| Abstract |
Chemical and isotopic data indicate that significant quantity of mineralized thermal fluids are present downstream of the Palinpinon thermal areas that are migrating into the shallow groundwater aquifer. Water district wells located in Camanjac, 3 kms. east from the Palinpinon springs, are tapping fluids that are relatively enriched in Na+K, Cl+SO4, B and Li concentrations. Fluids appear to be diluted towards the Sibulan area as waters become diluted to Ca+Mg-Cl+SO4 type. Shallow water southeast from the Palinpinon springs is composed of typical Ca+Mg-HCO3 groundwater. Isotopically, the shallow groundwater in Camanjac are relatively enriched in Σ18O and Σ2H while the wells southwest of the springs are relatively depleted. Slight variations in the stable isotope composition of the shallow groundwater was observed from 1999 to 2002, but the difference is not as distinct as the variations of the heavy isotopes in rainfall. Isotopic altitude gradient for Σ18O and Σ2H are 0.28 and 2.1ë per 100 meters change in elevation, respectively. These correspond to a calculated recharge elevation of at least 1000masl for the shallow groundwater. Relative age dating using Chlorofluorocarbon (CFC) reveals relative ages from 10 to older than 60 years old, which partly confirms with the previous Tritium age between 50 to 100 years old. Numerical simulation models confirm the migration of the thermal fluids from Palinpinon to the groundwater wells in Camanjac, with no significant seasonal variation in isotope, Cl and SO4 concentration indicating only minor dilution effect from precipitation. Drawdown in the deep geothermal reservoir have induced more than 500 meters of drawdown in the center of the resource but not enough to revert the naturally outflowing fluids into the Palinpinon thermal springs. Hence, there exists continuous natural migration of slightly mineralized geothermal fluids into the shallow groundwater aquifer of Dumaguete City. Even though the chemical species of the groundwater fall within the Philippine Standard Drinking for Water, future expansion of the cityÆs well fields should be diverted away from the path of the migrating diluted mineralized fluids to avert extraction of relatively Cl-rich waters. |