Groundwater resources in Bac Lieu province, Mekong Delta: current groundwater extraction and related issues

Groundwater in coastal cities is facing with various challenges such as contamination, salinization and land subsidence due to poor management and climate change. This research is aimed to summary groundwater issues such as groundwater quality, salinization and land subsidence in Bac Lieu City, a coastal city of Mekong Delta. A datasets of wells survey, wells logs and groundwater sampling were utilized for the study. The results show that saline groundwaters (TDS> 1.5 g/L) cover 99 and 54% area of shallow aquifers including Holocene, Upper Pleistocene and 48, 93, and 82% area of very deep aquifers like middle Pliocene, lower Pliocene, and upper Miocene, respectively. Fresh groundwater (TDS <1.5 g/L) is mainly found from 75 to 227 m in upper middle Pleistocene, lower Pleistocene and middle Pliocene aquifers. Groundwater extraction amount increases sharply during the last decades which induced groundwater level decline to 15 m below mean sea level (bmsl). The decline of groundwater level is even observed in the non-extracted aquifers suggesting that those aquifers are hydraulically connected. Land subsidence is observed in some groundwater extraction wells. Groundwater chemical data reflects that 73, 53, 28, 21, 18, 16, and 8% of total 120 groundwater samples show the concentrations of hardness, NO3-, Cl-, TDS, Na+, NO2-, and NH4+ are greater than those recommended in Vietnam National Technical Regulation on Domestic Water Quality (RDWQ; QCVN 01-1:2018/BYT) suggesting that groundwater in the area should not be used directly for domestic purposes. The occurrence of high Na+, TDS and Cl- concentrations in some groundwater samples may indicate salinization. The observation of high NO3- is also a strange phenomenon in this area. Finding the sources of NO3 high concentration and salinization are highly recommended for further studies to sustainable practices of groundwater protection and management in the area.