Journal of Cave and Karst Studies, August 1998 • 107 Carol M. Wicks and Joseph W. Troester-Groundwater Geochemistry of Isla de Mona, Puerto Rico. Journal of Cave and Karst Studies 60(2): 107-114. GROUNDWATER GEOCHEMISTRY OF ISLA DE MONA, PUERTO RICO CAROL M. WICKS Department of Geological Sciences, University of Missouri, Columbia, MO 65211 USA JOSEPH W. TROESTER U.S. Geological Survey, GSA Center, 651 Federal Drive, Suite 400-15, Guaynabo, PR, 00965 In this study,we explore the differences between the hydrogeochemical processes observed in a setting that is open to input from the land surface and in a setting that is closed with respect to input from the land surface. The closed setting was a water-filled passage in a cave. Samples of groundwater and of a solid that appeared to be suspended in the relatively fresh region of saline-freshwater mixing zone were collected. The solid was determined to be aragonite. Based on the analyses of the composition and sat- uration state of the groundwater,the mixing of fresh and saline water and precipitation of aragonite are the controlling geochemical processes in this mixing zone. We found no evidence of sulfate reduction. Thus, this mixing zone is similar to that observed in Caleta Xel Ha, Quintana Roo, also a system that is closed with respect to input from the land surface. The open setting was an unconfined aquifer underlying the coastal plain along which four hand-dug wells are located. Two wells are at the downgradient ends of inferred flowpaths and one is along a flow- path. The composition of the groundwater in the downgradient wells is sulfide-rich and brackish. In contrast, at the well located along a flow line, the groundwater is oxygenated and brackish. All ground- water is oversaturated with respect to calcite, aragonite, and dolomite. The composition is attributed to mixing of fresh and saline groundwater, CO2 outgassing, and sulfate reduction. This mixing zone is geo- chemically similar to that observed in blue holes and cenotes. In coastal carbonate aquifers, the saline-freshwater mixing zone is an area of enhanced calcite dissolution, aragonite neo- morphism, and dolomitization (Back et al. 1979; Randazzo & Bloom 1985; Randazzo & Cook 1987; Smart et al. 1988; Budd 1988; Stoessell et al. 1989; Whitaker et al. 1994). The water- rock interactions that occur in the mixing zone can lead to changes in the chemical composition of the groundwater, the mineralogical composition of the bedrock, and the porosity and permeability of the aquifer. Summarizing the geochemi- cal reactions in saline-freshwater mixing zones and their role in the evolution of carbonate-aquifer systems, Hanshaw and Back (1980) conclude that mineral dissolution and precipita- tion in the mixing zone could enhance, or at least redistribute, porosity and permeability. Physical heterogeneities, such as cenotes, blue holes, and caves, are common on the low-lying carbonate islands of the Caribbean. These features can be flooded by fresh, brackish, or saline water. Cenotes and blue holes are collapse depres- sions that are directly open to the earth’s surface, thus rainfall and detritus can enter cenotes and blue holes directly (Mylroie et al. 1995, Ford & Williams 1989). A subaqueous cave pas- sage has a roof and is not as open to the earth’s surface as are cenotes or blue holes. Therefore, subaqueous cave passages probably do not directly receive rainfall or detritus material inputs. In a series of studies of the blue holes of Andros Island, Bahamas, Smart et al. (1988) and Bottrell et al. (1991) demon- strated that pervasive dissolution of carbonate wall rock was driven by mixing of fresh and saline groundwater and by bac- terial processes. Organic matter was observed suspended on the saline-freshwater interface. In detailed studies of the cenotes and blue holes in the Yucatan, Stoessell et al. (1993) found that dissolution was enhanced by bacterial-driven processes. It seems that in cenotes and blue holes that are open to the earth’s surface, bacterial reactions influence processes within the mixing zone. From detailed sampling along a subaqueous cave passage, Back et al. (1986) determined that mixing of fresh and saline groundwater in the Yucatan Peninsula resulted in dissolution of the carbonate bedrock and the development of caves and cres- cent-shaped beaches. Three of the five groundwater samples collected from a cave system were in equilibrium with respect to aragonite (Back et al. 1986). Subsequently, Stoessell et al. (1989) showed that aragonite was dissolved in the Xcaret Cave. In the studies based on the Xcaret Cave, no mention of bacterial-driven processes is given. In subaqueous cave pas- sages, bacterial processes appear to be of minor importance. In our study, we explore the contrasts between the hydro- geochemical processes observed in open and closed settings and seek a better understanding of the controls on the geo- chemical processes that occur in these two distinct settings. In this study, we document the mixing and geochemical process- es that control the composition of groundwater in the saline- freshwater mixing zone on Isla de Mona, Puerto Rico.