Rainwater is considered a promising alternative water source for coastal areas where freshwater resources are increasingly under pressure. This study evaluates rain regime characteristics that influence the ability to exploit rainwater in the coastal provinces of the Vietnamese Mekong Delta (VMD). In particular, it considers the impact of climate change and El Niño–Southern Oscillation (ENSO) processes. We analyzed rainfall data from 102 monitoring stations across the VMD from 1989 to 2017. Using statistical methods, we explored (1) characteristics of the rainy season, including the dates of onset and cessation and season length; (2) average rainfall volumes; and (3) the maximum number of consecutive nonrainy days during the rainy season and over the year. We also analyzed changes in these characteristics over time, in parallel with ENSO processes and climate change. Trend lines were determined using nonparametric methods, utilizing Sen’s slope estimation and the Mann–Kendall test. Results showed a tendency for the rainy season to start earlier and end later in the western coastal zone of the study area, with season length gradually decreasing towards the East Sea and inland. The shortest rainy season was found in the estuary zone (in the northeast of the VMD). Rainfall was abundant on the west coast, again diminishing gradually towards the East Sea and inland. Rain was also quite plentiful during four dry season observation months in the study area, but it lacked the predictability needed for effective exploitation. The number of consecutive days without rain averaged 96 annually, with a difference of 29 days between the largest and smallest observations. The difference between the provinces with the longest and shortest periods without rain averaged 41 days. Although the impact of climate change on the rain regime is complex, we can say that the rainy season now tends to start earlier, end later, and be lengthier, though without exhibiting clear trends. ENSO processes significantly impacted rainfall regime characteristics, especially the dates of onset and cessation, and season length.

Groundwater in the Southeastern coastal region of Vietnam is exploited for many purposes, including irrigation, livestock, domestic and production activities. In this study, Groundwater Quality Index calculated by Entropy weight method (EWQI) and Geographic Information System (GIS) are applied for zoning water quality through determining the weight of each parameter in accordance with the variation of each value in terms of. Spatial distribution the suitability of groundwater use in the study area. Groundwater samples were collected from 46 wells in the dry and wet seasons in 2018 for analyzing seven selected water quality parameters, including: pH, total dissolved solids (TDS), total hardness (TH), ammonium (NH₄ ⁺-N), nitrate (NO₃ ⁻-N), sulphate (SO₄ ^2-) and ferrous (Fe²⁺). The groundwater quality is divided into five (05) categories corresponding to Entropy Water Quality Index (EWQI), comprising: excellent, good, medium, poor and extremely poor. The analysis results of Entropy weight indicated that pH, NH₄ ⁺-N, NO₃ ⁻-N are the most affected on the quality of groundwater at the study area. According to EWQI results, more than 41.30% of wells’ water quality are “very good” in both the dry and wet seasons, mainly in the coastal of Ba Ria - Vung Tau province and 13.04% of the total surveyed wells in the study area are “extremely poor”, mainly in Ho Chi Minh city. Accordingly, the zoning map of Entropy weighted Water Quality Index (EWQI), showed that the area with good quality of groundwater accounts for 34.3% and 37.72% of the total study area in the dry and the rainy season, respectively.

Sea-level rise, in the context of climate change, increases the likelihood of seawater intruding into coastal aquifers. This study assesses the vulnerability of the Pleistocene aquifer in the coastal area of Ba Ria-Vung Tau province, Vietnam. Data for calculation and analysis were collected from 99 boreholes in the study area. Using the vulnerability assessment index (GALDIT) with expanded weights, the vulnerability of the aquifer to the influence of coastlines was evaluated and visualized in a GIS environment. The set of Entropy weights used clearly shows the significance of the component parameters and indicates the characteristics of the risk partitioning of aquifer salinization. The Entropy-GALDIT results divided the Pleistocene aquifer in the coastal area of Ba Ria-Vung Tau province into three levels of vulnerability: high vulnerability zones (3.88% of the area), medium vulnerability zones (55.47%), and low vulnerability zones (40.65%). According to the GALDIT susceptibility zoning map, the western area of Phu My town (along the Thi Vai River), the southwest region of Vung Tau City, and the southeast region of Dat Do District are highly sensitive and not recommended for any purpose. This result provides useful insights into the vulnerability of aquifers in the coastal area of Ba Ria-Vung Tau province, with respect to factors such as the height of the groundwater level above sea level, the distance from the shore to the wells, and the impact of existing seawater intrusion. Accordingly, it is necessary to establish monitoring systems to warn of saltwater intrusion and to develop integrated resource management strategies to ensure the sustainability of groundwater resources in the area.

Groundwater in Xuyen Moc district is currently exploited for many purposes such as irrigation, domestic, production and livestock activities. In this study, the health risk assessment (HRA) method was used to determine the risks of public health for local people consuming groundwater for their domestic purposes by using the hazard quotient (HQ). This method calculated HQ of parameters exceeding the allowable limit in drinking water through monitoring data and the survey of households in the study area. Fourteen (14) groundwater samples were collected in the dry and wet seasons in 2017. Eleven (11) water quality parameters (pH, total hardness, TDS, SO₄ ^2-, Cl⁻, NO₃ ⁻, F⁻, Mn²⁺, Fe²⁺, Zn²⁺ and Pb) were selected for analysis. Results of calculation of HQ showed that fluoride is a parameter affecting the health of children when using groundwater for a long-term (HQ > 1). HQ values ranged from 1.9 to 2.2 through ingestion of water and HQ through the dermal absorption ranged from 4.710⁻⁴ - 8.610⁻⁴.