The chemical evolution of ground water and the diagenetic history of the rocks in a modern saline–fresh-water mixing zone were investigated to test the hypothesis that this zone is a chemically active environment in which carbonate mineral diagenesis occurs. The field site was in the unconfined section of the upper Floridan aquifer system in west-central Florida, a regionally extensive hydrostratigraphic unit composed of Eocene–Oligocene limestone and dolomite. The mixing zone, marked by a steep concentration gradient, is >90 m thick at the study site in New Port Richey, Pasco County, Florida. Interpretation of aqueous geochemical data indicates that the coastal zone is most likely a location where three waters of distinct composition mix. A fresh Ca-HCO3 water that derives its composition from calcite dissolution and CO2 influx in the up-gradient recharge area, a Na-Cl water that results from salt-water intrusion, and a Ca-SO4–rich water that upwells subsequent to contact with underlying gypsiferous beds of the Avon Park Formation are likely all converging in this coastal setting. Poor core recovery and an enlarged core hole provide evidence of dissolution. The ground water, however, is oversaturated with respect to calcite; thus, the large-scale dissolution features observed are unrelated to modern-day mixing. The mixing of Ca-SO4–rich ground water in the coastal region led to oversaturation with respect to calcite. At the petrographic scale, there is little evidence of diagenesis related to the modern mixing zone. Calcite crystals from the lower section of the cored interval have isotopic signatures that are different than those from the upper section, suggesting that the latter has undergone more water-rock interactions. Calcium-rich dolomite, which occurred relatively late in the paragenetic sequence, has thin cathodoluminescent zonations but shows no evidence of subsequent alteration. The paragenetic sequence includes marine micritization and cementation; meteoric mineralogic stabilization; dissolution enhancement of porosity; phreatic, equant calcite cements; and echinoderm overgrowths, all typical of the region and not unique to the mixing zone.