Pressure changes in rotorua geothermal aquifers, 1982–90

Abstract

Pressure measurements made in the shallow geothermal aquifers at Rotorua during 1982–1987 show a seasonal pressure cycle caused by seasonal changes in withdrawal and not seen in the overlying cold groundwater aquifers. Pressures rose sharply during the Bore Closure Programme in 1987–1988 and subsequently settled in a new steady state with most of the variation caused by rainfall changes. The winter withdrawal before the Bore Closure Programme was about 31 000 tonnes/day, and the summer withdrawal was about 20% less. The Bore Closure Programme reduced winter withdrawal to 11 000 tonnes/day and summer withdrawal was similarly reduced. Reinjection to production depth increased from less than 1000 tonnes/day to about 3500 tonnes/day. The size of the pressure rise during the Bore Closure Programme suggests that only about half the decline in pressure since 1960 can be attributed to withdrawal. A drop in groundwater pressure due to lower rainfall in the 1980s than in the 1960s and early 1970s is the most likely cause of the extra decline in geothermal pressures. All pressure changes in the Rotorua geothermal field are small compared to those in other exploited fields in New Zealand. The maximum estimated pressure drop is 0.5–0.7 bars, or 5–7 metres.