| Abstract |
Geoscientific investigations on the quality of groundwater in the Dumaguete aquifer reveal that it is influenced by the natural recharge of hot springs from the outflow of the geothermal system. Although the concentrations of the major ions are still within the drinking water standard, mixing of this hot spring water and meteoric water results to three types of groundwater in the study area, which are: (1) Na+K-Cl+SO4 waters, (2) Ca+Mg-HCO3+CO3 waters and (3) Ca+Mg-Cl+SO4 waters. Typical deep groundwater are found in the area of well 46 in the south, relatively Cl-rich waters are within well 54 and the rest of the wells to the northeast are discharging mixed hot spring and typical groundwater. Chemical simulation indicates that Okoy River contributes to the deep groundwater aquifer in the area of wells 49, 53, 54 and 55, and significant recharge comes from the hot spring and deep groundwater. Apparently, local precipitation does not seem to significantly contribute to the deep and shallow groundwater aquifers since recharge mainly comes from elevation between 1100 and 1300 masl. The different water types are also distinguished based on their isotopic composition, and the hot spring water appears to influence the quality of water in wells 49 and 54, while both wells 53 and 55 are being recharged dominantly by meteoric and river water, respectively, with only minor recharge from the hot spring. Moreover, isotopic data indicate that recharge for both shallow and deep aquifers occur only during the rainy season, with very minimum recharge, if at all, during the dry season. Groundwater dating using chlorofluorocarbon (CFC) provided significant information that further validated isotopic data. Wells in the south in the area of well 46 have older groundwater (>60 years old) than the wells within well 55, which have young groundwater of about ten (10) years old. There is binary mixing of old water and young water in the wells, where about 20% of the water in well 56, for instance, is less than forty (40) years old while the remaining 80% is more than fifty (50) years old. CFC data further confirm that well 55 is influenced by infiltration from both Okoy and Banica Rivers due to its proximity to the river systems. |