Reykjanes Grey Lagoon: Precipitate chemistry and implications for metal rich scale precipitation, submarine hydrothermal vents, and environmental impact
Author: Sarah Kezia Sternbergh
Year: 2016
Supervisor: Vigdís Harðardóttir
The project was conducted in cooperation with ÍSOR.
Abstract
Reykjanes geothermal system is a high temperature, basalt hosted geothermal system with saline geothermal fluid (3.3 wt.%). Geothermal fluid at Reykjanes is between 270 and 340°C, the geothermal fluid is near seawater composition (salinity 3.5 wt.%) with chemical modifications due to water-rock interactions metals have significantly higher concentrations compared to seawater; the fluid is greatly enriched in silica; the concentrations of Ca+2 and K+ are two to four times higher in the reservoir water compared to seawater; and SO42-and Mg have been nearly removed from the reservoir water. The chemistry of the geothermal fluid closely resembles that found in the highest temperature black smoker hydrothermal vent systems. Scales found in the geothermal wells and pipes at the Reykjanes power plant are similar in mineralogy and chemistry to chimney scales found in high temperature black smoker systems. Reykjanes can be considered an on land proxy for basalt hosted submarine hydrothermal venting systems. Some of the Reykjanes geothermal brine is released to the surface after flashing and forms the “Grey Lagoon”. The Grey Lagoon precipitates and brine are formed from waste fluid released from the venthouse.
Considering the formation of brine in the Reykjanes geothermal plant, the Grey Lagoon can be considered a proxy for a low temperature white smoker hydrothermal vent system. White smokers form as the high temperature system moves off the centre of the submarine rift and vent lower temperature hydrothermal fluid at the margins of these high temperature vent systems. The Grey Lagoon precipitates are primarily silica with some metals. Submarine hydrothermal vents have been shown to precipitate silica as high temperature fluids undergo conductive cooling. Grey Lagoon precipitates present an environmental concern as they have high concentrations of several metals including As, Pb, Hg and Zn. The precipitates can be considered toxic waste.
This thesis discusses the evolution of the fluid and precipitate chemistry downstream of the power plant process pipes, how the Grey Lagoon compares to white smoker environments and the environmental implications of the silica precipitates. Changes in the geothermal fluid between the wellhead at surface and release at the Grey Lagoon include decreasing metals and increasing salinity. Changes in precipitate chemistry include decreasing metal concentrations and increasing silica concentrations with distance from the discharge point. The Grey Lagoon fluid is also not sufficiently high in Mg and Ca concentration to form carbonates, which are found in some white smoker environments. The results is that though the Grey Lagoon represents a similar environment, fluid and precipitate chemistry can be related to some white smoker systems but have distinct chemistry due to lack of seawater mixing.