Modeling aquifer-system compaction and predicting land subsidence in central Taiwan

Abstract

Changhua is a county in central Taiwan that will house several major economic development projects. Extracting groundwater has caused large-scale land subsidence in Changhua, with the largest cumulative subsidence being 210cm over 1992–2010. A multi-sensor monitoring system consisting of continuous GPS stations, a leveling network, multi-layer compaction monitoring wells and groundwater wells is deployed to monitor land subsidence and its mechanism in Changhua. Data from the monitoring well CGSG in Dacheng Township of Changhua show a cumulative compaction of 110.6cm over 1997–2010, occurring mainly at two aquifers and one aquitard. A novel combination of GPS and monitoring well data was used to determine the stress–strain relations. The stratum compaction turns from plasticity to elastoplasticity after a long-term compaction at the second aquifer below the surface. Four hydrogeological parameters of the three sediment layers in Dacheng, vertical hydraulic conductivity, elastic skeletal specific storage, inelastic skeletal specific storage, and the initial maximum preconsolidation stress, in the one‐dimensional compaction model COMPAC, are estimated using the genetic algorithm. With the parameters, COMPAC predicts compactions to an accuracy consistent with in situ measurements, and the mean absolute percentage errors of prediction are below 10%. The result provides a key reference for water management in central Taiwan.