Rising carbon dioxide concentrations in Swedish groundwater during 1980−2020

Groundwater is one of the largest continental carbon reservoirs and tightly linked to globally significant carbon fluxes such as uptake on land, evasion from inland waters and delivery to oceans. Despite emerging evidence that these fluxes are sensitive to environmental changes, long-term trends in groundwater carbon dynamics remain widely unknown. Here I show that dissolved inorganic carbon and carbon dioxide concentrations in groundwater have increased on average by 29% and 48%, respectively, across Sweden (55−68°N) during 1980−2020. I attribute these changes mainly to a partial recovery from historic atmospheric sulfate deposition and associated shifts in weathering pathways in acid-sensitive bedrock, but also to enhanced soil respiration as a likely consequence of climate and land use changes. The results highlight previously neglected significant long-term and large-scale dynamics in groundwater carbon cycling and have implications for the pathways and time scales through which carbon is cycled through the land - inland water - ocean continuum. The observed dynamics should be included in carbon cycle models for accurate evaluations and predictions of the effects of environmental changes on regional and global carbon fluxes.