The role of groundwater recharge in groundwater exploitation of the Red river delta plain

The Red river delta plain (RRDP), the second largest delta in Vietnam, is situated in the northern region of the country and spans an area of 21,260 km² and supports a population of over 22.9 million inhabitants. Groundwater extraction primarily occurs from quaternary sedimentary aquifers, with a total discharge of approximately 1.5 million m3/day. However, certain localities, such as Hanoi and Nam Dinh, have exhibited signs of over-exploitation, leading to associated issues such as depletion, land subsidence, saltwater intrusion, and water pollution. Groundwater recharge in the study area primarily originates from various sources including rainfall, irrigation, wastewater, and the river system. The objective of this study is to evaluate the contribution of groundwater recharge to groundwater exploitation reserves within the Red river delta. To achieve this, a three-dimensional (3D) model employing the MODFLOW code was developed and refined through the comparison of modelled and actual groundwater levels within the surveillance network. The refined model’s budget analysis indicated that the recharge of quaternary aquifers by means of rainfall, irrigation, and effluent occurs year-round, with the peak recharge happening during the monsoon season (approximately 68% of rainfall) and the nadir during the arid season (approximately 10% of rainfall). Throughout the monsoon period, the river system predominantly replenishes the quaternary aquifers, contributing approximately 9.51-17.36% to the overall water balance of the aquifers. The influx from fractured aquifers at the fringe of the plain to the quaternary aquifers remains consistently minimal year-round.