DISCHARGE OF MERCURY FROM THE WAIRAKEI GEOTHERMAL POWER-STATION TO THE WAIKATO RIVER, NEW-ZEALAND

The Wairakei geothermal power station on the west bank of the Waikato River 10 km north of Lake Taupo, New Zealand, discharges a warm mixtur e of cooling water and geothermal steam condensate to the river. This discharge contains at least two chemical species of mercury (Hg): elem ental mercury (Hg-0) at a mean concentration of 42 mu g m(-3); and oth er unidentified species, not reducible by acidic stannous chloride, at a mean concentration of 79 mu g m(-3). The annual discharge to the ri ver of total Hg is 46.5 kg (CI95 42.0-51.8) of which 19.3 kg (CI95 18. 3-20.0) is Hg-0. On average 53 g d(-1) of Hg-0 is discharged from the Wairakei station to the river and 50 g d(-1) of Hg-0 flows out of Lake Aratiatia, the reservoir formed for the Aratiatia hydro-electric powe r station 3 km down-stream from the Wairakei station. The residence ti me of the condensate in Lake Aratiatia varies between 1.5 h at high ri ver flow and 12 h during winter when water is accumulated in the lake overnight. The warm condensate initially floats on the surface of the river but only a small proportion of the Hg-0 is lost to the atmospher e. Furthermore, the short residence time of the condensate in the lake probably limits the extent to which other mechanisms can remove Hg-0 from the river water. For these reasons it is concluded that most of t he Hg-0 discharged to the river from the Wairakei station remains as H g0 in the water flowing out of Lake Aratiatia. Fine-grained bed sedime nts in Lake Aratiatia are an insignificant sink for Hg because of thei r very small area and relatively low total Hg concentrations. It seems likely, therefore, that the non-reducible forms of Hg discharged with the condensate also remain in the river water and pass out of the lak e.