Characteristics of Point Recharge in Karst Aquifers

Nara Somaratne

doi:10.3390/w6092782

Abstract

Karstic groundwater basins are characterized by both point and diffuse recharge. This paper describes the hydrologic characteristics of point recharge and their influence on recharge estimation for four groundwater basins. Point recharge is highly transient and may occur in relatively short-time periods, yet is capable of recharging a large volume of water, even from a single extreme rainfall event. Preferential groundwater flows are observed in karst aquifers with local fresher water pockets of low salinity that develop around point recharge sources. Measurable fresh water plumes develop only when a large quantity of surface water enters the aquifer as a point recharge. In fresh water plumes, the difference in chloride concentrations in diffuse and point recharge zones decreases as the plumes become enriched through mixing. The relative contributions to total recharge from point sources using the measured gap between groundwater and rainwater chloride in the chloride vs. δ18O plot is not necessarily indicative of sinkholes not directly recharging the aquifer. In karst aquifers, recharge estimation methods based on groundwater age distribution; average annual rainfall and basin average chloride in the conventional chloride mass balance (CMB) method are questionable due to theoretical limitations and key assumptions of these methods not being met. In point recharge dominant groundwater basins, application of: watertable fluctuation, numerical groundwater modelling, Darcy flow calculation or water budget methods are more suitable for recharge estimation as they are independent of the particular mode of recharge. The duality of the recharge mechanism in karst aquifers suggests that modification to the CMB method may be required to include both point and diffuse recharge components.

Highlights

IntroductionGroundwater resource assessment is germane to estimation of net recharge to the aquifers
Groundwater resource assessment is germane to estimation of net recharge to the aquifers.A wide range of direct and indirect methods to estimate groundwater recharge, with varying degree of complexity and approximation on the range of spatial and temporal scales is available [1,2,3,4], but difficulties arise in complex karst hydrogeologic settings [5,6]
Point recharge via sinkholes that are directly connected to the watertable plane is usually of surface water quality, as these fluxes escape evapotranspiration

Summary

Introduction

Groundwater resource assessment is germane to estimation of net recharge to the aquifers. A wide range of direct and indirect methods to estimate groundwater recharge, with varying degree of complexity and approximation on the range of spatial and temporal scales is available [1,2,3,4], but difficulties arise in complex karst hydrogeologic settings [5,6]. Aquifer water level response time may differ as the point and diffuse recharge residence times vary. The presence of sinkholes gives a distinct recharge feature to karst systems via the duality of flow regimes, which can be separated into point (shaft and conduit dominated), and diffuse (matrix, meso-pore, and macro-pore dominated) infiltration and recharge

Methods

Results

Conclusion