Evidence of a water delta18O negative shift driven by intensive deep CO2 upflow at Shiwaga gas field (Rungwe, Tanzania)

Located on the flank of Ngozi volcanoes (Tanzania), the Shiwaga gas field is a spot of intense CO2(g) emanations. Physicochemical measurements on different types of waters (rivers, puddles, and springs) as water and gas sampling were discontinuously performed over 10 years for equilibrated partial CO2 pressure calculations and stable isotopic analyses. The most striking result shows that meteoric H2O and deep originated CO2(g) exchanges are responsible for a negative 18O-shift of the studied waters in relation with waters electrical conductivity, pH, and pCO2eq changes. In spring waters, a maximum shift of -11.2%o in delta18O was observed and pCO2eq values up to 1196 mbar were computed. Although this trend has already been reported around the world, such extended shift is rarely measured and requires an important amount of CO2(g), with a CO2(g)/ H2O ratio up more than 0.5 mol/mol. This approach is useful to better understand the hydro-geochemical processes involved in such environments. Moreover, this study evidences that an inventory as a monitoring of these gas fields are needed for the management of natural hazards and local resource