Genesis of Carbon Dioxide and Associated Characteristics of Selected Mofette Springs in the Eastern Mt. Kenya Region

Abstract

The Eastern Mt. Kenya region has a significant number of mofette or cold CO2-rich springs which have not been fully investigated previously. They include Gikumene, Kathathantu, Kiambogo, Mbwinjeru, Mulathankari, Nthungu, Rwarera-A, Rwarera-B, Tharu and Ukuu. This study evaluated the origin of CO2 and geochemistry of these springs in relation to their potential environmental, socio-economic and health impacts. A wide range of instruments were used for the analysis. Non-dispersive infra-red sensor was used for the measurement of ambient CO2 in the air, cavity ring-down spectroscopy for δ 13C and dissolved inorganic carbon determination, potentiometric titration for alkalinity, ion chromatography and graphite furnace atomic absorption spectrometry for major ions and inductively coupled plasmaoptical emission spectrometry for trace elements analysis in water. Host rocks analysis was done by atomic absorption spectrometry, X-ray fluorescence and X-ray diffraction methods. Questionnaires were used to obtain views about the springs from the local community. The springs had average ambient CO2 levels between 3,417 and 4,804 ppm which were far above the reported global atmospheric CO2 value of 410.24 ppm in August 2019. The dissolved inorganic carbon δ 13C ranged from -3.597 to +0.283 ‰, Vienna Pee Dee Belemnite. This indicated that the CO2 was either mantle or deep crust derived probably due to post volcanic degassing with a minor contribution from carbonate minerals. The springs had low temperatures (21.2-29.7 oC) and slightly acidic pH (5.83-6.81). The results indicated high dissolved inorganic carbon (39,184-89,013 μmol/kg), total alkalinity (14,925-61,810 μmol/L) and electrical conductivity (1,221-5,195 μS/cm). The predominant cation in the waters was Na+ at 121-1,273 mg/L which was largely counter-balanced by HCO3 - to form Na-HCO3 water-type. The levels of As, B, Mn, Ni, Se and F ions in some spring waters exceeded the Kenya Bureau of Standards guideline values. Only As and F ions exceeded the World Health Organization limits. Variations of data between the wet and dry seasons were not statistically significant. Essential trace elements such as Cu, Fe, Mn, Mo, Ni and Zn were present in the rocks. Among the toxic elements, As and Pb dominated in the rocks. From these findings, CO2 emissions is evident in Mt. Kenya region based on the high dissolved inorganic carbon in the waters and high levels of ambient CO2 in the air which could influence climate change. There is great socio-economic potential in the region for industrial extraction of CO2 and processing of the mineral waters. However, there is considerable health risk of long-term consumption of the waters contaminated with As which is a known carcinogen.