CARBONATE GEOCHEMISTRY AND THE CONCENTRATIONS OF AQUEOUS MG2+, SR2+ AND CA2+ - WESTERN NORTH COAST OF THE YUCATAN, MEXICO

We have measured concentrations of major cations, Sr2+ and alkalinity in groundwater, pore-water and surface-water samples collected from the western north coast of the Yucatan Peninsula, Mexico. Thermodynamic calculations indicate that almost all water samples from the study area are supersaturated with respect to calcite, aragonite, high-Mg calcite (HMC) and dolomite. Analysis of the data suggests that the geochemical processes controlling Mg2+, Sr2+ and Ca2+ concentrations differ in different environments. Mass-balance modeling has been used to calculate the amount of dissolution and/or precipitation of aragonite, low-Mg calcite (LMC) and HMC (and/or dolomite) required to produce waters with the observed Mg2+, Sr2+ and Ca2+ concentrations. Mass-balance calculations indicate that HMC and aragonite are dissolving, whereas LMC is precipitating from most groundwater and inland surface waters. The chemistry of surface waters from an extensive saline coastal swamp suggests that LMC and aragonite in surficial swamp sediments are converted to HMC or dolomite while swamp pore-water chemistry suggests that HMC is dissolving and LMC and aragonite are precipitating in deeper swamp sediments. The chemistry of pore waters suggests that they are derived from saline surface waters and that fresh to brackish groundwater from the shallow confined aquifer does not flow into swamp sediments.