Abstract:
Coastal karst aquifers have highly variable distributions of porosity and permeability. The ability to assess the volume of aquifer occupied by freshwater in coastal karst aquifers is limited by both the lack of understanding of the effect that regions of cavernous porosity and permeability have on the configuration of the saline-freshwater mixing zone and by the limited knowledge of the location of the cavernous regions. A dual-density ground-water flow and solute transport model was used to explore the effect that the depth, lateral extent, and proximity to the coast of zones of high porosity and permeability has on the configuration of the saline-freshwater mixing zone. These aquifer heterogeneities tend to shift the mixing zone upward relative to the position it would have in aquifers with homogeneous porosity and permeability, Zones of high porosity and permeability located at positions shallow in the aquifer or nearer to the coast had the greatest effect. In fact, for the conditions modeled, position was more important in modifying the configuration of the mixing zone than was changing the ratio of the intrinsic permeability of the cavernous zone to the aquifer matrix from 100 to 1000. Modeling results show that ground-water flow is concentrated into the zones of high porosity and permeability and that flow configuration results in steep salinity gradients with depth, Field observations of the location of the halocline and of step changes in ground-water composition coincident with regions of cavernous porosity in coastal karst aquifers corroborate the model results, In a coastal setting with saline water intruding into an aquifer, the effect of cavernous porosity and associated high permeability is to decrease the volume of aquifer in which freshwater occurs by a greater degree than would occur in an aquifer with homogeneous porosity and permeability