Influence of thick karst vadose zone on aquifer recharge in karst formations

Abstract

Vadose zone has been proven as an essential buffer zone for aquifer recharge. However, most previous researches associated with vadose zone influence of groundwater recharge concern granular porous media aquifers. Despite of the massive karst formations on surface of earth, the role of karst vadose zone on affecting recharge to the underneath karst aquifer is largely unknown. The objective of this study is to conceptualize and understand the influence of a thick karst vadose zone on groundwater recharge in a regional karst formation. In this work, an integrated field and numerical investigation on such a thick vadose zone above the regional karst aquifer is conducted, where the field component of the investigation involves survey and observation of hydraulic heads and spring discharge, and the numerical component of the investigation involves coupled unsaturated–saturated modeling exercises. The numerical simulation works are conducted by building the following three models: Model 1, a coupled saturated–unsaturated model; Model 2, a coupled saturated–unsaturated model without preferential infiltration; Model 3, a saturated flow model without vadose zone process. The influence of the vadose zone on groundwater recharge is analyzed by comparing the results of those three models. The comparison results of the spring discharge/groundwater level-time curves show that in karst area with a thick vadose zone like the study area, considering vadose zone will improve the simulation results significantly and the slow (diffusive) flow plays a major role in controlling groundwater recharge. The comparison of the recharge-time curves shows that the thick karst vadose zone has a significant buffer effect on pattern and timing of recharge. Parameters of the vadose zone, which controls the infiltration process, are analyzed and prioritized. Our analysis suggests that the infiltration rate has a great influence on groundwater recharge and the specific yield of the deep seated unconfined aquifer affects not only the amount of recharge but also its time lag.