Origin of Carbon dioxide in Selected Mofette Springs in the Eastern Mt. Kenya Region and Associated Characteristics

Abstract

The Eastern Mt. Kenya region has a significant number of mofette springs containing high content of dissolved carbon dioxide. They include Gikumene, Kathathantu, Kiambogo, Mbwinjeru, Mulathankari, Nthungu, Rwarera-A, Rwarera-B, Tharu and Ukuu. The origin of carbon dioxide and their geochemistry has not been understood. This work was aimed at establishing the origin of carbon dioxide and thereof the chemistry of the mofette springs as a significant contributor of carbon dioxide in the environment. The source of carbon dioxide was determined by measuring the δ¹³C for dissolved inorganic carbon using cavity ring-down spectroscopy. The δ¹³C which ranged from -3.394 to +0.283 ‰ Vienna Pee Dee Belemnite was above the mantle signatures of -6 ± 2.5 ‰. This indicated that the CO2 was mantle or deep crust derived probably due to post volcanic degassing with a minor contribution from carbonate minerals. The springs had low temperatures between 21.9-29.7 oC and slightly acidic pH of 5.83-6.57. Dissolved inorganic carbon ranged between 39,184-89,013 μmol/kg, total alkalinity 14,925-61,810 μmol/L and electrical conductivity 1,375-5,195 μS/cm. The predominant cation in the waters was Na+ at 5,261-55,348 μmol/L which was largely counterbalanced by HCO3-. Other ions found in lesser amounts included Mg²⁺, Ca²⁺, K⁺, Cl^- and SO₄^2-. The apparent carbon dioxide degassing phenomenon would be of interest in terms of understanding the geoscience of this region, climate change monitoring and harnessing the potential socio-economic benefits.