Environmental Impact of a Geothermal Power Plant

Abstract

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.

Lithium and rubidium in the Waikato River, New Zealand

“…The average mass flow past Lake Aratiatia, 22.8 g/s, agrees closely with the sum of the Lake Taupo outflow 7.7 g/s plus the estimated discharge from the Wairakei geothermal station, 16.2 g/s (Axtmann 1974). Below Lake Aratiatia the fLi mass flow increases gradually to 29.8 g/s at Lake Maraetai and this is associated with an average increase in river discharge of about 60 m 3 /s.…”

“…The influence of boron on phytoplankton community structure has been reported (Wilkinson 1985). Among the other trace elements added by geothermal fluids to the river, two groups are prominent: the rare alkalis lithium, rubidium, and caesium, and the halides fluoride, bromide, and iodide (Axtmann 1974). The behaviour of the rare alkalis in the Waikato River has not been specifically considered.…”

“…In the first 10 km down stream from the lake, the river receives further geothermal fluids from several small streams and from the Wairakei Geothermal Power Station. Axtmann (1974) reviewed this latter discharge, particularly the quantities of various salts added to the river. Further down stream, between the Wairakei power station and the Lake Ohakuri dam (Fig.…”

Dissolved coloured compounds and suspended matter in the waters of the middle Waikato River

“…The impoundments for hydroelectricity generation retain water which encourages algal growth (Coulter 1981), and effluents from geothermal development have dramatically changed the water chemistry (Axtmann 1974). Areas of intensive livestock agriculture contribute large quantities of plant nutrients (Coulter 1981) and population centres of the lower river add their wastes.…”

Inflows of geothermal fluid chemicals to the Waikato River catchment, New Zealand

“…Most of the natural geothermal fluid flows occur at the surface in tributary catchments but sub-surface flows into the river occur from active systems drowned by the formation of the reservoirs. Axtmann (1975) first estimated the discharge of geothermal chemicals from the Wairakei geothermal power station to the Waikato River. Included in his estimates were about 83 200 tonnes per annum (2638 g s" 1 ) of chloride (Cl) and 158 t p.a.…”