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Groundwater serves as the major source for drinking water and the only fresh water source in the dry season in Ca Mau province, the Vietnamese Mekong Delta. The study is performed to assess groundwater quality in the area. Groundwater Quality Index (GWQI), magnesium hazard (MH), sodium percentage (Na%), sodium adsorption ratio (SAR), permeability index (PI) combined with geostatistical tools were conducted by using groundwater quality data from 400 samples collected from private wells. GWQI values suggest that 7%, 63%, 28%, 2%, 1% of total samples are classified into excellent, good, poor, very poor and unsuitable for drinking water. High Cl and TDS are the main cause of high GWQI values suggesting that saltwater intrusion deteriorates groundwater quality. For irrigation purposes, 4%, 17%, 36%, 43% of total samples classify into Good, Permissible, Doubtful, Unsuitable respectively by Na% indicator; 55.0%, 30.9%, 11.9%, 2.2% of total samples is Excellent, Good, Doubtful and Unsuitable according to SAR, 54% samples are unsuitable by MH index, and all samples are good by PI. This study provides useful information on groundwater quality for various purposes in Ca Mau province, that could help water management and water users to identify the necessary approach for groundwater protection and water-treatment methods before utilising the groundwater source for specific purposes in the study area.

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Saltwater intrusion is widely observed in coastal aquifers because fresh groundwater is over exploited and abstraction wells were installed too close to saltwater intrusion zone. Identification of salt/fresh groundwater interface is important for fresh groundwater protection. This study presents an application of combination groundwater investigation methods including vertical electric sounding (VES), geochemistry and geographic information system (GIS) tools for delineation of salt/fresh groundwater interface in Mekong Delta. Twenty-seven groundwater samples are collected in existing wells for chemical analysis. Vertical electric sounding (VES) method was conducted for 37 sites to delineate freshwater zone (TDS <1g/L) and saline groundwater zone (TDS>1 g/L). Two new wells are also installed after VES measurement for groundwater sampling. Result of geochemical analysis show that TDS values of groundwater samples range from 0.25 g/L to 1.3 g/L and Cl from 8.8 to 758.6 mg/L. Electrical resistivity values range from 2 to 18.4 Ωm. The electrical resistivity values less than 10.4 Ωm is well correlated to saline groundwater. The study shows that even though VES can be an effective tool for identification of fresh/saline groundwater distribution, the method could not identify the saltwater intrusion zone. Combination of geochemical and GIS methods can clearly delineate saltwater intrusion zone. The study results show that fresh groundwater is limited in a small area of upper-middle Pleistocene aquifers. The fresh groundwater is highly affected by salinization. Therefore, the area urgently needs to take proper measures for sustainable groundwater development.