DISSOLUTION AND CAVE FORMATION IN EOGENETIC LIMESTONE AQUIFERS BY SPATIAL HETEROGENEITY IN PCO2 AT WATER TABLES

Abstract:

Dissolution processes are well-understood in limestone that has had its primary porosity occluded during burial and cementation (telogenetic limestone). Less is known about dissolution in unburied limestone with high primary permeability (eogenetic limestone). In telogenetic limestone, dissolution occurs where surface streams follow fractures that provide preferential flow paths through an impermeable aquifer matrix. Initial chemical undersaturation of surface water drives dissolution; caves form by enlargement of fractures. In contrast, eogenetic limestone’s high permeability precludes surface streams. Recharge is thought to be diffuse and water-rock interaction in porous vadose zones to result in chemical equilibration of recharge prior to reaching the water table. Despite these limitations on dissolution, phreatic conduits >100 km long occur in eogenetic limestone, suggesting active dissolution in the phreatic zone. Models of dissolution and cave formation in eogenetic limestone emphasize mixing, with mixing of fresh and saline water thought to be the primary dissolution mechanism. Mixing dissolution models consider pCO₂ to be homogeneously distributed within mixing waters. If this assumption is correct, mixing offers the only plausible dissolution mechanism. Here we report data from San Salvador Island, Bahamas that demonstrates pCO₂ is heterogeneously, rather than homogenously, distributed. The pCO₂ in wells varied from < log -2.0 to > log -1.0 atm over distances of less than 30 m. We hypothesize spatial variability in pCO₂ results from spatial variability in inputs of DOC to the water table; dissolution is focused to form caves where water flows from regions of low DOC inputs to regions of high DOC inputs. Oxidation of DOC to CO₂, enables water to dissolve 2.5 to 10 times more calcite than the maximum amount possible by mixing. Heterogeneous inputs of DOC to aquifers may be the dominant dissolution and cave formation mechanism in eogenetic karst aquifers.