Modelling CO2 degassing from small acidic rivers using water pCO2, DIC and δ13C-DIC data

Degassing of terrestrially-respired CO 2 from streams and small rivers appears to be a significant component in watershed carbon budgets. Here we propose an original approach to quantify CO 2 degassing in small headwater bodies using pCO 2 , DIC (or total alkalinity, TA) and δ 13 C-DIC data in stream waters that avoids the difficulty of measuring or choosing a gas transfer velocity. Our inversed model applies to acidic, non-buffered (humic-type) waters and relies on two main assumptions, i.e., the stable isotopic composition of DIC in groundwater seeping to surface water (CO 2 from respired soil organic carbon and HCO 3 - from weathering) and on kinetic fractionation at the water–air interface ( 12 CO 2 degases to the atmosphere more rapidly than 13 CO 2 ). We first consider both the soil organic matter isotopic composition and the isotopic fractionation of CO 2 in the soil, to derive the δ 13 C–CO 2 in that soil and groundwater. From the HCO 3 - concentrations in streams, we estimate the relative contribution of silicate and carbonate weathering (the latter being minor in these waters) to the HCO 3 - and its associated isotopic composition. Model calculations start from the δ 13 C-DIC value computed by the aforementioned method and consist of two interlocked iterative procedures. The first procedure simulates the decrease in pCO 2 and the increase in δ 13 C-DIC that occur along the stream watercourse during degassing, starting from an assumed initial soil pCO 2 and ending at the in situ pCO 2 or δ 13 C-DIC. The second iteration procedure consists of adjusting the initial soil pCO 2 until pCO 2 and δ 13 C-DIC simultaneously reach the in situ measured values. After convergence is obtained, the model computes a theoretical concentration of DIC, [DIC] ex. , that has been lost as CO 2 to the atmosphere from the headwater to the sampling point in the river. [DIC] ex. can be multiplied by the river discharge to derive the quantity of carbon degassed from the river surface. The model was tested on seasonal field datasets from three small rivers draining sandy podsols in southern France and gave annual areal degassing rates comparable to those reported in other studies, though somewhat larger (upper half range in two rivers, ∼10 times the average in one stream). Part of this discrepancy might have been caused by an intense degassing in the vicinity of groundwater seeps, which was accounted for our integrative method but not by classical methods based on stream water pCO 2 and gas transfer velocity. The sensitivity of the model results on the assumption of the importance of carbonate weathering might also explain part of this high degassing rate. The model reproduced consistent values and seasonal trends of soil pCO 2 (maximal in summer) and gas transfer velocity (maximal at high water flow). We discuss the sensitivity of the model to the different parameters and assumptions and propose some improvements including groundwater sampling, for better constraining the computed degassing rates.