Environmental impact of a geothermal power plant.

The Wairakei plant discharges approximately 6.5 times as much heat, 5.5 times as much water vapor, and 0.5 times as much sulfur, per unit of power produced, as would a modern coal plant in New Zealand. It also contaminates the Waikato River with H(2)S, CO(2), arsenic, and mercury at concentrations that have adverse but not calamitous effects. Designed and built at a time when environmental sensibilities were less acute and geothermal technology was less developed, Wairakei produces an overall environmental impact that would be neither acceptable nor necessary in a new plant. Despite its imperfections, however, the Wairakei plant has been under development or in operation for more than 20 years without presenting any serious environmental problems for the local population. Reinjection of the hot waste water, an as yet unproven procedure for liquid-dominated fields, would reduce the plant's environmental impact sharply. Ground subsidence is not a severe problem at Wairakei but may prove to be one at the nearby Broadlands field. There are several environmental characteristics that are unique to geothermal power: (i) pollutant formation may be independent of the power productionc rate; (ii) effluent pathways may change abruptly; (iii) preoperational testing and wild bores contribute significantly to the overall impact; and (iv) waste water may be discharged at temperatures high enough so that utilization of the waste heat becomes both practical and imperative.