The ebullition of hydrogen, carbon monoxide, methane, carbon dioxide and total gaseous mercury from the Cornwall Area of Concern

This paper reports the first ebullitive fluxes of hydrogen (H2), carbon monoxide (CO), methane (CH4), carbon dioxide (CO2) and total gaseous mercury (TGM) from the Cornwall Area of Concern (CAC). Although sediments were contaminated with mercury, bubbling was a negligible source of mercury for the global atmosphere. Indeed, the average emission of TGM through ebullition was 0.04 pg m−2 h−1. Measurements of H2, CO, CH4 and CO2 trapped gas concentrations and fluxes were used as indicators of diagenesis processes. The CAC represented a significant regional source of CH4 since the estimated ebullitive fluxes (3.5 mg m−2 h−1) were similar to the CH4 emissions measured above typical flooded freshwater wetlands. As molecular diffusion is known as the main pathway of CO2 transport from water to the atmosphere, CO2 ebullitive fluxes were weak (0.39 mg m−2 h−1). Bubbling of CO (1.6 μg m−2 h−1) was 10 folds less important than CO fluxes measured over flooded freshwater wetlands. Finally, H2 emissions (0.001 μg m−2 h−1) were negligible since the level of this trace gas is tightly regulated by microorganisms in anaerobic environments.