Fate Of Nitrate In Groundwater Research Articles

Investigating Nitrate with Other Constituents in Groundwater in Two Contrasting Tropical Highland Watersheds

Nitrate is globally the most widespread and widely studied groundwater contaminant. However, few studies have been conducted in sub-Saharan Africa, where the leaching potential is enhanced during the rainy monsoon phase. The few monitoring studies found concentrations over drinking water standards of 10 mg N-NO3− L−1 in the groundwater, the primary water supply in rural communities. Studies on nitrate movement are limited to the volcanic Ethiopian highlands. Therefore, this study aimed to evaluate the transport and fate of nitrate in groundwater and identify processes that control the concentrations. Water table height, nitrate, chloride, ammonium, reduced iron, and three other groundwater constituents were determined monthly in the groundwater in over 30 wells in two contrasting volcanic watersheds over two years in the Ethiopian highlands. The first watershed was Dangishta, with lava intrusion dikes that blocked the subsurface flow in the valley bottom. The water table remained within 3 m of the surface. The second watershed without volcanic barriers was Robit Bata. The water table dropped rapidly within three months of the end of the rain phase and disappeared except near faults. The average nitrate concentration in both watersheds was between 4 and 5 mg N-NO3− L−1. Hydrogeology influenced the transport and fate of nitrogen. In Dangishta, water was blocked by volcanic lava intrusion dikes, and residence time in the aquifer was larger than in Robit Bata. Consequently, nitrate remained high (in several wells, 10 mg N-NO3− L−1) and decreased slowly due to denitrification. In Robit Bata, the water residence time was lower, and peak concentrations were only observed in the month after fertilizer application; otherwise, it was near an average of 4 mg N-NO3− L−1. Nitrate concentrations were predicted using a multiple linear regression model. Hydrology explained the nitrate concentrations in Robit Bata. In Dangishta, biogeochemistry was also significant.

Sources and Fate of Nitrate in Groundwater in a Typical Karst Basin: Insights from Carbon, Nitrogen, and Oxygen Isotopes

Multiple isotopes (C, N, and O) and hydrochemical data were used to trace the sources and fate of nitrate in ground and surface waters of the Babu subterranean stream watershed in Guizhou Province. The origin of nitrate in the water samples was also quantitatively analyzed by the SIAR model. Approximately 38% of the groundwater samples were not drinkable because the nitrate exceeded the drinking water standard, thereby indicating that the groundwater was seriously polluted by nitrate. The ranges of δ15N-NO3, δ18O-NO3, and δ18O-H2O in groundwater were 2.30‰-30.33‰ (mean of 9.68‰), 2.65‰-13.73‰ (mean of 6.64‰), and -8.83‰﹣-7.37‰ (mean of -8.18‰), respectively. Based on the stable isotopic compositions (δ15N-NO3, δ18O-NO3, and δ18O-H2O), nitrification was the dominant process in the basin. The nitric acid produced by nitrification promoted the dissolution of carbonate rocks, thereby leading to a significantly negative correlation (P<0.001) between the carbon isotope of dissolved inorganic carbon (δ13CDIC) and δ15N-NO3 and indicating that δ13CDIC, combined with δ15N-NO3, is effective in exploring the fate of nitrate in karst groundwater. The nitrate in the ground and surface waters mainly originated from soil N, manure and sewage, and ammonium nitrogen fertilizer. The results of the SIAR model showed that the contributions of soil N, manure and sewage, and ammonium nitrogen fertilizer were 36.19%, 33.71%, and 30.10% in groundwater, respectively, and 39.15%, 36.08%, and 24.77% in surface water, respectively. Therefore, it would be more effective to reduce the nitrate recharge flux in groundwater by simultaneously removing nitrate and ammonium nitrogen during wastewater treatment and by adopting scientific fertilization technology in agricultural areas.

Sources and fate of nitrate in groundwater at agricultural operations overlying glacial sediments

Abstract. Leaching of nitrate (NO3-) from animal waste or fertilisers at agricultural operations can result in NO3- contamination of groundwater, lakes, and streams. Understanding the sources and fate of nitrate in groundwater systems in glacial sediments, which underlie many agricultural operations, is critical for managing impacts of human food production on the environment. Elevated NO3- concentrations in groundwater can be naturally attenuated through mixing or denitrification. Here we use isotopic enrichment of the stable isotope values of NO3- to quantify the amount of denitrification in groundwater at two confined feeding operations overlying glacial sediments in Alberta, Canada. Uncertainty in δ15NNO3 and δ18ONO3 values of the NO3- source and denitrification enrichment factors are accounted for using a Monte Carlo approach. When denitrification could be quantified, we used these values to constrain a mixing model based on NO3- and Cl− concentrations. Using this novel approach we were able to reconstruct the initial NO3−N concentration and NO3-N/Cl- ratio at the point of entry to the groundwater system. Manure filtrate had total nitrogen (TN) of up to 1820 mg L−1, which was predominantly organic N and NH3. Groundwater had up to 85 mg L−1 TN, which was predominantly NO3-. The addition of NO3- to the local groundwater system from temporary manure piles and pens equalled or exceeded NO3- additions from earthen manure storages at these sites. On-farm management of manure waste should therefore increasingly focus on limiting manure piles in direct contact with the soil and encourage storage in lined lagoons. Nitrate attenuation at both sites is attributed to a spatially variable combination of mixing and denitrification, but is dominated by denitrification. Where identified, denitrification reduced agriculturally derived NO3- concentrations by at least half and, in some wells, completely. Infiltration to groundwater systems in glacial sediments where NO3- can be naturally attenuated is likely preferable to off-farm export via runoff or drainage networks, especially if local groundwater is not used for potable water supply.

Relationship between land-use and sources and fate of nitrate in groundwater in a typical recharge area of the North China Plain

Identification of different nitrate sources in groundwater is challenging in areas with diverse land use and multiple potential inputs. An area with mixed land-uses, typical of the piedmont-plain recharge area of the North China Plain, was selected to investigate different nitrate sources and the impact of land use on nitrate distribution in groundwater. Multiple environmental tracers were examined, including major ions, stable isotopes of water (δ2H-H2O, δ18O-H2O) and nitrate (δ15N-NO3− and δ18O-NO3−). Groundwater was sampled from four land-use types; natural vegetation (NV), farmland (FL), economic forestland (EF) and residential areas (RA). A mixing model using δ18O and Cl− concentrations showed that groundwater recharge predominantly comprises precipitation and lateral groundwater flow from areas of natural vegetation in the upper catchment, while irrigation return water and wastewater from septic tanks were major inputs in farmland and residential areas, respectively. Land use variation is the major contributing factor to different nitrate concentrations. In total, 80%, 49% and 86% of samples from RA, FL and EF, respectively exceeded the WHO standard (50mg/L NO3−), compared to 6.9% of samples from NV. Isotopes of δ15N-NO3− and δ18O-NO3− verified that nitrate in groundwater of the NV (with δ15N ranging from 1.7‰ to 4.7‰) was sourced from soil and precipitation. Examination of δ15N-NO3− vs δ18O-NO3− values along with multivariate statistical analysis (principle component and cluster analysis) helped identify sources with overlapping isotopic values in other land-use areas (where δ15N values range from 2.5‰ to 10.2‰). Manure and septic waste were dominant sources for most groundwater with high NO3− and Cl− concentrations in both farmland and residential areas. The lack of de-nitrification and fact that the area is a recharge zone for the North China Plain highlight the importance of controlling nitrate sources through careful application of manure and fertilizers, and control of septic leakage.

Effect of wheat-maize straw return on the fate of nitrate in groundwater in the Huaihe River Basin, China

Straw return is becoming a routine practice in disposing of crop residues worldwide. However, the potential effect of such operation on the chemistry of local groundwater is not well documented. Here, shallow groundwater in an area where wheat-maize straw return is practiced was analyzed, and the seasonal changes in the nitrate concentration and the isotope compositions of NO3− and H2O were determined along two flow paths. Measured δD and δ18O in waters indicated that the groundwater was mainly recharged by atmospheric precipitation, while measured δ15N and δ18O in nitrate suggested that the sources for groundwater NO3− included urea fertilizer, soil nitrogen, and sewage/manure. Reduced NO3− concentrations coincided with an enrichment of organic matter in the groundwater of the straw return area, revealing an environmental condition that facilitates nitrate reduction, whereas increased δ15N-NO3− and δ18O-NO3− along the flow path suggested the occurrence of denitrification. Further analyses showed that, compared to the cases in the absence of straw return, as much as 80% and 90% of groundwater nitrate was removed in low and high water seasons in the straw return area, pointing to a potential positive effect of straw return to groundwater quality.

The influence of bedrock hydrogeology on catchment-scale nitrate fate and transport in fractured aquifers

Characterising catchment scale biogeochemical processes controlling nitrate fate in groundwater constitutes a fundamental consideration when applying programmes of measures to reduce risks posed by diffuse agricultural pollutants to water quality. Combining hydrochemical analyses with nitrate isotopic data and physical hydrogeological measurements permitted characterisation of biogeochemical processes influencing nitrogen fate and transport in the groundwater in two fractured bedrock aquifers with contrasting hydrogeology but comparable nutrient loads. Hydrochemical and isotopic analyses of groundwater samples collected from moderately fractured, diffusely karstified limestone indicated nitrification controlled dissolved nitrogen fate and delivery to aquatic receptors. By contrast nitrate concentrations in groundwater were considerably lower in a low transmissivity highly lithified sandstone and pyrite-bearing shale unit with patchy subsoil cover. Geophysical and hydrochemical investigations showed shallower intervals contained hydraulically active fractures where denitrification was reflected through lower nitrogen levels and an isotopic enrichment ratio of 1.7 between δ15N and δ18O. Study findings highlight the influence of bedrock hydrogeological conditions on aqueous nitrogen mobility. Investigation results demonstrate that bedrock conditions need to be considered when implementing catchment management plans to reduce the impact of agricultural practices on the quality of groundwater and baseflow in receiving rivers.Nitrate isotopic signatures in the groundwater of a freely draining catchment underlain by a karstified aquifer and a poorly draining aquifer with a low transmissivity aquifer.

Quantifying the fate of agricultural nitrogen in an unconfined aquifer: Stream‐based observations at three measurement scales

AbstractWe compared three stream‐based sampling methods to study the fate of nitrate in groundwater in a coastal plain watershed: point measurements beneath the streambed, seepage blankets (novel seepage‐meter design), and reach mass‐balance. The methods gave similar mean groundwater seepage rates into the stream (0.3–0.6 m/d) during two 3–4 day field campaigns despite an order of magnitude difference in stream discharge between the campaigns. At low flow, estimates of flow‐weighted mean nitrate concentrations in groundwater discharge ([ ]FWM) and nitrate flux from groundwater to the stream decreased with increasing degree of channel influence and measurement scale, i.e., [ ]FWM was 654, 561, and 451 µM for point, blanket, and reach mass‐balance sampling, respectively. At high flow the trend was reversed, likely because reach mass‐balance captured inputs from shallow transient high‐nitrate flow paths while point and blanket measurements did not. Point sampling may be better suited to estimating aquifer discharge of nitrate, while reach mass‐balance reflects full nitrate inputs into the channel (which at high flow may be more than aquifer discharge due to transient flow paths, and at low flow may be less than aquifer discharge due to channel‐based nitrate removal). Modeling dissolved N2 from streambed samples suggested (1) about half of groundwater nitrate was denitrified prior to discharge from the aquifer, and (2) both extent of denitrification and initial nitrate concentration in groundwater (700–1300 µM) were related to land use, suggesting these forms of streambed sampling for groundwater can reveal watershed spatial relations relevant to nitrate contamination and fate in the aquifer.

An investigation of nitrate and iron concentrations and their relationship in shallow groundwater systems of Kathmandu

Groundwater is vulnerable to contamination by chemicals, including nitrate, which can pass through soil to the shallow groundwater systems. Nitrate-N is commonly used as an environmental indicator to trace the impact of anthropogenic activities on groundwater. An understanding of the fate of nitrate in groundwater is vital for managing risks associated with nitrate pollution to safeguard groundwater supplies. Hence, groundwater samples were collected in ninety wells of shallow groundwater systems of Kathmandu, Nepal including shallow tube wells, dug wells and stone spouts (locally called Dhunge Dharas). The samples were analyzed for nitrate-nitrogen (NO3-N), iron (Fe) along with pH and temperature. The nitrate-nitrogen and iron (Fe) concentrations ranged from 0.0 to 26.4 and 0.0 to 5.24 mg/L, respectively. An understanding of the relationship between nitrate and iron in groundwater is crucial to explore the mechanism of natural denitrification and its ability to reduce nitrate concentrations. In general, elevated nitrate concentrations were not found in sampled groundwater sources in study area where elevated iron concentrations were common. The observed negative correlations between nitrate and iron suggest that the nitrate in shallow groundwater is being consumed. Moreover, this study highlights the current status and trends of the nitrate and iron in shallow groundwater systems and their relationship to water depth and well types.

Understanding the origin and fate of nitrate in groundwater of semi-arid environments

Though nitrate enrichment in groundwater is a worldwide phenomenon and mainly related to human impact, processes leading to nitrate enrichment in scarcely inhabited semi-arid regions are not yet well understood. In those regions, elevated nitrate concentrations put additional pressure on the scarce water resources, as they pose a serious health risk. This study applies a multidisciplinary approach (hydrogeology, isotope hydrology, and geochemistry) to understand the origin and fate of nitrate in groundwater of the semi-arid Kalahari of Botswana. Our investigations suggest that nitrate in groundwater of the study area is of natural origin, leached from a pool in the unsaturated zone that was actively involved in the soil nitrogen cycle. The presence of active (minor) recharge was found, showing that nitrate may be transported into the groundwater under the present conditions. Yet, slow travel times of replenishing water and the low recharge amounts render the thick unsaturated zone into a long-term reservoir for nitrate. Being only little influenced by reactive processes, nitrate has a high persistency in the observed groundwater system. Concentration increases induced by the present land-use do not yet appear to affect the groundwater quality but may within decades.