EXPLORING THE CONTROL OF SUB-AERIAL EXPOSURE HORIZONS ON AQUIFER HYDRODYNAMICS AND FRESH WATER LENS MORPHOLOGY

Karst systems are characteristically heterogeneous, resulting in flow behaviours very different from those of other aquifers. Meaningful representation of scale-dependent diagenetic features in groundwater models of such systems remains a challenge. Here we consider the impact of paleo-exposure horizons (laterally continuous zones of contrasting permeability) on the hydrogeology of an eogenetic coastal carbonate aquifer on North Andros island, The Bahamas. In common with many limestone islands, effective management of groundwater resources is challenged by a combination of a thin freshwater lens (FWL), permeability heterogeneity and conflicting demands for groundwater and aggregate.

The FWL on North Andros is significantly thinner than predicted by the Dupuit-Ghyben-Hertzberg approximation. This has previously been explained by high permeability at depth or by thin but extremely low permeability horizons within the FWL. We have developed a 2D numerical model of density-dependent groundwater flow using TOUGH2, which was parameterised using core logs and data from laboratory and field scale permeability tests. We incorporate the effects of different types of sub-aerial exposure horizon, some cemented and others with high secondary porosity, as well as permeability variation in the intervening carbonates and a karstified fracture zone. Predictions are compared with borehole salinity profiles which predate significant groundwater abstraction.

Near the coast the FWL is locally thinned by a high-permeability fracture zone which parallels the platform margin. Inland the FWL geometry is strongly controlled by diagenetic surfaces, including caliche crusts and evaporitic cements which form baffles to flow, and more permeable horizons due to focused meteoric karstification. In particular, low permeability horizons located considerably deeper than the base of the FWL exert a strong control on groundwater flow patterns and thereby on FWL thickness. These have substantially greater impact than more laterally-extensive alteration horizons within the FWL. Whilst these surfaces are clearly an important control on the hydrogeology of eogenetic aquifers, their properties and lateral continuity remain a significant source of uncertainty in predicting the FWL response to anthropogenic activities.