Export, not evasion: the fate of groundwater-derived CO2 in a boreal stream

Groundwater is increasingly recognised as a significant source of carbon dioxide (CO₂) to streams. However, the fate of this terrestrial CO₂, whether it is released locally to the atmosphere or transported downstream, remains unclear. This uncertainty stems from the difficulty of quantifying groundwater inflows on a fine spatial and temporal scale. In this study, we examine the fate of groundwater-derived CO₂ along a 400 m reach of a boreal headwater stream by combining high-resolution measurements of groundwater CO₂ inputs, CO₂ evasion, and downstream CO₂ export during the ice-free period (April to September). Groundwater CO₂ inputs exhibited strong spatial heterogeneity, spanning more than two orders of magnitude across gaining stream segments (median: 13 g C m^-2 d^-1, interquartile range (IR): 0.00 – 50 g C m^-2 d^-1). This variability was primarily driven by differences in groundwater inflow rates associated with local catchment characteristics, such as stream slope. Over time, groundwater CO₂ inputs varied by more than one order of magnitude, with pronounced peaks in late April (108 g C m^-2 d^-1, IQR: 58 – 126 g C m^-2 d^-1) and late July (136 g C m^-2 d^-1, IQR: 46 – 175 g C m^-2 d^-1). In contrast, groundwater CO₂ inputs remained consistently low during baseflow conditions in mid-July (10 g C m^-2 d^-1, IQR: 7.1 – 18 g C m^-2 d^-1) and late August (16 g C m^-2 d^-1, IQR: 6.6 – 31 g C m^-2 d^-1). The seasonal variability in groundwater CO₂ inputs was driven by two contrasting mechanisms: a spring peak mainly caused by increased groundwater discharge during snowmelt, despite relatively low CO₂ concentrations in the groundwater, and summer and autumn peaks linked to rainfall events and higher CO₂ concentrations in the groundwater, likely reflecting increased soil respiration. Throughout the study period, the median value of groundwater CO₂ inputs exceeded the median value of CO₂ evasion (3.0 g C m^-2 d^-1, IQR: 1.9 – 3.0 g C m^-2 d^-1) by a factor of 20 and was of the same order of magnitude as downstream CO₂ export (76 g C m^-2 d^-1, IQR: 46 – 300 g C m^-2 d^-1). These results demonstrate that a substantial proportion of the CO₂ derived from groundwater in headwater streams is not immediately emitted, but instead redistributed along the stream network, where it can contribute to downstream emissions or biogeochemical processing. Our findings highlight the need for integrative assessments of CO₂ fluxes, which explicitly account for groundwater inflows, atmospheric emissions, and downstream export, particularly in the context of climate-driven changes to hydrology and terrestrial carbon cycling.