Groundwater discharge contribution to dissolved inorganic carbon and riverine carbon emissions in a subarctic region

Rivers act as a source of CO 2 to the atmosphere and some of the implied inorganic carbon comes from the aquifer-river connectivity through groundwater discharges to surface water. This study aims to quantify groundwater discharge entering the stream and to estimate this external input to the riverine inorganic carbon cycle, as both dissolved inorganic carbon (DIC) and CO 2 in the Matane River (Qc, Canada). Two approaches based on radon ( 222 Rn) mass balance models, DIC, total alkalinity (TA), pH and P CO 2 measurements were developed to quantify groundwater discharges and associated DIC and CO 2 fluxes at a high- (< 1 km) and low- (> 1 km) resolution scales. Groundwater discharges were heterogeneous along the riverbed with mean linear inputs varying from 3.1 to 51.9 m 3 ·day −1 ·m −1 depending on the scale. The associated fluxes of DIC ranged between 7.4 and 132.9 mol·day −1 ·m −1 and corresponded to only less than 13% of the total DIC transported by the river. Regarding CO 2 , however, the contribution of groundwater to CO 2 emission fluxes reached 81% to 287% of the river's internal CO 2 production. Exceeding groundwater-derived CO 2 flux compared to the total CO 2 flux from the river probably highlights the instantaneous degassing of CO 2 as soon as groundwater discharges to the surface water. These results shed light on the key role of groundwater in the riverine inorganic carbon cycle in a subarctic region, and specifically in the CO 2 evasion to the atmosphere. Such quantifications are particularly important in northern systems where important changes in hydroclimatic conditions and terrestrial carbon storage are undergoing and are expected to continue to undergo.