Concentrations of sulphur and trace elements in semi-arid soils and plants in relation to geothermal power plants at Olkaria, Kenya

Exploitation of geothermal energy is considered to have minimal ecological impacts. However, this assumption has not been widely studied. We tested the hypothesis that emitted elements from geothermal power plants would be enriched in both plant tissue and soil close to the power plants with consequences for plant health. The concentrations of sulphur, arsenic, boron, antimony and mercury in the soil and leaves of the dominating shrub, Tarchonanthus camphoratus, were assayed and associated foliar injury and growth traits assessed at variable distances and directions from two geothermal power plants in Kenya, Olkaria I (operated since 1981) and Olkaria II (since 2003). Sulphur concentration in the leaves was elevated close to the power plants and decreased with increasing distance, implying atmospheric input of sulphur to the ecosystem from the power plants. Similar trends were not detected in soil and with the other elements. Our study design did not support the observed higher degree of leaf injury close to the power plants. Similarly, any association of growth traits with distance or location was not detected. The results were compared with data from a reference site well out of the range of element deposition from the power plants. Overall, the levels of sulphur, arsenic, boron and antimony in leaves of T. camphoratus and sulphur, and boron concentration in soil around the Olkaria I and Olkaria II geothermal power plants were higher than at the reference site. Furthermore, the number of healthy leaves per shrub and stem circumference were lower around the power plants than the reference site, while leaf damage and other plant growth traits did not differ. In spite of relatively weak indication of the harmful effects of the geothermal power plants on the dominating shrub species, follow-up experimental studies and studies on more sensitive ecosystem components are recommended to advise existing mitigation measures against chronic exposure from the emitted gases and associated impacts.