Direct Aquatic Application of Crushed Dolomite Reduces CO2 Evasion in an Acidified River

Acidified rivers may have increased CO₂ emissions because their low pH transforms inorganic carbon in the form of bicarbonate anions to CO₂, which can evade to the atmosphere, thus interrupting the delivery inorganic carbon to the oceans, a key flux in the long-term carbonate silicate cycle. Enhanced weathering (EW) is a carbon dioxide removal (CDR) strategy aiming to increase drawdown of atmospheric CO₂ through accelerated carbonation weathering of crushed minerals with targeted carbonate sequestration in oceanic stores. To date, EW research has been focused on terrestrial application of crushed minerals, and the CDR capability of enhancing weathering via addition of crushed minerals to rivers from lime dosers is essentially unexplored. Lime dosers have been used for decades to directly deposit crushed carbonate rock to rivers as a function of river flow in Norway and Nova Scotia, Canada, yet their potential as a CDR tool has yet to be verified in the field. In this study, we adapt CO₂ flux sensors (eosFD) designed for soils to be deployed in rivers. We conducted field trials on the Killag River, Nova Scotia, upstream and downstream of a lime doser over a period of six weeks in the autumn of 2020. Preliminary analysis shows elevated CO₂ evasion rates upstream of the lime doser and decreased evasion rates downstream. Aside from flood waves, CO₂ evasion at the downstream (treated) site is reduced to almost zero for extended periods of time. Next steps are to identify whether the reduced CO₂ evasion is due to CO₂ drawdown via increased carbonation weathering of the crushed dolomite or through reduced CO₂ evasion due to increased pH, or from a combination of the two processes. The results of this study may have implications for carbon credit programs for acidification mitigation and may encourage more widespread use of enhanced weathering as a CDR tool in rivers.