Multi-tracer approach to understand nitrate contamination and groundwater-surface water interactions in the Mediterranean coastal area of Guerbes-Senhadja, Algeria

Revealing nitrate sources seasonal difference between groundwater and surface water in China's largest fresh water lake (Poyang Lake): Insights from sources proportion, dynamic evolution and driving forces

Tracing the source of nitrate is the key path to solve the problem of nitrogen pollution. However, the seasonal difference of nitrate sources in groundwater and surface water and its dynamic evolution process and mechanism in large fresh water lake area are still not clear. In this study, 126 water samples were collected from groundwater and surface water in China's largest fresh water lake (Poyang Lake) region from 2022 to 2023. Bayesian stable isotope mixing model, absolute principal component score-multiple linear regression, ion ratio coefficients and uncertainty index (UI90) were used to investigate the nitrate sources variation in groundwater and surface water as well as its uncertainty in Poyang Lake area. Results showed that anthropogenic influence had significant influence on nitrate sources, which was mainly affected by chemical fertilizer (CF), soil nitrogen (SN) and manure and sewage input (M&S). Specifically, from 2022 to 2023, CF contributed 16.6 % to 32.4 %, SN contributed 26.0 % to 38.1 %, M&S contributed 26.5 % to 48.2 % to groundwater. CF contributed 38.8 % to 43.9 %, SN contributed 37.6 % to 40.6 %, M&S contributed 12.3 % to 18.6 % to surface water. The sources and proportion of nitrate in groundwater and surface water exhibited obvious difference. Temporal heterogeneity, land use type, population density and vegetation cover type had influence on nitrate sources. UI90 results showed that there was uncertainty in nitrate sources tracing process, with SN (mean 0.78), CF (mean 0.64), M&S (mean 0.35) and AD (mean 0.09), respectively. These results will provide vital references for understanding nitrate sources variation, controlling and removing nitrate surplus in groundwater-surface water system in the similar large fresh water lake areas.

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.

Hydrogeochemical processes controlling surface water quality for irrigation in a Mediterranean wetland ecosystem, Northeast Algeria

Fetzara Lake, considered one of the most important wetlands in northeastern Algeria, was designated a Ramsar site in 2002. The waters in its watershed are affected by salinity, which influences their suitability for irrigation. To identify the factors influencing the quality of these surface waters, geochemical and statistical analyses were carried out on the basis of the results of chemical analyses of 51 samples collected, during two monitoring campaigns, from all the tributaries in the watershed. The findings show the dominance of three hydrochemical facies over the two campaigns: Na-Cl facies (55.17% and 22.73%) characterizes the waters water from Fetzara Lake outlet (drainage channel and wadi Meboudja), in relation to the influx of saliferous elements due to water evaporation in the lake. Ca-Mg-Cl (27.59% and 40.91%) and Ca-Mg-HCO3 (13.79%. and 13.79%) facies characterize the waters of the remaining tributaries, reflecting the dissolution of carbonate formations and the alteration of the Edough metamorphic basement. Multivariate statistical analysis, using principal component analysis (PCA), shows three water types: highly mineralized (EC > 3000 µS/cm), moderately mineralized (1000 < EC < 3000 µS/cm), and weakly mineralized (EC < 1000 µS/cm). Evaporation and silicate weathering are the main mechanisms controlling water mineralization according to the different bivariate plots. Furthermore, cation exchange indices (CAI-I and CAI-II) reveal that these reactions involve the adsorption of Na+ and K+ onto clay minerals, as well as the simultaneous release of Ca2+ and Mg2+ ions. Finally, the various quality indices (SAR, %Na, RSC and KR) revealed that the water in 36% of tributaries is unsuitable for irrigation. These findings will provide important information on surface water quality in the study area, particularly for irrigation purposes, and will contribute to the thoughtful and sustainable management of this resource.

Nitrate dynamics in the streamwater-groundwater interaction system: Sources, fate, and controls

Nitrate (NO3-) pollution has attracted widespread attention as a threat to human health and aquatic ecosystems; however, elucidating the controlling factors behind nitrate dynamics under the context of changeable hydrological processes, particularly the interactions between streamwater and groundwater (SW-GW), presents significant challenges. A multi-tracer approach, integrating physicochemical and isotopic tracers (Cl-, δ2H-H2O, δ18O-H2O, δ15N-NO3- and δ18O-NO3-), was employed to identify the response of nitrate dynamics to SW-GW interaction in the Fen River Basin. The streamwater and groundwater NO3- concentrations varied greatly with space and time. Sewage and manure (28 %-73 %), fertilizer (14 %-36 %) and soil organic nitrogen (12 %-28 %) were the main NO3- sources in water bodies. Despite the control of land use type on streamwater nitrate dynamics in losing sections, SW-GW interactions drove NO3- dynamics in both streamwater and groundwater under most circumstances. In gaining streams, streamwater nitrate dynamics were influenced either by groundwater dilution or microbial nitrification, depending on whether groundwater discharge ratios exceeded or fell below 25 %, respectively. In losing streams, groundwater nitrate content increased with streamwater infiltration time, but the influence was mainly limited within 3 km from the river channel. This study provides a scientific basis for the effective management of water nitrate pollution at the watershed scale.

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

In coastal salinized groundwater systems, contamination from various nitrate (NO3) inputs combined with complex hydrogeochemical processes make it difficult to distinguish NO3 sources and identify potential NO3 transformtation processes. Effective field-based NO3 studies in coastal areas are needed to improve the understanding of NO3 contamination dynamics in groundwater of such complex coastal systems. This study focuses on a typical Mediterranean coastal agricultural area, located in Tunisia, experiencing substantial NO3 contamination from multiple anthropogenic sources. Here, multiple isotopic tracers (δ18OH2O, δ2HH2O, δ15NNO3, δ18ONO3, and δ11B) combined with a Bayesian isotope MixSIAR model are used (i) to identify the major NO3 sources and their contributions, and (ii) to describe the potential NO3 transformation processes. The measured NO3 concentrations in groundwater are above the natural baseline threshold, suggesting anthropogenic influence. The measured isotopic composition of NO3 indicates that manure, soil organic matter, and sewage are the potential sources of NO3, while δ11B values constrain the NO3 contamination to manure; a finding that is supported by the results of MixSIAR model revealing that manure-derived NO3 dominates over other likely sources. Nitrate derived from manure in the study area is attributed to organic fertilizers used to promote crop growth, and livestock that deposit manure directly on the ground surface. Evidence for ongoing denitrification in groundwaters of the study area is supported by an enrichment in both 15N and 18O in the remaining NO3, although isotopic mass balances between the measured and the theoretical δ18ONO3 values also suggest the occurrence of nitrification. The simultaneous occurrence of these biogeochemical processes with heterogeneous distribution across the study area reflect the complexity of interactions within the investigated coastal aquifer. The multiple isotopic tracer approach used here can identify the effect of multiple NO3 anthropogenic activities in coastal environments, which is fundamental for sustainable groundwater resources management.

Nitrogen in surface aquifer - Coastal lagoons systems: Analyzing the origin of eutrophication processes

Coastal lagoons can act as sinks and sources of a wide range of substances, including nutrients, and pollutants. In these ecosystems, primary production is limited more by nitrogen than by phosphorus. For this reason, they are significantly vulnerable to nitrate pollution. In this study, a joined analysis of surface and ground water was conducted to determine the origin, occurrence and processes affecting nitrogen fate in a Mediterranean coastal aquifer-lagoon system. This included the analysis of water levels, as well as hydrochemical and isotopes data evolution for a 4-year period, which revealed two important insights of nitrogen cycling within the system. Firstly, we detected different origins for nitrate pollution (a mixture of sewage, manure and chemical fertilizers), as well as their nearly complete attenuation in the alluvial aquifer due to heterotrophic and autotrophic processes, favoured by the presence of organic matter and Fe-minerals in its sediments. Secondly, due to its rapid assimilation, inorganic nitrogen peaks in the lagoons were mainly detected after storm events. While nitrate peaks may be attributed to surface water runoff, ammonium peaks may be related to organic nutrient cycling. In contrast, we did not detect continuous and low N inputs, associated to groundwater flow. These results depict the need of a more integrated management strategy of these aquifer-lagoon systems.