Chemistry of the Rotorua Geothermal Field - Update on Spring and Well Compositions

Abstract

The monitoring undertaken over the last decade and comparison with historical data suggests that, with only a few exceptions, the compositional spring changes appear within natural variability.

The change after the bore closure programme is exemplified by THC Blowout spring (S 952) as well the Kuirau Park Tarewa Springs (S649 to S653). THC Blowout spring reached a stable composition by 1990 and the Tarewa Springs which in 1984 were dry sinter lined craters and recommenced overflowing in 1998. However there is no explanation as to why the Whakarewarewa springs, which became acid features prior to the bore close programme, have not reactivated. Also difficult to explain is the decrease, after an initial increase in activity and discharge, especially around Lake Roto-a-Tamaheke but also elsewhere at Whakarewarewa. After the bore closure programme water levels (pressures) rapidly recovered in the monitor bores. Timing suggests the latter decease in activity may be due to other causes.

The well compositions in the Western and Government Garden areas are still changing, most containing less SO4 and more HCO3. This is what would be expected given an increase in fluid pressures but there has been little increase in the fraction of deep reservoir higher chloride fluids. There is sufficient data to show that the Western wells are related to each other along a dilution mixing trend of cooler fluid, inferred by Stewart et al. (1992) to be high SO4 surface water penetrating down into the reservoir. The influence of the shallow fluid is much less for the Government Garden area wells and the changes here are typically an increase or decrease in the HCO3 fraction with little change in enthalpy. The only Kuirau well sampled, RR913, shows no change in composition since 2002. The geothermal reservoir in these areas could be within the normal range expected relative to the current use of the geothermal resource.