Abstract. Land subsidence rates of ∼ 1-4 cm yr −1 are measured in the low-lying Vietnamese Mekong Delta. These relatively high subsidence rates are attributed to groundwater extraction, which has increased drastically over the past decades due to growing domestic, agricultural and industrial demands. As a result, hydraulic heads in aquifers are dropping, on average 0.3-0.7 m yr −1 . There is an urgent need to go from measurements to predictions in order to test possible future groundwater management scenarios and to reduce the increase of flood risk, salt water intrusion and, on the longer term, prevent the delta from drowning. In this study, we aim to assess the subsidence potential of the multi-aquifer subsurface of the Mekong delta due to groundwater extraction. The first step is to gain a thorough understanding of the complex sedimentary architecture of the heterogeneous subsurface. Combined with the related geotechnical properties, the subsurface build-up determines the subsidence potential. Here, we present our approach to develop a 3-D geo-hydrological model based on lithological borehole data, geophysical sedimentary properties, palaeogeography and conceptual models of delta evolution.
This study designed and assessed preliminary monitoring networks to fulfill the urgent need for essential information to formulate plans for sustainable groundwater resources development and management in the coastal Tra Vinh province of Vietnam. Groundwater overexploitation has resulted in aquifer depletion and saltwater intrusion, predominantly originating from formation water trapped in clays from past marine transgressions. A limited number of monitoring wells is operated and is insufficient to delineate the cone of depression and fresh/salt water interfaces. Since data from the existing monitoring were insufficient to explore variogram analysis and subsequent Kriging interpolation for the network design, groundwater flow and saltwater transport models were constructed to create groundwater level and salinity distributions. We followed an iterative procedure to select monitoring locations based on the contour maps of groundwater levels and total dissolved solids (TDS) constructed from the groundwater models. Priorities for the locating groundwater level monitoring wells were targeted in areas with cones of depression and boundaries, while priorities for salinity monitoring were given to coastal zone and fresh/saltwater interfaces. The standard deviation of Kriging interpolation error was used to assess the quality of the designed monitoring networks. Once the designed networks are implemented, observed data from the networks will provide valuable information for sustainable groundwater resources management in Tra Vinh province.
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