| Abstract |
The Waikato River as it flows (152 m3/s annual mean) from Lake Taupo in North Island, New Zealand, to the Tasman Sea receives approximately 270000 t/a of dissolved salts from geothermal fluids including 190 t/a of As, 1400 t/a of B, 710 t/a of Li, 88t/a of Rb and 50 kg/a of Hg, from both natural fluid emissions and the Wairakei Geothermal Power Station. This discharge of Hg has been considered as partly responsible for high concentrations of Hg, commonly exceeding 0.5 mg/kg (wet flesh), in trout from the river. An estimated 1020 kg of Hg is retained in the fine-grained river sediments but this accounts for only a part of the lifetime discharge of 1500 kg from the Wairakei Geothermal Power Station. Elemental mercury entering the Wairakei Station with geothermal steam is only partly oxidised in the turbine condensers and it is postulated that the elemental fraction remaining in the cooling water discharge is lost to the atmosphere and so does not contribute to the load on the river. Mass flow measurements at Aratiatia, the dam immediately downstream from Wairakei, confirms the loss of Wairakei Hg from the river water, but' some of this loss may be deposition into sediment. Loss to the atmosphere, is a mechanism which might be utilised for removing Hg from condensate wastes and so protecting the aquatic environment from geothermal Hg. |