Groundwater resource challenges and abstraction-induced land subsidence in the Vietnamese Mekong Delta

Groundwater is a crucial source for water supply in the Vietnamese Mekong Delta (VMD). However, its availability and accessibility face many challenges such as population growth and the associated increase in water demand, deterioration of river water quality, climate change (droughts), and sea-level rise. Overexploitation of groundwater is the main cause for groundwater salinization and land subsidence, which is a big environmental issue in the VMD. In this chapter, the present and future status of the VMD groundwater system will be described, and a critical review on groundwater problems and its relation to land subsidence will be discussed based on the literature and collected data. The analysis shows that groundwater abstraction rates have increased and are expected to increase in the future due to higher agricultural and domestic demands, an inadequate water supply system, and salinization of surface water. Most fresh groundwater resources are old and from fossil origin, and once these fresh groundwater resources are depleted, they cannot be replenished easily. Poor management of groundwater resources and the lack of alternative water sources place even more pressure on the groundwater situation. This results in sharply declined groundwater pressure heads, groundwater contaminated by trace metals, arsenic, and saltwater intrusion, and aquifer-system compaction leading to extraction-induced land subsidence. Recent studies suggested that the recovery of the groundwater pressure head by reduced fresh groundwater abstractions and/or aquifer storage and recovery can be the promising approach to partially restore the fresh groundwater resources and reduce extraction-induced land subsidence problems. This chapter also points out that the groundwater management strategy could seriously be improved through sustainable groundwater abstractions in combination with the application of various alternative methods such as conjunctive water use, rainwater harvesting techniques, managed aquifer recharge, and (agricultural) water saving practices.