HTO (10 Ci) was injected into well 1R8D on 12 July 2006. Ten production wells were chosen for tracer breakthrough monitoring. These are wells 101, 103, 105D, 109D, 202, 209A, 212, 214, 2R2, 2R3D and 2R4D. Wells 105D, 103 and 109D were selected on the basis of their rapid physicochemical response, employing 1R8D as a reinjection well. Wells with considerable steam fraction (enthalpy of 1800-2700 kJ/kg) were selected deliberately to determine the behaviour of tritium in the vapour phase. However, a few watery wells were also included in the programme to determine tritium’s fractionation between the water and vapour phases.

As a basis for selecting samples from ‘watery’ wells, 1,5-naphthalene disulphonate (NDS) was injected on 14 June 2006, one month prior to the injection of tritium in 1R8D. The watery wells that showed NDS breakthroughs were chosen for tritium analysis (i. e. 2R3D, 2R4D, 202, 214). Moreover, the results of NDS tracer were also used to compare and/or confirm the tritium data collected.

Continuous sampling was conducted for a year in the selected monitor wells. However, analysis was terminated for the wells which did not yield positive tritium returns after six months from injection. Tritium was analysed at the Philippine National Research Institute and at CAIRT, Jakarta, Indonesia. The NDS analysis, on the other hand, was done at the LGPF Geoservices Laboratory using high performance liquid chromatography.

Data reduction, both for tritium and NDS, was achieved using the Anduril 2.3 package specifically designed for radioactive tracers. NDS data were processed using the ICEBOX software package (United Nations University Geothermal Training Programme, 1994) and Anduril 2.3. Results from both software packages revealed almost identical values.