Geochemical significance of groundwater discharge and carbonate solution to the formation of Caleta Xel Ha, Quintana Roo, Mexico

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Abstract
Xel Ha is a lagoon whose location and morphology are partly controlled by the fracture pattern in upper Pleistocene coral reef limestone. Chemical, isotopic, and hydrologic observations were made nearby where groundwater could be sampled. In addition, similar observations along vertical profiles were made at stations within the lagoon. A thin lens of rapidly moving fresh groundwater overlies a regional thick body of nearly stagnant saline groundwater that is close to ocean water in chemical composition. These two chemically distinct groundwaters are separated by a thin zone of dispersion, and both are at or slightly beyond saturation with respect to calcite. Where these two end‐member groundwaters mix to form the brackish dispersion zone, the resulting solution is undersaturated with respect to calcite. This was confirmed by mass transfer and isotopic modeling which show that the mixing of two solutions of different ionic strength, each saturated with respect to calcite, can result in an undersaturated solution. Model calculations indicate that as much as 1.23 mmol of CaCO3 may be dissolved per kilogram of water. We hypothesize that many of the cuspate beaches along the east coast of the Yucatan formed in this manner and that mixing of waters of different chemistry may be an important geomorphic process in similar geologic‐hydrologic settings.