Origin of nitrate and sulfate sources in volcano-sedimentary aquifers of the East Africa Rift System: An example of the Ali-Sabieh groundwater (Republic of Djibouti)

Spatial distribution, geochemical processes of high-content fluoride and nitrate groundwater, and an associated probabilistic human health risk appraisal in the Republic of Djibouti

In the northern East African Rift System, the Republic of Djibouti relies exclusively on groundwater, with levels of fluoride (up to 14 mg/L) and nitrate (up to 256 mg/L) posing potential health risks. To address this, 362 samples were considered, including 133 shallow groundwater samples, along with new and previously published data dating back to 2012 on deep (88) and thermal (141) groundwater samples. To understand the enrichment mechanisms, dissolved anion and cation constituents, geochemical and thermodynamic tools, and stable isotope ratios, such as δ2H(H2O), δ18O(H2O), δ15N(NO3-), and δ18O(NO3-), were used. In particular, two activity diagrams (Mg2+ vs. Ca2+ and Na+ vs. Ca2+), focused on aqueous and solid fluoride species in an updated thermodynamic dataset of 15 fluoride-bearing minerals, are shown for the first time. The dataset offers new and valuable insights into fluoride geochemistry (classic thermodynamic datasets combined with geochemical codes rely solely on fluorapatite and fluorite F-bearing minerals). Activity diagrams and geochemical modeling indicate that mineral dissolution primarily drives groundwater fluoride enrichment in all water types, whereas the elevated nitrate levels may stem from organic fertilizers like animal manure, as indicated by nitrate isotopes and NO3-/Cl- vs Cl- diagrams. Despite the arid climate and 2H18O enrichment in shallow waters, evaporation seems to play a minor role. Monte Carlo simulations and sensitivity analysis were used to assess the health risks associated with elevated F- and NO3- concentrations. Mapping-related spatial distribution analysis identified regional contamination hotspots using a global Moran's I and GIS tools. One fluoride and three nitrate contamination hotspots were identified at a p-value of 0.05. Groundwater chemistry revealed that 88 % of groundwater being consumed exceeded the permissible levels for fluoride and nitrate, posing potential health risks, particularly for teenagers and children. This study pinpoints specific areas with excessive nitrate and fluoride contamination, highlighting a high non-carcinogenic risk.

Tracing sulfate sources and transformations of surface water using multiple isotopes in a mining-rural-urban agglomeration area

Rapid urbanization and mining activities are exacerbating sulfate (SO42-) pollution in surface water, and the information on its sources and transformations is crucial for understanding the sulphur cycle in mining areas. In this study, the SO42- in the surface water of Huaibei mining area were monitored and the main sources of pollution and biogeochemical processes were identified using stable isotopes (δD, δ18O-H2O, δ34S-SO42- and δ18O-SO42-) and water chemistry. The results demonstrated the SO42- content in the Huihe River and Linhuan subsidence water area (SWA) is higher than that in other rivers and SWAs, which exceeded the environmental quality standard of surface water. The SO42- content of different rivers and SWAs showed seasonal differences, and the dry season was higher than the wet season. In addition, the SO42- in Tuohe River and Suihe River is primarily caused by urban sewage and agriculture activities, while in Zhonghu and Shuoxihu SWA is mainly contributed by natural evaporate dissolution. Notably, the input of SO42- in the Huihe River and Linhuan SWA caused by mining activities cannot be disregarded. The aerobic environment and isotopic fractionation of surface water indicate that sulfide oxidation is not the major cause of SO42- formation. This work has revealed the multiple sources and transformation mechanisms of SO42-, and provided a reference for the development of comprehensive management and effective remediation strategies of SO42- contamination in surface water around mining areas.

Source identification and potential health risks of fluoride and nitrate in groundwater of a typical alluvial plain

A comprehensive understanding of the role of natural and anthropogenic factors in groundwater pollution is essential for sustainable groundwater resource management, especially in alluvial plains with intensive anthropogenic activities. Numerous studies have focused on the contribution of individual factors on groundwater pollution in alluvial aquifers, but distinguishing the effects of natural and anthropogenic factors is limited. In this study, 64 wells were sampled in different seasons from the Yellow River alluvial plain in China for hydrochemical and isotopic analysis to investigate the spatiotemporal distribution, sources and health risks of fluoride and nitrate in alluvial aquifers. Results showed that fluoride contamination was widely distributed without significant seasonal variation, and 78.1 % of the dry season samples and 65.6 % of the wet season samples showed fluoride concentrations above the permissible limit (1.5 mg/L). High-F- groundwater was generally accompanied by Na-HCO3 and Na-HCO3·SO4 water types. Fluoride was from a natural origin mainly associated with mineral dissolution, competitive adsorption, cation exchange, and evaporation. Groundwater nitrate contamination was spatially sporadic and showed significant seasonal differences. Only 13.6 % of the dry season samples and 3.2 % of the wet season samples had NO3- concentrations exceeded the permissible limit of 50 mg/L. The hydrochemical phase evolved from bicarbonate or sulfate type to chloride type with increasing nitrate concentration. Manure and sewage attributed to agricultural activities contributed the most nitrogen to groundwater, followed by soil organic nitrogen and chemical fertilizers, revealing the anthropogenic origin of nitrate. Nitrification was the dominant nitrogen transformation process in the wet season, and denitrification was prevalent in the dry season. Oral ingestion of high fluoride groundwater was a major threat to human health, especially for infants. This study provided a significant reference for water resources management in alluvial aquifers.

Hydrochemical and isotopic fingerprints of groundwater origin and evolution in the Urangulan River basin, China's Loess Plateau

The origin and evolution of groundwater in the Urangulan River basin area under growing concern as its situated in an economically and ecologically crucial area of China. In the present study, a combination of different methods (i.e. self-organizing maps (SOM), piper diagrams, ionic ratios, multiple isotopic analyses and Bayesian isotope mixing model) provided an efficient way for analysing groundwater origin and evolution. The hydrochemical type was found to be Ca-HCO₃ in low TDS and Na + K-Cl or Na + K-SO₄ in high TDS groundwater. According to the δ²H and δ¹⁸O_water values, groundwater in the study area mainly originated from atmospheric precipitation and was influenced by evaporation. In addition, the rock weathering in conjunction with the cation exchange completely dominated the geochemical evolution process. The dual SO₄^2- isotope and Bayesian isotope mixing model showed that gypsum dissolution, fertilizer input and sewage input were the main sources of SO₄^2- in the study area, accounting for an average of 30.2 %, 28.5 %, and 17.3 % of SO₄^2- in the groundwater, respectively. Other than water-rock interactions, human activity (mining and irrigation) distributed throughout the study area in combination with the spatial characteristics was the dominant factor controlling the hydrochemical evolution. The results of this study provided a basis for understanding groundwater origin and evolution while facilitating the effective management and utilization of groundwater.

Hydrogeochemical and isotopic characterization of the Gioia Tauro coastal Plain (Calabria - southern Italy): A multidisciplinary approach for a focused management of vulnerable strategic systems

This work pursues the hydro-geochemical and isotopic characterization of the complex groundwater system of the Gioia Tauro Plain, one of the most important industrialized and agricultural coastal areas of southern Italy. The anthropic pressure exposes the water resources at risk of depletion and quality degradation making the plain groundwater a system of high scientific and social interest. The plain is characterized by a shallow aquifer, mostly recharged by local rains and a deep aquifer apparently less influenced by local precipitation. Both aquifers are mainly Ca-HCO₃ waters except for localized sectors where Na-HCO₃, Na-Cl and Ca-SO₄ waters are present. In deep aquifer, both prolonged interaction with sedimentary rocks, mainly deriving from the erosion of crystalline rocks, and direct cation exchange represent the primary factors controlling the formation of Na-HCO₃ waters. Mixing processes between these waters and either connate brine and/or deep thermal waters contribute to the formation of isolated high salinity Na-Cl-rich waters. In shallow aquifer, inputs of N-rich sewage and agriculture-related contaminants, and SOx emissions in proximity of the harbor are responsible of the increasing nitrate and sulphate concentrations, respectively. The Cl/Br and NO₃/Cl ratios highlight contamination mainly linked to agricultural activities and contribution of wastewater. Along the northern boundary, the warmest groundwater (Na-Cl[SO₄]) were found close to a bend of the main strike-slip fault system, locally favouring the rising of B- and Li-rich deep waters, testifying the influence of geological-structural features on deep water circulation. Despite the high-water demand, a direct marine intrusion is localized in a very restricted area, where we observed an incipient groundwater-seawater mixing (seawater contribution ≤7 %). The qualitative and quantitative conditions of the shallow aquifer still have acceptable levels because of the relatively high recharge inflow. A reliable hydrogeochemical conceptual model, able to explain the compositional variability of the studied waters, is proposed.

Contamination Assessment and Temporal Evolution of Nitrates in the Shallow Aquifer of the Metauro River Plain (Adriatic Sea, Italy) after Remediation Actions

Over the last decades, groundwater resources at global level have suffered a significant deterioration due to nitrate pollution, mainly related to the input of agricultural fertilizers, manure, sewage, and untreated urban and industrial effluents. The most impacted waters are those forming surface and shallow reservoirs, which usually play a key role in supplying waters to civil, agricultural, and industrial activities. The terminal portion of the Metauro River plain, located in central Italy along the Adriatic Sea coastline, hosts a strategic phreatic aquifer that, along with the surface water of the Metauro River, supplies water to the local population (i.e., about 60,000 people). This shallow coastal aquifer experiences a long-lasting story of nitrate contamination since the 1970s when the increase in the use of agricultural fertilizers contributed to very high levels of pollution (NO₃^- > 100 mg/L). This fact prompted the local authorities to carry out remediation actions that involve a pumping system to inject the NO₃^--poor waters from the Metauro River course directly into the shallow aquifer. The present work was aimed at defining the contamination of nitrates in this important water resource. The main geochemical characteristics and the temporal evolution of NO₃^- concentrations (between 2009 and 2020), in the shallow coastal aquifer of the Metauro River plain, were analyzed by means of classical geochemical analyses and multivariate methods accounting for the compositional nature of the data, to assess the efficiency of the in-situ remediation over time.

Application of hydrochemical and multi-isotopic (87Sr/86Sr, δ13C-DIC, δ2H-H2O, δ18O-H2O) tools to determine contamination sources and processes in the Guadalhorce River Basin, southern Spain

The integrated use of multi-isotopic (⁸⁷Sr/⁸⁶Sr, δ¹³C-DIC, δ²H-H₂O, δ¹⁸O-H₂O) and hydrochemical data was applied in the highly anthropized Guadalhorce river basin, southern Spain, to improve the knowledge about water contamination sources and processes and to achieve improved water resource management. The results obtained highlight the importance of the use of isotopes as tracers of pollutants. DIC, δ²H-H₂O, δ¹⁸O-H₂O and δ¹³C-DIC allowed differentiating two water recharge end members: direct rainwater, infiltrated into the upper and lower detritic aquifers of the sub-basins, and the Guadalhorce dam system, which act as a source in some groundwater and surface waters of the lower sub-basin. ⁸⁷Sr/⁸⁶Sr data supported the existing conclusions in relation to pollution sources in the study area. The Triassic basement (evaporites) of the carbonate and detritic aquifers of the basin generally controls the natural ⁸⁷Sr/⁸⁶Sr composition in waters of the upper sub-basin. Only one groundwater sample reflects the influence of a human organic source (sewage) in its composition. On the other hand, mixing of human inorganic (fertilizers and detergents) strontium sources is required to explain the ⁸⁷Sr/⁸⁶Sr contents of the lower sub-basin waters. Discriminating the use of domestic detergents as another anthropogenic source of strontium and sulphate in waters is a novel finding in this research. The conclusions reached can be extrapolated to other anthropized basins.

Geochemical and isotope tracing of groundwater salinity in the coastal Gujarat alluvial plain, India

This study attempts to learn the groundwater salinization mechanism in the large and well-populated coastal Gujarat alluvial plain under growing anthropogenic pressures. For this purpose, geochemical compositions and ⁸⁷Sr/⁸⁶Sr were analyzed in 53 groundwater samples collected in the three different seasons during 2016-2017. Additionally, surface water samples collected in each season from two nearby large rivers (Narmada and Tapi) and urban sewage discharge of Surat city were also analyzed. Excluding a few salinity hotspots (EC >8-18 mS/cm), the seasonal average of EC (~2.3-2.5 mS/cm) in groundwater shows little fluctuations negating evaporation as the major salinization process. Localized anthropogenic inputs as traced using NO₃ also fail to explain the widespread groundwater salinity. A strong marine influence on the groundwater salinity is revealed by an average Cl/Br of ~655 ± 104 mol/mol in groundwater (except a few outliers) falling close to the seawater composition. The lateral seawater intrusion at shallow depths is generally not supported by the freshening of coastal groundwater samples and generally seaward movements of shallow groundwater as indicated by regional piezometric heads. However, few (Ca)-Mg-Cl type groundwaters collected from north of the Narmada River fall in the piezometric depressions and hint at seawater intrusion and/or upconing of deep saline aquifers. The Sr isotope budget in most of the alluvial groundwater samples is controlled by the seepage groundwater-seawater mixing. Exceptional ⁸⁷Sr/⁸⁶Sr values owing to Sr inputs from different age terrains are found in the northeastern groundwater and a few groundwater samples collected downslope of ancient deposits in the central alluvium. The upconing of salty waters having marine signatures partly altered by aquifer-water interactions primarily governs the regional groundwater salinity, which makes the study area vulnerable to continued salinization by excessive groundwater extraction.