Use of CFCs in Groundwater Investigations in Dumaguete City And Initial Application in Philippine Geothermal Fields

J.A. Caranto¹ and L. Han²

¹PNOC-EDC Fort Bonifacio, Makati City, Philippines
²Isotope Hydrology Laboratory, IAEA, Vienna, Austria

Abstract

The use of Chlorofluorocarbon (CFC) tracing technique in Dumaguete City provided significant information on the groundwater flow dynamics of the Okoy and Banica watersheds. Water district wells 54 and 55 are producing groundwater with 11% to 80% “young water”with relative age ranging from 10 to 36 years old. Towards the southeast in the area of well 47, groundwater becomes older than 60 years old. CFC and geochemical data confirmed that the wells within 54 are hydrologically connected to the River waters of both Okoy and Banica while the wells to the south are recharged from the upper slope of the Banica watershed. With very low tritium concentrations in Philippine atmosphere, CFC becomes a useful alternative tracer to determine relative groundwater ages.

Results of the initial application of the CFC technique in the Mt. Apo (MGPF) and Southern Negros Geothermal Production Field (SNGPF) suggest that there are about 7 to 42% recent groundwater recharge into the geothermal reservoirs with relative ages of 28 to 39 years old, respectively. The groundwater in the wells affected by injection fluid returns and cool water inflows were quantified using CFC data, which was correlated with existing geochemical, hydrological and stable isotope data. The use of the technique in geothermal wells, however, should be treated with caution since the geothermal reservoir is subjected to different physical and geochemical processes.