Title: Assessment of geothermal wastewater disposal effects. Case studies: Nesjavellir (Iceland) and Olkaria (Kenya) fields

Abstract

Every geothermal installation produces a unique set of chemical effluents, which depend
intricately upon the geochemistry and the exploitation history of the hydrothermal reservoir
and on the individual plant design. Potentially adverse effects of geothermal energy
utilization are chemical, - from gaseous components in steam that are discharged into the
atmosphere and from aqueous components in spent water - and physical in the form of
thermal stress, particularly in aquatic ecosystems. An environmental assessment of potential
effects of current geothermal wastewater disposal options in the exploitation of the
Nesjavellir geothermal field (Iceland) and the eastern sector of the Olkaria geothermal field
(Kenya) is made. At Nesjavellir co-generation power plant hot wastewater from the plant is
either pumped into shallow drill holes or disposed off in the Nesjavellir stream, which
disappears into the lava and finds its way into Lake Thingvallavatn, a rift lake of high
conservational value. Measurement of quantities of chemical constituents in effluents and
lakeshore outflow sites (springs), and in the biota at outflow sites, and at a control site,
Vatnskot, was done. Also an assessment of thermal stress at lake shoreline outflow sites was
done. Recent increase in the volume of wastewater has caused a 15-17/C rise in water
temperature at lake shoreline outflow sites. In calm periods during the ice free period and
when the lake freezes over in the winter this causes temporal thermal stress near the outflow
sites. Concentrations of SiO2, As, Al and B are elevated in geothermal effluents but are
diluted before the wastewater reaches the lake. There is no detectable rise or accumulation
of trace elements in the biota at 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 thermal influence, chemical constituents in
effluents and lake outflow sites water should be adopted. Olkaria East geothermal field is one
of the sectors of the Greater Olkaria geothermal area in the central sector of the Kenya Rift
Valley. The field supports the 45 MWe power station. Monitoring of trace elements in the
wastewater has been an ongoing process as one of the pollution control measures since 1993.
Results indicate that concentration level of most trace elements in wastewater from most
wells is low in relation to plant and animal water quality criteria except for As, Mo and B.
With the reinjection of the wastewater, which has also been an ongoing reservoir
management strategy, any potential ecotoxicological effects that might emanate from
elevated levels would be avoided.