Karst Water Resources (Proceedings of the Ankara - Antalya Symposium, July 1985). IAHSPubl.no. 161 GEOCHEMISTRY OF GROUNDWATER IN THE MIXING ZONE ALONG THE EAST COAST OF MALLORCA, SPAIN Janet S. Herman Department of Environmental Sciences, University of Virginia Charlottesville, VA 22903, USA William Back 432 National Center, U.S. Geological Survey Reston, VA 22092 > USA Luis Pomar Departamento de Geologia, Facultad de Ciencias, Universidad de Palma de Mallorca Palma de Mallorca, SPAIN ABSTRACT Intruding seawater mixes with discharging freshwater in coastal carbonate aquifers creating an extremely active geocnemical environment. The reactivity of the brackwish water results from the nonlinearity of mineral solubility with regard to differences in ionic strength, partial pressure of carbon dioxide, and ionic activities in the endmember solu- tions. The processes of carbonate mineral dissolution, precipitation, and replacement are enhanced in the groundwater mixing zone. Fourteen groundwater samples were collected across the mixing zone along the east coast of Mallorca, Spain. Samples were obtained from caves and wells in both the Upper Miocene reef complex near the coast and the Jurassic limestone in the more mountainous northern part of the island. The samples covered a range in salinities from 1.0 to 35.0 o/oo. The observed chemical character of these water samples is presumably the result of 1) mixing of Na-CI Mediterranean seawater with local fresh Ca-HCQ 3 groundwater, 2) mineral dissolution, precipitation, and replacement, and 3) C0 2 gas exchange. Conserv- ative mixing and chemical reactions were simulated with the geochemical model PHREEQE, a computer program that calculates equilibrium ionic speciation and mass transfer along a reaction pathway. Simple mixing of a fresh groundwater which is supersaturated with respect to calcite, aragonite, and dolomite and Mediterranean seawater which is highly supersaturated with respect to all three minerals generates a solution that is subsaturated with respect to aragonite over a broad range of salinity. In order to simulate observed chemical compos- itions, conservative mixing, dissolution and neomorphism of aragonite, dissolution and precipitation of calcite, C0 2 gas flux, and dolomitization are considered as a function of the salinity of the groundwater in the mixing zone. 467