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Global water resources affected by excessive nitrate (NO 3 −) have caused a series of human health and ecological problems. Therefore, identification of NO 3 − sources and transformations is of pivotal significance in the strategic governance of widespread NO 3 − contaminant. In this investigation, a combination of statistical analysis, chemical indicators, isotopes, and MixSIAR model approaches was adopted to reveal the hydrochemical factors affecting NO 3 − concentrations and quantify the contribution of each source to NO 3 − concentrations in surface water and groundwater. The findings revealed that high groundwater NO 3 − concentration is concentrated in the southwestern region, peaking at 271 mg/L. NO 3 − concentration in the Wei River and Yuxian River exhibited an increase from upstream to downstream, but in the Shidi River and Luowen River, its concentration was highest in the upstream. Groundwater NO 3 − has noticeable correlation with Na+, Ca2+, Mg2+, Cl−, HCO 3 −, TDS, EC, and ORP. In surface water, NO 3 − level is significantly correlated with NH 4 + and ORP. Major sources of NO 3 − in surface and groundwater comprise manure & sewage and soil nitrogen. Source contribution for surface water was calculated by MixSIAR model to obtain soil nitrogen (57.7%), manure & sewage (23.8%), chemical fertilizer (12%), and atmospheric deposition (6.4%). In groundwater, soil nitrogen and manure & sewage accounted for 19% and 63.8% of nitrate sources, respectively. Both surface water and groundwater exhibited strong oxidation, with nitrification the primary process. It is expected that this study will provide insights into the dynamics of NO 3 − and contribute to the development of effective strategies for mitigating NO 3 − contaminant, leading to sustainable management of water resources. [Display omitted] • Multiple approaches were used to identify nitrate sources and proportional contribution. • Manure & sewage and soil nitrogen are main sources of NO 3 − in Huazhou District. • Nitrification is the dominant process in surface water and groundwater. [ABSTRACT FROM AUTHOR]

[Display omitted] • Lithology of glacial deposits affect pore water isotope and hydrochemical profiles. • Variable nature of glacial deposits controls hydrological response to storm events. • NO 3 − source from nitrified NH 4 + evident in baseflow in areas of glacial sands. • Denitrification with δ15N NO3 enrichment in baseflow in areas of glacial till. • Mitigation measures recommended to control NO 3 − in catchments with glacial deposits. Mitigating NO 3 − pollution requires an understanding of the hydrological processes controlling contaminant mobilisation and transport, particularly in agricultural catchments underlain by Pleistocene glacial deposits. Focusing on the Wensum catchment in East Anglia, UK, precipitation (n = 20), stream water (n = 50), field drainage (n = 22) and groundwater (n = 84) samples collected between February–March 2011 and April–September 2012 were variously analysed for water stable isotopes (δ2H H2O and δ18O H2O), the dual-isotopes of NO 3 − (δ15N NO3 and δ18O NO3), groundwater residence time indicators (CFCs and SF 6) and hydrochemical parameters. The residence time indicators suggested a component of modern (post-1960) groundwater throughout the sequence of glacial deposits that corresponds with the penetration of agricultural NO 3 −. Denitrification and lower NO 3 − concentrations (<8 mg L−1) are observed in the glacial tills, compared with higher NO 3 − concentrations (<90 mg L−1) observed under more oxidising conditions in the glacial sands and gravels. Storm hydrograph separation for two storms in April and September 2012 using two- and three-component mixing models showed a faster response with field drainage (36–38 %) and baseflow (5–37 %) contributing to the total stream discharge in areas of clay loam soils over glacial tills. In these areas, the dual stable isotopes of NO 3 − (δ15N NO3 = +11.8 ‰ and δ18O NO3 = +7.1 ‰) indicated a denitrified source of nitrogen from field drainage and groundwater. In comparison, a dampened response and a higher percentage of baseflow (29–80 %) was observed in areas of sandy clay loam soils over glacial sands and gravels. In these areas, mean NO 3 − isotopic signatures (δ15N NO3 = +7.8 ‰ and δ18O NO3 = +5.0 ‰) indicated a source of nitrified NH 4 +. In conclusion, understanding hydrological processes in catchments underlain by variable glacial deposits can inform nutrient management plans and cultivation practices to reduce the risk of agricultural NO 3 − contamination. [ABSTRACT FROM AUTHOR]

Traditional site characterization and laboratory testing methods are insufficient to quantify and conceptualize subsurface contaminant source-pathway-receptor heterogeneity issues, as they hamper groundwater risk assessment and water resource management using mathematical modeling. To address these issues, we propose an adaptive observation-based conceptual site modeling framework, which emphasizes the need for the iterative testing of hypotheses centered on specific questions with clearly defined objectives using interdisciplinary tools (including, but not limited to, geology, microbiology, hydrogeology, geophysics, and the chemistry of solute fate and transport). Under this framework, we present a case study aimed at a goal-oriented investigation of the source and occurrence of a groundwater nitrate plume previously identified using chemical concentration data from sparsely distributed, conventional, and regional groundwater monitoring wells. These investigations occurred in stages, with the first comprehensive outcome of cost-efficient, non-invasive surface geophysical surveys localizing subsurface heterogeneities laying the groundwork for collaborative, minimally invasive, direct push-based investigations followed by groundwater chemical and stable isotope analyses for source fingerprinting and bioprocess evaluation. Despite the obvious need for further refinement of the conceptual site model as new data become available, we illustrate that the step-by-step integrative framework was useful for systematic maximization of the strengths of different investigation methodologies. Such frameworks and approaches should be encouraged for successful environmental site characterization, monitoring, and modeling. [ABSTRACT FROM AUTHOR]

In this study, we assess the validity of an African-scale groundwater pollution model for nitrates. In a previous study, we identified a statistical continental-scale groundwater pollution model for nitrate. The model was identified using a pan-African meta-analysis of available nitrate groundwater pollution studies. The model was implemented in both Random Forest (RF) and multiple regression formats. For both approaches, we collected as predictors a comprehensive GIS database of 13 spatial attributes, related to land use, soil type, hydrogeology, topography, climatology, region typology, nitrogen fertiliser application rate, and population density. In this paper, we validate the continental-scale model of groundwater contamination by using a nitrate measurement dataset from three African countries. We discuss the issue of data availability, and quality and scale issues, as challenges in validation. Notwithstanding that the modelling procedure exhibited very good success using a continental-scale dataset (e.g. R2 = 0.97 in the RF format using a cross-validation approach), the continental-scale model could not be used without recalibration to predict nitrate pollution at the country scale using regional data. In addition, when recalibrating the model using country-scale datasets, the order of model exploratory factors changes. This suggests that the structure and the parameters of a statistical spatially distributed groundwater degradation model for the African continent are strongly scale dependent. [ABSTRACT FROM AUTHOR]

Abstract Identification of the nitrate sources that adversely impact groundwater quality is a necessary first step in the control of this major worldwide pollutant. The impact of nitrate leachate from urea-ammonium nitrate (UAN) (50% urea-N, 25% ammonium-N, 25% nitrate-N) fertilizer, whose use has increased dramatically in the last three decades largely because it can be applied through sprinkler irrigation systems to corn in all growth stages, is investigated. The dual isotopes δ15N NO3 and δ18O NO3 were measured in groundwater samples from 39 irrigation wells in two intensively sprinkler-irrigated, corn-growing areas of Nebraska with nitrate-contaminated (N > 10 mg/L) groundwater and documented UAN use to ascertain whether nitrified ammonia and nitrate fertilizers can be distinguished in the High Plains aquifer. The areas, which are highly vulnerable to nitrate leaching and differ only in the composition and thickness of their unsaturated zones, are uniquely suited to provide scientific evidence of the feasibility of identifying nitrate fertilizer leachate in groundwater and thereby add significantly to the small body of existing and inconclusive data. The dual isotope method (DIM) results indicate that the nitrate contamination in 38 wells is mostly nitrified ammonium fertilizer. Most importantly, nitrate fertilizer from UAN was not identified isotopically in groundwater beneath almost all fields with documented heavy UAN use. This could be a potentially valuable finding for fertilizer management or it could convey limitations on the appropriateness of the DIM for nitrate fertilizer source identification in groundwater. Slightly enriched δ15N NO3 values in a few wells coincide with the practice of wintering cattle on corn stubble, which reportedly occurred more frequently in one focus area. The absence of natural soil-N leachates and denitrification in groundwater enabled an apparently reliable identification of manure leachates in both areas. Highlights • DIM did not identify UAN fertilizer nitrate in 38 of 39 groundwater samples. • Crop uptake and/or soil biological activity may control nitrate transport. • Denitrification was not responsible for isotopic enrichment in most samples. • The primary nitrate source was the nitrification of ammonium fertilizers. • Manure leachate appeared to be a minor source of nitrate in several samples. [ABSTRACT FROM AUTHOR]

Landscape may result in uneven nutrient loads within a field. The objective of this study was to determine the influence of landscape on soil carbon and nutrient levels, and on levels of nitrate-N in groundwater. Soil samples were collected in three fields, two transects each, 30 sites in each field. The soil morphology was characterized for the profile, and soil organic carbon and nutrient levels were determined for 0–0.15 and 0.15–0.3 m depths. Each field had wells installed at three of the sites. One field showed a wide range of landscape variability, and significant effects of curvature on soil carbon and nutrient levels. Another field showed no significant effect of slope or curvature on soil carbon and nutrient levels because the nutrient levels were quite variable, including high spikes. The third field had less variable landscape trends but still showed a few significant effects on soil carbon and nutrient levels. Nitrate-N levels remained high in two of the nine wells (20 to 50 mg L −1 ), suggesting that additions of nitrate-N at the concave or converging sites replaced any losses. Median nitrate-N levels at the other seven well sites were lower, ranging from 8 to 17 mg L −1 . Influence of landscape on soil carbon and nutrients was more detectable when the landscape factors were highly variable without excessive variability in soil nutrient properties. [ABSTRACT FROM AUTHOR]

Agriculture in southeastern Oregon and southwestern Idaho known collectively as the Treasure Valley has depended on furrow irrigation using heavy inputs of water and nitrogen (N) fertilizer. Crop rotations include onion, corn, wheat, sugar beet, potato, bean, and other crops. By 1986 groundwater had become contaminated with nitrate and residues of the herbicide chlorthal-dimethyl (DCPA); an official groundwater management area was established by the Oregon Department of Environmental Quality along with an action plan and well monitoring network. The action plan allowed for a trial period to see whether voluntary changes would improve trends. Researchers, producers, and agencies cooperated to develop production options that had the possibility of being both environmentally protective and cost effective. Options were tested to improve irrigation practices, increase N fertilizer use efficiency on several rotation crops, and find a cost effective replacement for DCPA. Research demonstrated the opportunity for increased productivity through both irrigation scheduling and the adoption of drip and sprinkler systems. Fertilizer research demonstrated that smaller, more frequent N applications were more efficient than a single large application. Effective, lower cost herbicides replaced DCPA. Research results were effectively delivered through many means and voluntarily adopted. Both groundwater nitrate and DCPA residues are declining. Productivity has increased.

Elevated groundwater nitrate concentrations have been linked to deleterious health and environmental effects. A significant source of the nitrate is nitrogen fertilizers applied to agricultural landscapes. Beneficial Management Practices (BMPs), including the optimization of fertilizer use and selective crop rotations, have proven to be effective in some cases. The city of Woodstock in southern Ontario relies on public wells for all of its municipal supply. Several of the wells have experienced chronic increases in nitrate concentrations exceeding the maximum allowable limit of 10 mg/L N-NO 3 −. While BMPs are established, an interim reduction plan based on enhanced in situ denitrification (Cross Injection System, CIS) in a 15 m thick zone of high nitrate mass flux within the aquifer zone was evaluated. Based in the results of preliminary acetate injection experiments, a C:N ratio of 2.35, (approximately 260 mg acetate/L), was selected to optimize the denitrification reaction. Injections were performed for six hours a day every day for a period of approximately two months. Dissolved oxygen (DO) and nitrate concentrations recorded over time indicated that reduction of both commenced within a few days of the beginning of the acetate injections and reduced levels were maintained for the remainder of the two-month injection period. Denitrification occurred throughout the profile although nitrate reduction was the highest in the lower groundwater velocity zones. An overall reduction of nitrate of 50% was achieved through the treated section of the aquifer. It is estimated that an upscaled treatment system utilizing a treatment width of only 70 m would be sufficient to reduce the nitrate concentrations to below the drinking water limit demonstrating the potential for the CIS method to functions as an interim groundwater nitrate reduction strategy. • In situ denitrification is a viable groundwater treatment strategy in contaminated aquifers. • Heterogeneous aquifers can be treated by adjusting the C:N ratio upward with acetate. • Enhanced in situ denitrification can act as interim treatment while BMPs are established. [ABSTRACT FROM AUTHOR]

Groundwater nitrate (NO 3 -) pollution has attracted widespread attention; however, accurately evaluating the sources of NO 3 - and their contribution patterns in regional groundwater is difficult in areas with multiple sources and complex hydrogeological conditions. In this study, 161 groundwater samples were collected from the Poyang Lake Basin for hydrochemical and dual NO 3 - isotope analyses to explore the sources of NO 3 - and their spatial contribution using the Positive Matrix Factorization (PMF) and Bayesian stable isotope mixing (MixSIAR) models. The results revealed that the enrichment of NO 3 - in groundwater was primarily attributed to sewage/manure (SM), which accounted for more than 50 %. The contributions of nitrogen fertilizer and soil organic nitrogen should also be considered. Groundwater NO 3 - sources showed obvious spatial differences in contributions. Regions with large contributions of SM (>90 %) were located in the southeastern part of the study area and downstream of Nanchang, which are areas with relatively high population density. Nitrogen fertilizer and soil organic nitrogen showed concentrated contributions in paddy soil in the lower reaches of the Gan and Rao Rivers, and these accumulations were mainly driven by the soil type, land use type, and topography. This study provides insight into groundwater NO 3 - contamination on a regional scale. [Display omitted] • Contributions and spatial pattern of NO 3 sources were deciphered in a typical region. • Four contributors to groundwater NO 3 were firstly quantified in Poyang Lake Basin. • More than 50% of groundwater NO 3 content originates from sewage/manure. • Hydrogeological condition and human activities control spatial pattern of NO 3 sources. [ABSTRACT FROM AUTHOR]

Effective regulations may help reduce nitrate contamination of groundwater from agriculture. Dairy farmers in California must maintain a ratio below 1.4 of total nitrogen (N) applied to total N-removed (N-Ratio) on cropland receiving manure application. In annual reports to the regulatory agency, farmers detail nutrients applied to cropland, removed in harvests, and exported off farm. Data were extracted from all available annual reports for 62 dairies from 2011, 2012, and 2013. Excretions of N, phosphorus (P), and potassium (K) were calculated using reported herd demographics and standard excretion equations from the American Society of Agricultural and Biological Engineers. Calculated nutrient excretion values were compared to the reported values of manure nutrients applied to cropland and exported off farm. Reported N-Ratios were compared to mass balance simulations exploring variable crop yields and alfalfa management. In the nutrient excretion balance, the distribution of the percent of N and P recovered in manures applied or exported peaked at 24% (median = 31%) and 26% (median = 53%) of excreted, respectively. The distribution of recovered K was fairly uniform from 0% to 300% (median = 146%) of excreted K. In N-ratio simulations, 62% and 66% of all reported N-ratios were lower than their respective simulated N-ratio, assuming alfalfa crops received no N fertilization and minimal fertilization (26% of N-removed in harvest) respectively. When simulated crop yields were normally (sd = 0.25) or Student's t distributed (df = 154) around expected crop yields, 28% and 57% of all reported ratios fell within the 95% confidence interval of the simulations, respectively. Low and erratic recovery rates of excreted P and K existed. Additionally, reported N-Ratios were generally lower and more varied than necessary for farmers to maintain crop yields while complying with regulations. Greater understanding of low recovery rates is needed before data are used to assess the impact of regulations. [ABSTRACT FROM AUTHOR]

The importance of corn production to the midwestern United States cannot be overestimated. However, high production requires high nitrogen fertilization, which carries costs to environmental services such as water quality. Therefore, a trade-off exists between the production of corn yield and water quality. We used the Groundwater Vulnerability Assessment for Shallow depths and Crop Environment Resource Synthesis-Maize models to investigate the nature of this trade-off while testing the Simple Analytic Framework trade-offs featured in this Special Feature. First, we estimated the current levels of yield and water quality production in northeastern Iowa and southern Minnesota at the 1-square-kilometer, county, and regional scales. We then constructed an efficiency frontier from optimized nitrogen application patterns to maximize the production of both yield and water quality. Results highlight the context dependency of this trade-off, but show room for increasing the production of both services to the benefit of all stakeholders. We discuss these results in the context of spatial scale, biophysical limitations to the production of services, and stakeholder outcomes given disparate power balances and biophysical contexts. [ABSTRACT FROM AUTHOR]

• A quasi-3D integrated model for the vadose zone-groundwater system was developed. • Groundwater level recovery lead to more nitrate within vadose zones into the aquifer. • Groundwater level recovery elevate the groundwater nitrate-N concentration. Groundwater level (GWL) recovery in some semiarid regions, attributed to mitigation countermeasures for groundwater depletion, potentially causes nitrate accumulated in the vadose zone to be introduced into the aquifer. However, the extent to which GWL recovery affects interactions between the vadose zone and saturated aquifers, migration pathways of soil nitrogen and groundwater nitrate dynamics have not been explicitly determined. This study established a quasi-3D feedback model for the vadose zone-groundwater coupled system in a typical GWL recovery area and quantitatively evaluated the effects of GWL recovery on nitrate-N leaching fluxes via the vadose zone and groundwater nitrate-N dynamics. Within the framework of the integrated model, both the water/contaminant leaching fluxes and the depth to groundwater were exchanged at each flow time step. The obtained results reveal that the temporal changes in nitrate-N leaching fluxes depended on the behaviors of precipitation, farmland irrigation and lithology of the vadose zone, while its spatial patterns were determined by both the GWL undulation and the vertical profiles of nitrate-N content. Furthermore, the GWL recovery caused the magnitude of the nitrate-N leaching fluxes into the aquifer to increase by 44.4%. Along with the GWL recovery, the phreatic aquifer volume increased by 7.47%, and the nitrate-N mass herein increased by 40.06%, which was largely driven by the nitrate-N leaching flux. Consequently, the average groundwater nitrate-N concentration in the GWL recovery region increased by approximately 2.4 mg/L, apart from the artificial recharge route. This finding suggests that the intensified leaching of soil contaminants, given the circumstances of GWL recovery, has a negative effect on groundwater quality. An appropriate groundwater management scheme is therefore urgently required to achieve an optimal balance between GWL recovery and groundwater environment. [Display omitted] [ABSTRACT FROM AUTHOR]

Nitrate loading from an intensively managed commercial red raspberry field to groundwater in the Abbotsford-Sumas Aquifer, British Columbia was estimated over a 1 yr period and compared with the nitrogen surplus calculated using a simple nitrogen budget. Nitrate loading was estimated as the product of recharge (estimated from climate data as total precipitation minus potential evapotranspiration (PET)) and monthly nitrate concentration measured at the water table. Most nitrate loading occurred when nitrate, accumulated in the root zone over the growing season, was leached following heavy autumn rainfall events. Elevated groundwater nitrate concentrations at the water table during the growing season when recharge was assumed to be negligible suggested that the nitrate loading was underestimated. The estimate of annual nitrate loading to the water table was high (174 kg N ha-1) suggesting that the tools currently available to growers to manage N in raspberry production are not adequate to protect groundwater quality. The calculated nitrogen surplus from the nitrogen budget (180 kg N ha-1) was similar to the measured nitrate loading suggesting that simple nitrogen budgets may be relatively effective indices of the risk of nitrate loading to groundwater. [ABSTRACT FROM AUTHOR]

Summary: It is critical that stakeholders are aware of the lag time necessary for conservation practices to demonstrate a positive impact on surface water quality. For solutes like nitrate that are transported primarily by the groundwater pathway, the lag time is a function of the groundwater travel time distribution (TTD). We used three models of varying levels of complexity to estimate the steady-state TTD of a shallow, unconfined aquifer in a small Iowa watershed: (a) analytic model, (b) GIS approach, and (c) MODFLOW model. The analytic model was the least input-intensive, whereas the GIS and MODFLOW approach required detailed data for model development. The resulting TTDs displayed an exponential distribution with good agreement among all the three methods (mean travel times ranging from 16.2years in the analytic model, 19.6years in GIS model and 20.5years in MODFLOW model). The greater deviation in the analytic model was attributed to the difficulty in estimation of a representative saturated thickness in an unconfined aquifer. The correspondence between the spatial travel time distributions generated by GIS and MODFLOW was a function of the landscape position, with greater correspondence in uplands compared to floodplains. In the floodplains the land surface slope is a poor approximation of the water table gradient that is captured by the MODFLOW model but not the GIS that uses the land surface as a surrogate for the water table. Study results indicate that except for cases where there are marked differences between water table surface and land surface, simpler approaches (analytic and GIS) can be used to estimate TTDs required for the design and optimal placement of conservation practices and communicating lag times issues to the public. [Copyright &y& Elsevier]

By employing chemical and isotopic tracers (15N and 18O in NO3 −), we investigated the main processes controlling nitrate distribution in the unsaturated zone and aquifer. Soil water was extracted from two soil cores drilled in a typical agricultural cropping area of the North China Plain (NCP), where groundwater was also sampled. The results indicate that evaporation and denitrification are the two major causes of the distribution of nitrate in soil water extracts in the unsaturated zone. Evaporation from unsaturated zone is evidenced by a positive correlation between chloride and nitrate, and denitrification by a strong linear relationship between and ln(NO3 −/Cl). The latter is estimated to account for up to 50% of the nitrate loss from soil drainage. In the saturated zone, nitrate is reduced at varying extents (100 mg/L and 10 mg/L at two sites, respectively), largely by dilution of the aquifer water. [Copyright &y& Elsevier]

The present study was conducted to compare the behavior of wheat straw, sawdust and biodegradable plastic (BP) as potential carbon sources for denitrification in groundwater remediation. The results showed that a greater amount of nitrogen compounds were released from wheat straw and sawdust than from BP in leaching experiments. In batch experiments, BP showed higher nitrate removal efficiency and longer service life than wheat straw and sawdust, which illustrated that BP is the most appropriate carbon source for stimulation of denitrification activity. In column experiments, BP was able to support complete denitrification at influent nitrate concentrations of 50, 60, 70, 80, and 90 mg NO3--N/L, showing corresponding denitrification rates of 0.12, 0.14, 0.17, 0.19, and 0.22 mg NO3--N.L-1.d-1.g-1, respectively. These findings indicate that BP is applicable for use as a carbon source for nitrate-polluted groundwater remediation. [ABSTRACT FROM AUTHOR]