Estimating land subsidence based on hydrogeological and lithological data from 1991-2016 in the Mekong Delta, Vietnam

Abstract

Many deltas in the world are experiencing an increasing vulnerability to geohazards such as flooding, salinization and land subsidence. Land subsidence in the Mekong Delta is a relatively new hazard caused by the extensive groundwater extraction in the delta. Since the exact effect of groundwater on land subsidence is unknown, feasible groundwater policies and proper water management are difficult to achieve in the Mekong Delta. This study provides land subsidence estimates of the Mekong Delta for the years 1991-2016 based on detailed characterization of the subsurface, geotechnical parameterization of the different hydrogeological layers and groundwater extraction data. By integrating lithological & hydrogeological data a detailed database of the subsurface is created. This database is equipped with geotechnical parameters for all the different boreholes in the Mekong Delta. Accompanied with groundwater extraction rates of each aquifer land subsidence rates are estimated for each aquifer and aquitard for each borehole. The result shows an average land subsidence rate of 43.3 cm in 25 years in the Mekong Delta. The hotspots are located in the urban areas of Ca Mau (70 cm) and Soc Trang (50-60 cm) Over time the average land subsidence equals 17,33 cm in 5 years, 26,45 cm in 10 years, 32,87 cm in 15 years and 38,14 cm in 20 years. Primary subsidence causes 30% of the total land subsidence while secondary subsidence causes 70% of the total land subsidence. The results are exceeding previous research due to the high clay fractions in the hydrogeological layers which cause relatively a high amount of creep. Spreadsheet models are developed to estimate land subsidence in different years over 25 years. The spreadsheet model and results in this study contribute to a more thorough understanding of the different parameters and the quantification of the effect of these parameters on land subsidence.