Assessing the Environmental Impact of Geothermal Power Utilization Using a Multi Isotope Approach (C, N, S, Pb) in Moss (Rhacomitrium Lanuginosum)

Abstract

Conventional geothermal power plants are sources of industrial waste including spent fluids and non-condensable gases. Increasingly spent fluids are being re-injected in to the geothermal systems reducing significantly the environmental impact of power production and sustaining pressure within the system. While efforts are afoot to sequester some of the non-condensable gases (e.g. CO2 and H2S) a significant portion of the gases are currently vented to the atmosphere. Little is known of the effects of long term exposures to low levels of hydrogen sulfide. However, it has been suggested that H2S plays a role in the decline of vegetation around the geothermal power plant in Hellisheiði. In this study we examine the stable isotopic composition of woolly fringe moss (Rhacomitrium lanuginosum) from lava-fields around geothermal power plants in south-west Iceland. A sample suite originally collected for trace element analysis was analyzed for the isotopic composition of sulfur (S), nitrogen (N), carbon (C) and lead (Pb). The isotope ratio of sulfur varies by about 16 per mil and is controlled by binary mixing, with the most probable end members being i) hydrothermal/magmatic (“mantle-derived”) sulfur and ii) marine-derived sulfur. The isotopic composition of N (15N) varies from -10.3 to -6.0 (‰ PDB). The more enriched (less negative -values) samples are more variable in terms of N content and are have comparatively high Pb content. The isotopic composition of C (13C) varies from -26.1 to -23.0 (‰ PDB). The more enriched samples have typically lower C content possibly reflecting addition of dust (carbon free) to the samples. The dust may contribute Pb to the moss samples. The change in discrimination for the heavy isotopes (15N and 13C) may be a result of environmental factors such as reduced precipitation or due to an increased anthropogenic influence. The results do not support the contention that geothermal H2S is responsible for the decline in vegetation around geothermal power plants. Other inputs are suggested by the data, including locally derived bedrock dust (e.g. construction activities), possibly heavy traffic (car exhaust, road dust) or other environmental factors such as reduced precipitation. environmental factors such as reduced precipitation or due to an increased anthropogenic influence. The results do not support the contention that geothermal H2S is responsible for the decline in vegetation around geothermal power plants. Other inputs are suggested by the data, including locally derived bedrock dust (e.g. construction activities), possibly heavy traffic (car exhaust, road dust) or other environmental factors such as reduced precipitation.