Title: The impacts of geothermal power plant emissions on terrestrial ecosystems in contrasting bio-climatic zones

Abstract

Very little is known on the ecosystem impacts of emissions from geothermal power plants. The
emissions, comprising mainly of non- condensable gases (NCGs) i.e. carbon dioxide, hydrogen
sulphide, methane and trace elements such as arsenic, boron, antimony and mercury, have the
potential to deposit and accumulate in ecosystems. At elevated levels, some NCGs can cause
ecosystem stress, especially H₂S and the trace elements. The aim of this thesis is to assess the
effects of these elements on terrestrial ecosystems around two geothermal areas in contrasting
biomes i.e. Kenya and Iceland.
Dominant plant species around each geothermal study area, Tarchonanthus camphoratus shrub
in Kenya and Racomitrium lanuginosum moss in Iceland, were used as bio-indicators and
concentrations of sulphur, arsenic, boron, antimony and mercury were mapped in their tissues and
soils at increasing distances from the power plants along the prevailing wind direction in field
surveys. Patterns of plant growth and health along the same distances and wind direction gradients
were also studied to assess any potential effects related to the power plants. Controlled
experiments were thereafter carried out on the same plant species to assess in detail the effects
of the most abundant phytotoxic NCG, i.e. H₂S gas, on plant growth and health.

Results of the field surveys and experiments indicated that the main geothermally emitted
component, H₂S gas, deposits and accumulates in plants and soils. The measured trace element
concentrations in plants and soils (from the field surveys): arsenic, boron, antimony and mercury,
did not show strong patterns attributable to the geothermal power plant emissions. Further, results
of the surveys in relation to geothermal power plant emissions showed weak indications of effects
on Tarchonanthus camphoratus shrub growth and health around the Olkaria geothermal power plants in
Kenya, while in Iceland, the growth of Racomitrium lanuginosum moss was reduced around the Hengill
geothermal power plants. Additionally, the experiments showed that, 30 µg/L aqueous H₂S

(10.96 ppm in air) may be a tolerable limit for plants around geothermal
power plants in Kenya and Iceland. These findings serve as important
baseline data toward environmental monitoring and management around
both geothermal power plant areas in Kenya and Iceland; this information
is also of utmost importance in advising the public and decision makers in
Kenya and Iceland on the ecosystem (terrestrial) impacts of geothermal
power plant emissions.