Modelling spring discharge and solute transport in conduits by coupling CFPv2 to an epikarst reservoir for a karst aquifer

摘要

Karst aquifers differ from unconsolidated granular aquifers with respect to their groundwater flow behavior because of the dual-permeability characteristics (discrete conduit network coupled to a fissured matrix), the duality of recharge (point vs. diffuse recharge) as well as the duality of flow (laminar vs. turbulent state). In many karst aquifers, the epikarst frequently plays an important role in redistributing precipitation as diffuse recharge into the fissured matrix and as point recharge into the conduit network as well as for the temporal distribution of recharge input. In this paper, a coupled groundwater model CE (CFPv2 + ERCH) is developed, consisting of a combination of a simple reservoir model generating the recharge source function assuming the concept of an epikarst horizon (ERCH) with CFPv2, a discrete conduit-continuum groundwater flow model of the phreatic karst aquifer. The CE model is applied to simulate spring discharge and a tracer breakthrough in a karst aquifer near Guilin City in southwest China. In order to examine the effect of an added epikarst reservoir model, simulations are carried out with CFPv2 but without including ERCH. Simulation results of both, spring hydro-graph and tracer breakthrough, indicate that CFPv2 with ERCH performs much better. A sensitivity analysis of the CE reveals the relative importance of parameters characterizing the epikarst reservoir. Point recharge is a main factor determining the large variation in spring discharge and its proportion to total recharge can reach 58%. These simulation results highlight the importance of the temporal and spatial distribution of recharge to karst spring discharge response. The coupled model provides an effective approach to include the effect of an epikarst zone in CFPv2 and it can easily be applied for the simulation of karst aquifers with similar characteristics as the study site.