Chemical and isotopic trends in the Southern Negros geothermal field and the shallow groundwater aquifer in Dumaguete City due to long term exploitation

The Southern Negros Geothermal Production Field was put into commercial operation in 1983 initially producing 112.5 MWe and was expanded to the present capacity of 192.5 MWe in 1995. From the start of production until 1988, separated brine is injected to wells located less than 1 km from the production wells. Hydrological connection through permeable structures allowed for the resulted to the immediate return of injected brine to the production wells resulting to thermal declines and consequent reduction in production output. The use of geochemistry and isotopes were invaluable in understanding long term reservoir changes and were used as basis in establishing appropriate strategies. Shift in injection farther from the production wells, controlled injection and prioritization of high enthalpy production wells proved effective to sustain the production capacity of the geothermal field. On the shallow groundwater system of Dumaguete City, the long-term effect of production is not as pronounced compared to the geothermal system. Looking at the interaction of the two systems through the geothermal outflow, mineralized fluids from the natural outflow in Palinpinon extend farther to the east as manifested in the chemistry of wells 55, 49, 53 and 54. Some of the mineralized fluids cross the contact of the volcanic terrane and the Quaternary alluvium, mixes with the groundwater and flows on the subsurface towards the northeast. The mixing of mineralized geothermal fluids in the MDWD wells 55, 49, 53 and 54, possibly through Palinpinon Fault, is a natural occurrence extending from the geothermal outflow in Palinpinon. Further northeast the fluid transport is already governed by convection and dispersion. Long term changes in the shallow groundwater reservoir are controlled only by seasonal variations. (author)