[A-HW28] Hydrology and Water Environment

[AHW28-P07] The impacts of unsaturated zone geological materials on the relationship between rainfall and groundwater level

*Tzu-Kuan Chang¹, Jui-Sheng Cheng¹ (1.Graduate Institute of Applied Geology, National Central University, Taiwan)

Keywords:Rainfall, Groundwater level, Material, Hydraulic conductivity, Porosity, Storativity

Precipitation first enters the unsaturated zone due to the force of gravity and then infiltrates downwards through the pores to reach the shallow unconfined saturated zone, causing the groundwater level to rise. During this process, the movement of water is influenced by the permeability of the unsaturated zone materials. In Taoyuan, the unsaturated zone often contains a lateritic gravel layer, primarily composed of unconsolidated gravel mixed with sand or silt-sized convex bodies. The layer has a dense texture and poor permeability, forming a barrier to rainfall infiltration and affecting the groundwater recharge process. However, our research in Longtan District of Taoyuan City found that rainfall has an immediate effect on the local groundwater level, which is significantly different from the response observed in other areas. It is currently speculated that the difference in recharge is related to the presence or thickness of the lateritic gravel layer. Therefore, this study aims to use THMC software to establish multiple 2D vertical models with different material compositions and varying rainfall intensities, analyzing the differences in rainfall recharge and groundwater level changes under the presence or absence of lateritic gravel, and exploring the role of lateritic gravel in hindering the infiltration process. The results show that, due to its hydraulic conductivity being much lower than other geological materials, the presence of lateritic gravel indeed increases the difficulty of rainfall infiltration.