Groundwater is a primary freshwater source for various domestic, industrial and agricultural purposes, especially in coastal regions where there are lacking surface water supply. However, groundwater quality in coastal regions is often threatened by seawater intrusion and contamination due to both anthropogenic activities and natural processes. Therefore, insights into groundwater geochemistry and occurrences are necessary for sustainable groundwater management in coastal regions. The main aim of this study is to investigate the hydrogeochemical characteristics and their influencing factors in a coastal area of the Mekong Delta, Vietnam (MD). A total of 286 groundwater samples were taken from shallow and deep aquifers for analyzing major ions and stable isotopes. The results show that deep groundwater is dominated by Ca-HCO 3 , Ca-Na-HCO 3 , Ca-Mg-Cl, and Na-HCO 3 while shallow groundwater is dominated by the Na-Cl water type. In this region, the main geochemical processes controlling groundwater chemistry are ion exchanges, mineralization and evaporation. Groundwater salinization in coastal aquifers of the Mekong Delta is caused by (1) paleoseawater intrusion and evaporation occurring in the Holocene and Pleistocene aquifers, (2) dissolution of salt sediment/rock and leakage of saline from upper to lower aquifers due to excessive groundwater exploitation and hydraulic connection. High nitrate concentrations in both shallow and deep aquifers are related to human activities. These results imply that groundwater extraction may exacerbate groundwater qualityrelated problems and suitable solutions for sustainable Electronic supplementary material The online version of this article (
To enable sustainable management of groundwater resources, knowledge of dominant hydrogeological processes is fundamental. In this study, stable isotopes of water [δ
18O and δD] and major inorganic ions were used to investigate recharge and groundwater flow processes in a catchment underlain by fractured and faulted hornblende-biotite-gneiss. Spatial and temporal geochemical distributions consistently showed Mg-Ca-HCO 3 dominated water facies. Evaporation was established to be the main process affecting isotopic enrichment in the study area. Stable isotopic and geochemical data revealed that a combination of thin overburden soil of up to 30 mm thickness and presence of fractures seem to enable localized rapid preferential recharge processes of isotopically enriched rainwater in shallow groundwater around the ridge section. However, the thicker overburden soil (up to 3 m) along the valley seems to allow only isotopically depleted large rain events to recharge deeper groundwater. The isotopically enriched small rain events seem to be allowed to evaporate before recharging groundwater in areas with thicker overburden soils. It was further established that the valley section also receives regional lateral groundwater recharge from high altitude areas. Regional groundwater flow system in the northwest-southeast (NW-SE) direction was thus established with local flows confirmed around the ridge section. It was also revealed that Ntcheu Fault acts as a conduit of regional groundwater flow in the NW-SE direction. Inter-aquifer connectivity and surface water and groundwater interaction were construed around the ridge section and around B12 and R8, respectively. The rapid recharge and flow phenomena in this type of geological media make the resource susceptible to pollution and inter-annual climatic variabilities. It is prudent therefore to consider such information when implementing other developmental plans in the catchment.
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