Production of electricity and hot water for district heating by Nesjavellir geothermal power plant in SW-Iceland utilizes high temperature steam, which contains various trace elements. The waste fluid from the plant is either pumped into shallow drill holes that connect to underground water or disposed of in the Nesjavellir stream, which disappears into the lava and finds its way into Lake Thingvallavatn, a rift lake of high conservational value. Here we evaluate data on temperature and quantities of traceelements in the geothermal wastewater discharged from the power plant and at lakeshore springs, and in biological samples of an aquatic plant, a gastropod snail, in the salmonid fish arctic charr and in sediments at Varmagja and at a control site, Vatnskot. Before the wastewater reaches the lake, trace elements are modified through chemical reactions or diluted to such an extent that there is little reason for concern except for arsenic. All trace elements in lake shoreline springs were within the international water quality criteria for protection of aquatic life except for arsenic. Aluminium was also found in concentrations that cause some concern. In most situations wave action ensures efficient mixing of the spring water at geothermally influenced sites with cold lake water precluding detrimental effects of elevated temperature and trace elements. However, following the development of electricity production at Nesjavellir since 1998 the disposal large amounts of 42?C cooling water has caused a rise in temperature at shoreline springs from around 12?C to around 20?C. This is of some concern at sheltered sites. There was no detectable rise or accumulation of trace elements in biological samples taken at Varmagj??, one of the geothermally influenced sites. However, taking into account the conservational value of Lake Thingvallavatn, sound wastewater management by deep re-injection and regular monitoring of trace elements and spring water temperature should be adopted.

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