Application of Gas Chemistry to Geohazard Investigation in the Central and North Rift Geothermal Prospects, Kenya

Abstract

Sampling of vapors from selected fumarolic vents in part of Central and North Rift geothermal prospects in Kenya was carried out with the aim of establishing a baseline for future monitoring of volcanic and/or geothermal activities in these areas. The fumaroles sampled were those that indicated relatively higher mass discharges, suggesting a rapid and direct migration to the surface. The survey program was undertaken towards the end of the dry season (March, 2014), just before the onset of major rains in the region in order to obtain samples least affected by mixing with surface/rain water. Two fumaroles in Silali, four in Paka, and one in Korosi, Chepchuk, Lake Bogoria and Arus were sampled. This paper also incorporates results from earlier surveys in Suswa, Emuruangogolak and Barrier geothermal prospects. The results indicate that fumarole KF-3 in Korosi and SF-6 in Silali experienced severe air contaminations, most probably due to inflow of atmospheric gases in the fumarolic conduits upstream of the sampling point given the relatively low flow rates witnessed. CO2 is the dominant non-condensable gas in the fumarolic samples with H2S being markedly low possibly due to secondary processes like oxidation that affect H2S gas resulting to an increase in the CO2/H2S ratios. Hydrogen is present in varying amounts and notably appears to be unstable in air-contaminated samples. The results further indicate that Silali, Paka and Suswa reservoirs have redox conditions close to those governed by the fayalite-Hematite-Quartz (FHQ) redox buffer, with reservoir temperatures of between 250C and 350C being estimated. Moreover, vapor separation in the upflow of Paka and Suswa seems to have either occurred at high temperatures ranging from 250 to 300C, or that Paka and Suswa geothermal systems are possibly vapor dominated.