RE-EXAMINING THE ROLE OF MIXING DISSOLUTION ON CONDUIT ENLARGEMENT IN COASTAL CARBONATE AQUIFERS

Conduits replace rivers as the dominant link between uplands and estuaries in coastal carbonate landscapes and, similar to rivers, conduits can evolve in size and distribution. Understanding how conduits form, evolve, and effect water compositions along flow paths, however, lags understanding of similar processes in rivers. Conduit formation and evolution has long been considered to relate to dissolution as fresh and salt water mix, which causes undersaturation with respect to carbonate minerals even if both end members are at equilibrium. Evidence for mixing dissolution comes primarily from observations of cave morphology and reactive transport models that assume idealized chemical compositions of fresh and saline end members. Only a few studies have used measured, chemical compositions of mixed water at haloclines to estimate saturation states. Here we report results from water samples collected in depth profiles through the mixing zones of two cenotes and use this data to directly evaluate causes, controls, and extents of dissolution from mixing. Saturation states of waters were controlled primarily by carbon fluxes in karst windows rather than mixing of fresh and salt water, which did not produce undersaturation. The upper portion of the freshwater lens is supersaturated as a result of CO₂ degassing from karst windows, and this supersaturation limits undersaturation when mixed with salt water. Karst windows also allow organic carbon to enter the aquifer where it can accumulate in saline portions of the aquifer. Microbial oxidation of this organic carbon increases pCO₂ and leads to undersaturation in waters that that otherwise have compositions similar to seawater. Increased pCO₂, however, does not lead to undersaturation where saline groundwater has elevated Ca²⁺ concentrations from dissolved gypsum. Our results suggest that dissolution by saline groundwater with high pCO₂ may be the dominant mechanism of conduit enlargement in the Yucatan Peninsula and that mixing is likely to play no, or a subordinate, role. Our results also highlight the importance of considering how carbon fluxes into, from , and through karst windows may impact dissolution in eogenetic karst aquifers.