Discharge of mercury from the Wairakei geothermal power station to the Waikato River, New Zealand

Abstract

The Wairakei geothermal power station on the west bank of the Waikato River 10 km north of Lake Taupo, New Zealand, discharges a warm mixture of cooling water and geothermal steam condensate to the river. This discharge contains at least two chemical species of mercury (Hg): elemental mercury (Hg0) at a mean concentration of 42 μg m−3; and other unidentified species, not reducible by acidic stannous chloride, at a mean concentration of 79 μg m−3. The annual discharge to the river of total Hg is 46.5 kg (CI95 42.0–51.8) of which 19.3 kg (CI95 18.3–20.0) is Hg0. On average 53 g d−1 of Hg0 is discharged from the Wairakei station to the river and 50 g d−1 of Hg0 flows out of Lake Aratiatia, the reservoir formed for the Aratiatia hydro‐electric power station 3 km down‐stream from the Wairakei station. The residence time of the condensate in Lake Aratiatia varies between 1.5 h at high river 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 Hg0 is lost to the atmosphere. Furthermore, the short residence time of the condensate in the lake probably limits the extent to which other mechanisms can remove Hg0 from the river water. For these reasons it is concluded that most of the Hg0 discharged to the river from the Wairakei station remains as Hg0 in the water flowing out of Lake Aratiatia. Fine‐grained bed sediments in Lake Aratiatia are an insignificant sink for Hg because of their 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 lake.