STUDY OF GEOTHERMAL WATER-GROUNDWATER INTERACTION AND EVOLUTION IN THERMAL FIELDS OF COSTA RICA

In spite of several investigations, mobilization and fate of trace elements (As, B, Li, Mn etc.) in geothermal waters are still enigmatic. The hydrogeochemical evolutionary pathway from the deep-seated thermal source to surface water encounters a number of thermodynamic and redox environments. In the present study, we investigated the hydrogeochemistry of natural waters from different stages of this evolutionary path, ranging from geothermal well waters to groundwater in two geothermal fields in northwestern Costa Rica. We tried to decipher the hydrochemical processes and interactions with and within the environments, with special emphasis on As kinematics.

Fifty water samples were collected from 49 different locations in the vicinity of the Miravalles and the Rincon de la Vieja volcanic areas. Water samples were collected using standard procedures from three types of sources: geothermal well fluids, thermal springs, and cold springs representing groundwater. The geothermal reservoir temperature ranged between 229-276°C. The geothermal waters were generally highly mineralized, of Na-Cl type, with slightly alkaline to weakly acidic pH. The thermal springs, with pH in the range 1.97-3.25, can be classified into neutral thermal waters of Na-Ca-HCO₃-Cl type and acidic thermal waters of Na-Ca-SO₄-Cl type. The cold spring waters are circum-neutral with dominant Ca-HCO₃ chemistry.

About 70% of the samples exceeded the WHO limit for safe drinking water for As concentrations. The median concentrations of trace metals (e.g. As and B) decrease from geothermal wells (As: 25.9 mg/L, B: 51.8 mg/L) to thermal springs (As: 0.1 mg/L, B: 1.8 mg/L) to cold springs (As: 0.06 mg/L, B: 1.2 mg/L). Measurements of ORP and other redox parameters indicate increasing oxidizing environment from depth to surface. Hence, it is evident that there are processes affecting As mobilization and fate in solution with mixing and evolution from geothermal sources to groundwater. Further geochemical and thermodynamic studies are being pursued for understanding of the processes.