Quantifying CO₂ leak rates in aquatic environments

The Daylesford region of Victoria (Australia), is a region of natural CO₂ seepage. Small bubble streams of CO₂ are released into ephemeral river beds proximal to mineral springs that contain high dissolved CO₂ content. We study four sites of CO₂ degassing to (i) establish the characteristics of CO₂ seepage caused by transport to surface of CO₂-rich water, (ii) provide an estimate of CO₂ flux in the region, and (iii) investigate seasonal effects on CO₂ seepage. We observe that bubbling behavior varies considerably between sites, including the number and distribution of bubbling points, and bubble stream the continuity. Total CO₂ seep rates at each site were low (< 20 kg/d) but varied substantially between different sites. There were no obvious indicators of total emission rate; the bubble density or other characteristics at the highest emission seep were not remarkably different to the smaller seeps. We find that the total CO₂ emission varies inconsistently with season, with some seep rates increasing and other decreasing in the dry season when water levels are lower. We find there are challenges in quantifying the total gas leakage at sites of highly localized and intermittent degassing. Our work has implications for detecting and quantifying leaks from engineered CO₂ storage sites which emerge in aqueous environments, which could be these are marine or terrestrial (lakes or rivers).