The evaluation of freshening and salinization in the Luy river coastal aquifers from Southern Central region of Vietnam

With an average annual rainfall of 800-1150 mm/year, the Binh Thuan province is one of the driest places in Vietnam. The quantity and quality of groundwater play a significant role in the agriculture, aquaculture development, and daily life of the local communities. Recently, prolonged droughts combined with sea level rise and over-extraction of groundwater increased dramatically the seawater intrusion process. In 2012, the national center for water resources delineated the seawater intrusion extent in Binh Thuan based on water samples taken from shallow boreholes. The salinity thresholds of 3g/L and 1.5g/L were exceeded in the estuaries of the Luy, Long Song, and Ca Ty rivers. The geochemistry of groundwater in the Luy River catchment was studied to investigate the contamination of the aquifers. From 1991 to 2015, 98 water samples had been taken mostly from the shallow (< 10 m) wells in the area in both dry and rainy seasons. 71% of the water samples were fresh while 21% and 5% were lightly saline and moderately saline respectively. In summer 2020, 110 new water samples from both shallow and deep wells were collected in the Luy river catchment in wells from 3m to 40m. The TDS values are ranging from 105 to 23080 mg/L among which 48% are fresh (mostly at depth < 10 m), 40% slightly saline, 8% moderately saline, and 4% very saline. The samples show that the seawater intrusion expands not only horizontally at shallow depth along the river but also deeper down the aquifer in most of the study area, with some strong variations at short distance, which is confirmed by geophysical data. The chemical composition of water samples was analyzed showing evidence of seawater intrusion, but also the occurrence of freshening processes within the study area. The sediments in the Luy’s river catchment consist of sand, clay, marine fossils, gravel, and pebble and are quite heterogeneously distributed. They also underwent cyclic transgressive and regressive events from the Pleistocene to the Holocene. Therefore, the presence of fossil seawater trapped in the heterogeneous sediments in the Luy’s river delta is a possible hypothesis. Together with the presence of saltwater at larger depths, this points towards a situation more complex than previously thought. Saltwater intrusions are likely not only related to interaction with the river estuary, but also to the presence of fossil saltwater in the aquifer, and groundwater pumping and irrigation practices.