Depth of Greatest Dissolution in Carbonate Aquifers

Substantial permeability due to dissolution is often found deep below the water table in carbonate aquifers, but existing models have provided little explanation for this phenomenon. An analytical model was used to investigate the influence of pertinent factors to understand the depth at which the greatest dissolution will occur. Results show that dissolution close to the water table is favored where flow paths are short and where vertical hydraulic conductivity is low. Dissolution deep below the water table is favored where flow paths are long and where vertical hydraulic conductivity is high. Deep flow is also promoted by low ambient temperatures, high geothermal gradients, high reaction orders, and small decreases in initial fracture aperture as a function of depth. Data from both wells and cave studies have shown that enhanced dissolution frequently occurs along a limited number of bedding planes which are often spaced 10-30 m apart and have been called inception horizons. These horizons are often at cycle boundaries and have contrasting lithology. Impurities adsorbed onto the surface of limestone result in high reaction orders and these favor preferential dissolution at these horizons. The model explains why the greatest dissolution in carbonate aquifers varies with depth and how geothermal heating is a major factor in promoting dissolution deep below the water table in many hydrogeological settings