Analysis and quantification of groundwater recession characteristics in regolith-bedrock aquifers: a case study in the mid- and upper-Choshuei river basin in central Taiwan

Abstract

Investigating the groundwater recession characteristics and its relation with geological conditions is of critical importance to the estimation of groundwater recharge in regolith-bedrock aquifers, which commonly occur in the mountainous areas of Taiwan. This paper examines the role of geological and hydrogeological factors in controlling rainfall-induced recharge processes. Groundwater and rainfall data were compiled from 29 monitoring wells and rainfall stations situated in the mid- and upper-Choshuei river basin in central Taiwan. In light of analyzing the outcome of the groundwater-level response to rainfall, the relation between the effective rainfall and groundwater-level fluctuation tends to be positive, and rainfall events with high intensity firmly control the groundwater recharge process in Taiwan mountainous areas. Permeable geological units likely dominate the groundwater-level variations. Besides, recession rates in the vertical direction varying with geological conditions were obviously found in each well site. To improve the efficiency of the water table fluctuation (WTF) method on extrapolating recession rates, two rating systems for evaluating hydraulic properties of regolith and fractured bedrock, respectively, were proposed and applied to develop the effective techniques linked to the quantification of recession characteristics varying with depths for each well. By regression analysis, the dependence of the hydraulic property rating system on the recession rate was performed, and the derived regression equation for each well can be used to estimate the recession rate with depths. Finally, the recharge estimation with the above techniques in Taiwan mountainous areas was established, and the present method of estimating recharge was compared with the existing method. The difference between the two methods was obviously found. Thus, recharge estimation in mountainous areas should take account of complex geological conditions.