Concentrations of selected trace elements and ions in spent geothermal liquid and associated impacts on groundwater, soil and vegetation in the Menengai geothermal field

Abstract

To assess the effect of spent geothermal liquids from discharging wells at the Menengai geothermal field on the environment, the concentrations of Na, Cl, K and F ions and the trace elements B, As, Hg, Cd, Cr and Pb in spent geothermal liquid from six geothermal wells and groundwater from four boreholes were analysed. Soils and vegetation in the vicinity of two of the six geothermal wells were also assessed for the concentrations of the trace elements B, As, Hg, Cd, Cr and Pb. Furthermore, the concentrations in spent liquids and groundwater were compared to the local standards for effluent released into the environment and the World Health Organization (WHO) drinking water standards. The soil and vegetation were compared to a reference site 2.5 km away from the discharging wells and further compared to the Food and Agricultural Organization (FAO) standards.

The results showed concentrations of the ions Na, Cl and F and the trace elements As, Hg, Cr and Pb in the spent geothermal liquid from some the wells above the requirements of the local effluent standards. In groundwater, the fluoride concentration was double the WHO drinking water standard of 1.5 mg/l, which is typical for groundwater in the Kenyan Rift Valley. All the trace element concentrations were below the WHO drinking water standards except for Pb in two boreholes. In soil and vegetation, all the trace element concentrations were within the reference site levels and the FAO standards. To estimate the effect of the spent geothermal liquid on the groundwater, soils and vegetation, separate linear models were run. The model results indicated no contamination of groundwater, surrounding soils and vegetation by spent geothermal liquid. However, there was limited data for this study and therefore more data is needed to evaluate future impacts. More adequate measures on proper management of the spent geothermal liquid need to be emphasised before reinjection. Measures, such as routine maintenance of High Density Polyethylene (HDPE) liners to avoid seepage of the spent geothermal liquid into the environment, should be considered. Moreover, continuous monitoring of ions and trace elements with more ecosystem components such as other dominant vegetation should also be implemented.