Title: The speciation of trace elements in spent geothermal fluids and implications for environmental health around Olkaria, Kenya

Abstract

The greatest environmental challenge in the Olkaria, Kenya, has been the large volume of spent
geothermal fluids and the constituent chemicals generated during geothermal development. The
potential environmental health risks of trace elements in spent geothermal fluids from Olkaria have
been studied fairly intensively but based on the total recoverable and dissolved concentrations.
However, there has been extensive evidence that neither total nor dissolved aqueous metal
concentrations are good predictors of metal mobility, bioavailability, and toxicity. The current study
is, therefore, an attempt to investigate the distribution of the chemical species of selected toxic trace
elements in spent geothermal fluids and assess the environmental health risks posed on the ecosystems
around Olkaria. The study involved sampling and analysis of the major ions and trace elements in
spent geothermal fluids from 4 discharging and 2 shut-in geothermal wells, outfall fluids from the
Olkaria I geothermal power station, and the Infiltration Lagoons located in the Eastern sector of the
Olkaria. The Ol’Njorowa Gorge thermal spring fluids and Lake Naivasha were used as control
samples. A chemical equilibrium model, EQ3/6 solubility-speciation code, was applied to predict the
probable distribution of the aqueous Al, As, Cd, Pb, Hg, and Zn with inorganic ligands at in-situ pH
and temperatures with the results of analysis of the sampled fluids as input data. The analytical results
reveal that the dissolved toxic trace element concentrations are significantly low (in μg/L) and in
compliance with the Kenyan maximum permissible limits for recreational water and effluent discharge
to terrestrial environment and the Canadian maximum acceptable concentrations for livestock
watering and protection of plants, except for As, Hg, Mo, and F. The speciation modelling with
inorganic ligands predicts occurrence of labile and toxic Cd in all the fluids sampled, Cu, Pb, and Zn
species in acidic to neutral fluids from Olkaria I outfall, shut-in geothermal wells, and the Infiltration
Lagoons and non-toxic aqueous Al, As, and Hg species in all the sampled fluids. The aqueous Cd
concentrations in all fluids were below the detection levels and in all probability low. Therefore, the
model predicted toxic Cd species is of relatively small environmental health concern. The speciation
modelling results confirm that the total or dissolved trace element concentrations adopted in the
national standards and international criteria or guidelines for environmental health quality are not
reliable predictors of trace element toxicity. The results also indicate that the fluids from discharging
geothermal wells are chemically benign. With increased hot fluid re-injection and incorporation of
detoxification and recovery procedures (fluid pH adjustment and use of appropriate organic complexes
or chelating chemicals) and use of constructed wetlands for acidic to neutral fluids from Olkaria I
outfall, shut-in geothermal wells, and the Infiltration Lagoons, the environmental health risks of the
model predicted toxic Cu, Pb, and Zn species and other elements Al, As, Cd, Hg, and Mo in spent
geothermal fluids are made negligible.