OP, groundwater discharge was an important pathway of OP transport to streams when BFI values were very high and when geochemical conditions favored OP mobility in groundwater.

Whole-stream response to nitrate loading in three streams draining agricultural landscapes

Physical, chemical, hydrologic, and biologic factors affecting nitrate (NO3(-)) removal were evaluated in three agricultural streams draining orchard/dairy and row crop settings. Using 3-d "snapshots" during biotically active periods, we estimated reach-level NO3(-) sources, NO3(-) mass balance, in-stream processing (nitrification, denitrification, and NO3(-) uptake), and NO3(-) retention potential associated with surface water transport and ground water discharge. Ground water contributed 5 to 11% to stream discharge along the study reaches and 8 to 42% of gross NO3(-) input. Streambed processes potentially reduced 45 to 75% of ground water NO3(-) before discharge to surface water. In all streams, transient storage was of little importance for surface water NO3(-) retention. Estimated nitrification (1.6-4.4 mg N m(-2) h(-1)) and unamended denitrification rates (2.0-16.3 mg N m(-2) h(-1)) in sediment slurries were high relative to pristine streams. Denitrification of NO3(-) was largely independent of nitrification because both stream and ground water were sources of NO3(-). Unamended denitrification rates extrapolated to the reach-scale accounted for <5% of NO3(-) exported from the reaches minimally reducing downstream loads. Nitrate retention as a percentage of gross NO3(-) inputs was >30% in an organic-poor, autotrophic stream with the lowest denitrification potentials and highest benthic chlorophyll a, photosynthesis/respiration ratio, pH, dissolved oxygen, and diurnal NO3(-) variation. Biotic processing potentially removed 75% of ground water NO3(-) at this site, suggesting an important role for photosynthetic assimilation of ground water NO3(-) relative to subsurface denitrification as water passed directly through benthic diatom beds.

Limited occurrence of denitrification in four shallow aquifers in agricultural areas of the United States

The ability of natural attenuation to mitigate agricultural nitrate contamination in recharging aquifers was investigated in four important agricultural settings in the United States. The study used laboratory analyses, field measurements, and flow and transport modeling for monitoring well transects (0.5 to 2.5 km in length) in the San Joaquin watershed, California, the Elkhorn watershed, Nebraska, the Yakima watershed, Washington, and the Chester watershed, Maryland. Ground water analyses included major ion chemistry, dissolved gases, nitrogen and oxygen stable isotopes, and estimates of recharge date. Sediment analyses included potential electron donors and stable nitrogen and carbon isotopes. Within each site and among aquifer-based medians, dissolved oxygen decreases with ground water age, and excess N(2) from denitrification increases with age. Stable isotopes and excess N(2) imply minimal denitrifying activity at the Maryland and Washington sites, partial denitrification at the California site, and total denitrification across portions of the Nebraska site. At all sites, recharging electron donor concentrations are not sufficient to account for the losses of dissolved oxygen and nitrate, implying that relict, solid phase electron donors drive redox reactions. Zero-order rates of denitrification range from 0 to 0.14 micromol N L(-1)d(-1), comparable to observations of other studies using the same methods. Many values reported in the literature are, however, orders of magnitude higher, which is attributed to a combination of method limitations and bias for selection of sites with rapid denitrification. In the shallow aquifers below these agricultural fields, denitrification is limited in extent and will require residence times of decades or longer to mitigate modern nitrate contamination.

National, holistic, watershed-scale approach to understand the sources, transport, and fate of agricultural chemicals

This paper is an introduction to the following series of papers that report on in-depth investigations that have been conducted at five agricultural study areas across the United States in order to gain insights into how environmental processes and agricultural practices interact to determine the transport and fate of agricultural chemicals in the environment. These are the first study areas in an ongoing national study. The study areas were selected, based on the combination of cropping patterns and hydrologic setting, as representative of nationally important agricultural settings to form a basis for extrapolation to unstudied areas. The holistic, watershed-scale study design that involves multiple environmental compartments and that employs both field observations and simulation modeling is presented. This paper introduces the overall study design and presents an overview of the hydrology of the five study areas.

Comparative study of transport processes of nitrogen, phosphorus, and herbicides to streams in five agricultural basins, USA

Agricultural chemical transport to surface water and the linkage to other hydrological compartments, principally ground water, was investigated at five watersheds in semiarid to humid climatic settings. Chemical transport was affected by storm water runoff, soil drainage, irrigation, and how streams were linked to shallow ground water systems. Irrigation practices and timing of chemical use greatly affected nutrient and pesticide transport in the semiarid basins. Irrigation with imported water tended to increase ground water and chemical transport, whereas the use of locally pumped irrigation water may eliminate connections between streams and ground water, resulting in lower annual loads. Drainage pathways in humid environments are important because the loads may be transported in tile drains, or through varying combinations of ground water discharge, and overland flow. In most cases, overland flow contributed the greatest loads, but a significant portion of the annual load of nitrate and some pesticide degradates can be transported under base-flow conditions. The highest basin yields for nitrate were measured in a semiarid irrigated system that used imported water and in a stream dominated by tile drainage in a humid environment. Pesticide loads, as a percent of actual use (LAPU), showed the effects of climate and geohydrologic conditions. The LAPU values in the semiarid study basin in Washington were generally low because most of the load was transported in ground water discharge to the stream. When herbicides are applied during the rainy season in a semiarid setting, such as simazine in the California basin, LAPU values are similar to those in the Midwest basins.

Using heat to characterize streambed water flux variability in four stream reaches

Estimates of streambed water flux are needed for the interpretation of streambed chemistry and reactions. Continuous temperature and head monitoring in stream reaches within four agricultural watersheds (Leary Weber Ditch, IN; Maple Creek, NE; DR2 Drain, WA; and Merced River, CA) allowed heat to be used as a tracer to study the temporal and spatial variability of fluxes through the streambed. Synoptic methods (seepage meter and differential discharge measurements) were compared with estimates obtained by using heat as a tracer. Water flux was estimated by modeling one-dimensional vertical flow of water and heat using the model VS2DH. Flux was influenced by physical heterogeneity of the stream channel and temporal variability in stream and ground-water levels. During most of the study period (April-December 2004), flux was upward through the streambeds. At the IN, NE, and CA sites, high-stage events resulted in rapid reversal of flow direction inducing short-term surface-water flow into the streambed. During late summer at the IN site, regional ground-water levels dropped, leading to surface-water loss to ground water that resulted in drying of the ditch. Synoptic measurements of flux generally supported the model flux estimates. Water flow through the streambed was roughly an order of magnitude larger in the humid basins (IN and NE) than in the arid basins (WA and CA). Downward flux, in response to sudden high streamflows, and seasonal variability in flux was most pronounced in the humid basins and in high conductivity zones in the streambed.