Nitrate sources and transformation processes in groundwater of a coastal area experiencing various environmental stressors

New insights into nitrate sources and transformations in riparian groundwater of a sluice-controlled river: An integrated approach using major ions, stable isotopes and microbial gene methods

Nitrate pollution in water environment is a serious problem worldwide. Identifying nitrate sources and transformations in the riparian aquifer is critical for effectively controlling and mitigating nitrate contamination, especially in sluice-controlled rivers. This study employs an integrated approach combining hydrochemical analysis, isotopes (δ18O-H2O, δ2H-H2O, δ15N-NO3- and δ18O-NO3-), quantification of nitrogen (N) functional genes and a Bayesian mixing model (MixSIAR) to comprehensively investigates nitrate sources and transformation processes in the riparian groundwater of a sluice-controlled Shaying River, China. Results revealed severe nitrate contamination in both the river (mean: 2.33-5.25 mg/L) and the riparian groundwater (mean: 0.42-24.46 mg/L). Manure and sewage were the primary sources (66.20-91.20 %) of nitrate contamination in both river and riparian groundwater. Key processes influencing nitrate dynamics in riparian groundwater included mixing with river water, external N supply, and transformation processes such as nitrification, vegetation uptake and anammox. We found that when sluices are closed, the nitrate concentration in riparian groundwater decreases. In contrast, during the flood season with sluices open, the nitrate concentration in the river water increases. This study also developed the first conceptual model illustrating the impact of sluice regulation on riparian nitrate dynamics, highlighting the complex interplay between sluice operations, hydrological conditions, and biogeochemical processes that govern nitrate behavior. These findings provide valuable insights into nitrate dynamics in riparian aquifers of sluice-controlled rivers, offering a robust scientific foundation for targeted nutrient management strategies in the Shaying River Basin and similar regulated environments globally.

Multiple environmental tracers combined with a constrained Bayesian isotope mixing model to elucidate nitrate and sulfate contamination in a coastal groundwater system

Several groundwater quality investigations have been conducted in coastal regions that are commonly exposed to multiple anthropogenic stressors. Nonetheless, such studies remain challenging because they require focused-diagnostic approaches for a comprehensive understanding of groundwater contamination. Therefore, this study integrates a multi-tracer approach to acquire comprehensive information allowing for an improved understanding of the origins of groundwater contamination, the relative contribution of contaminants, and their biogeochemical cycling within a coastal groundwater system. This multi-tracer approach, focusing on nitrate (NO3) and sulfate (SO4) groundwater contamination, is applied to a Mediterranean coastal aquifer underlying an important economically strategic agricultural area. Dissolved NO3 in groundwater has concentrations up to 89 mg/L, whereas SO4 concentrations in groundwater are up to 458 mg/L. By integrating isotope tracers (i.e., δ15NNO3, δ18ONO3, δ11B, δ34SSO4, and δ18OSO4), NO3 and SO4 in the groundwater are found to have originated from multiple anthropogenic and natural sources including synthetic fertilizers, manure, sewage, atmospheric deposition, and marine evaporites. Chemical and isotopic data are coupled to identify the dominant hydro(geo)logic processes and the major subsurface biogeochemical reactions that govern the NO3 and SO4 occurrences. Nitrate and SO4 concentrations are identified to be respectively controlled by nitrification/denitrification and by bacterial dissimilatory SO4 reduction. Identifying these subsurface biogeochemical processes constrained the Bayesian isotope MixSIAR model, that is used for apportioning the relative contributions of the identified groundwater contamination sources, by informed site-specific isotopic fractionation effects. Results from MixSIAR indicate that manure is distinguished as the predominant source for NO3 (61 %), whereas SO4 in groundwater is mostly supplied from two sources (i.e., synthetic fertilizers and soil-derived sulfate) identified with similar contributions (30 %). This study particularly demonstrates the utility of initially describing the subsurface processes, not only to predict the fate of NO3 and SO4 concentrations within the groundwater system, but also to constrain the MixSIAR model with justified site-specific isotopic fractionation effects for subsurface transformation processes affecting NO3 and SO4.

Source, transport, and fate of nitrate in shallow groundwater in the eastern Niger Delta

The eastern Niger Delta region in Nigeria is a hotspot for reactive nitrogen pollution due to extensive animal husbandry, pit latrine usage, and agricultural practices. Despite the high level of human activity, the sources and processes affecting nitrogen in groundwater remain understudied. Groundwater nitrate (NO3-) concentrations are highly variable, with some areas recording values well above the safe drinking water threshold of 50 mg/L. This is particularly true near municipal sewage systems. Elevated nitrite (NO2-) and ammonium (NH4+) concentrations were also detected in the study area. Sewage analysis revealed NO3- concentrations ranging from 1 to 145 mg/L, NO2- from 0.2 to 2 mg/L, and notably high NH4+ concentrations. A comparison of major ions indicated that 71%, 90%, 87%, and 92% of groundwater samples surpassed reference site levels for calcium (Ca2+), sodium (Na+), potassium (K+), and chloride (Cl-), respectively, pointing to sewage as a likely source of contamination. The NO3-/Cl- ratios at several sites suggested that most groundwater NO3- originates from human waste. Stable isotope analysis of NO3- showed a general enrichment in 15N and, in some cases, a depletion in 18O, indicating that the NO3- originates from sewage-derived NH4+ nitrification. Although denitrification, a process that reduces NO3-, is present, the high dissolved oxygen (DO) and NO3- levels in the groundwater suggest that denitrification is insufficient to fully mitigate NO3- pollution. Consequently, there is a risk of NO3- leaching from shallow aquifers into the Niger Delta's surface waters and ultimately into the coastal ocean.

Chemical and isotopic tracers combined with mixing models for tracking nitrate contamination in the Pampa de Pocho aquifer, Argentina

In recent years, it has become evident that human activities have significantly disrupted the nitrogen cycle surpassing acceptable environmental thresholds. In this study, chemical and isotopic tracers were combined with a mathematical mass balance model (EMMA), PHREEQC inverse mixing model, and statistical analyses to evaluate groundwater quality, across an area experiencing substantial human activities, with a specific focus on tracing the origin of nitrate (NO3-) with potential water mixing processes. This multi-technique approach was applied to an unconfined aquifer underlying an agricultural area setting in an inter-mountain depression (i.e., the "Pampa de Pocho Plain" in Argentina). Here, the primary identified geochemical processes occurring in the investigated groundwater system include the dissolution of carbonate salts, cation exchange, and hydrolysis of alumino-silicates along with incorporating ions from precipitation. It was observed that the chemistry of groundwater, predominantly of sodium bicarbonate with sulfate water types, is controlled by the area's geology, recharge from precipitation, and stream water infiltration originating from the surrounding hills. Chemical results reveal that 60% of groundwater samples have NO3- concentrations exceeding the regional natural background level, confirming the impact of human activities on groundwater quality. The dual plot of δ15NNO3 versus δ18ONO3 values indicates that groundwater is affected by NO3- sources overlapping manure/sewage with organic-rich soil. The mathematical EMMA model and PHREEQC inverse modeling, suggest organic-rich soil as an important source of nitrogen in the aquifer. Here, 64 % of samples exhibit a main mixture of organic-rich soil with manure, whereas 36 % of samples are affected mainly by a mixture of manure and fertilizer. This study demonstrates the utility of combining isotope tracers with mathematical modeling and statistical analyses for a better understanding of groundwater quality deterioration in situations where isotopic signatures of contamination sources overlap.

Integrated approach to understand the multiple natural and anthropogenic stresses on intensively irrigated coastal aquifer in the Mediterranean region

Understanding the major factors influencing groundwater chemistry and its evolution in irrigation areas is crucial for efficient irrigation management. Major ions and isotopes (δD-H2O together with δ18O-H2O) were used to identify the natural and anthropogenic factors contributing to groundwater salinization in the shallow aquifer of the Wadi Guenniche Plain (WGP) in the Mediterranean region of Tunisia. A comprehensive geochemical investigation of groundwater was conducted during both the low irrigation season (L-IR) and the high irrigation season (H-IR). The results show that the variation range and average concentrations of almost all the ions in both the L-IR and H-IR seasons are high. The groundwater in both seasons is characterized by high electrical conductivity and CaMgCl/SO4 and NaCl types. The dissolution of halite and gypsum, the precipitation of calcite and dolomite, and Na-Ca exchange are the main chemical reactions in the geochemical evolution of groundwater in the Wadi Guenniche Shallow Aquifer (WGSA). Stable isotopes of hydrogen and oxygen (δ18O-H2O and δD-H2O) indicate that groundwater in WGSA originated from local precipitation. In the H-IR season, the δ18O-H2O and δD-H2O values indicate that the groundwater experienced noticeable evaporation. The enriched isotopic signatures reveal that the WGSA's groundwater was influenced by irrigation return flow and seawater intrusion. The proportions of mixing with seawater were found to vary between 0.12% and 5.95%, and between 0.13% and 8.42% during the L-IR and H-IR seasons, respectively. Irrigation return flow and the associated evaporation increase the dissolved solids content in groundwater during the irrigation season. The long-term human activities (fertilization, irrigation, and septic waste infiltration) are the main drives of the high nitrate-N concentrations in groundwater. In coastal irrigation areas suffering from water scarcity, these results can help planners and policy makers understand the complexities of groundwater salinization to enable more sustainable management and development.

Investigating the effects of surface water recharge on groundwater quality using hydrochemistry and ANFIS model: A case study Minia Governorate, Egypt

Monitoring and assessing groundwater quality and quantity lays the basis for sustainable management. Therefore, this research aims to investigate various factors that affect groundwater quality, emphasizing its distance to the primary source of recharge, the Nile River. To this end, two separate study areas have been considered, including the West and West-West of Minia, Egypt, located around 30 and 80 km from the Nile River. The chosen areas rely on the same aquifer as groundwater source (Eocene aquifer). Groundwater quality has been assessed in the two studied regions to investigate the difference in quality parameters due to the river's distance. The power of machine learning to associate different variables and generate beneficial relationships has been utilized to mitigate the cost consumed in chemical analysis and alleviate the calculation complexity. Two adaptive neuro-fuzzy inference system (ANFIS) models were developed to predict the water quality index (WQI) and the irrigation water quality index (IWQI) using EC and the distance to the river. The findings of the assessment of groundwater quality revealed that the groundwater in the west of Minia exhibits suitability for agricultural utilization and partially meets the criteria for potable drinking water. Conversely, the findings strongly recommend the implementation of treatment processes for groundwater sourced from the West-West of Minia before its usage for various purposes. These outcomes underscore the significant influence of surface water recharge on the overall quality of groundwater. Also, the results revealed the uncertainty of using sodium adsorption ratio (SAR), Sodium Percentage (Na%), and Permeability Index (PI) techniques in assessing groundwater for irrigation and recommended using IWQI. The developed ANFIS models depicted perfect accuracy during the training and validation stages, reporting a coefficient of correlation (R) equal to 0.97 and 0.99 in the case of WQI and 0.96 and 0.98 in the case of IWQI. The research findings could incentivize decision-makers to monitor, manage, and sustain groundwater.