Evidence of a water δ 18 O negative shift driven by intensive deep CO 2 upflow at Shiwaga gas field (Rungwe, Tanzania)

Located on the flank of Ngozi volcanoes (Tanzania), the Shiwaga gas field is a spot of intense CO 2(g) emanations. Physico-chemical measurements on different types of waters (rivers, puddles, and springs) as water and gas sampling were discontinuously performed over 10 years for equilibrated partial CO 2 pressure calculations and stable isotopic analyses. The most striking result shows that meteoric H 2 O and deep originated CO 2(g) exchanges are responsible for a negative 18 O-shift of the studied waters in relation with waters electrical conductivity, pH, and p CO 2 eq changes. In spring waters, a maximum shift of − 11.2‰ in δ 18 O was observed and p CO 2 eq 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 CO 2(g) , with a CO 2(g) /H 2 O 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 resources.