The study on the groundwater flow patterns at the structural boundary between the Douliu Hills and the Changhua Terrace

In plain areas, groundwater is the main source of public water. Understanding groundwater recharge is essential for sustainable groundwater usage. Recharge phenomena mainly occurs in mountainous, foothill, and proximal fan areas. Therefore, it is necessary to investigate the behavior of groundwater recharge and migration from mountain (foothill) to plain (basin). Addressing potential land subsidence concerns, comprehending the interactive hydromechanical behavior resulting from groundwater recharge proves vital. This study aims to investigate this coupled interaction in the Douliu foothill to Choushui River Alluvial Fan. A two-dimensional geological model from Nantou County to Yunlin County across Douliu foothill was construct based on the study results of geophysical survey and literature. The geological model was then transferred to construct a numerical model for simulation. A coupled hydromechanical model with the poroelastic theory was developed in the COMSOL Multiphysics software. Various-complex geological models were considered to discuss their influence in the coupled system. The simulation results identify two primary flow patterns: front slope flow and regional flow from mountain to plain. These flows significantly contribute to upstream recharge, especially during the transition from mountain block to basin-fill. Moreover, the model result highlights natural compaction in unconsolidated sediments driven by gravity, whereas it does not significantly occur in bedrock. The coupled model contributes to the field of engineering geology and expands our understanding of coupled processes in geological formations by enhancing our comprehension of the interaction between hydraulic and mechanical processes. The study results may help the Water Resources Agency to well manage groundwater resource, mitigate land subsidence, and ensure sustainable water supply.