Understanding and modelling dissolved gas transport in the bedrock of three Fennoscandian sites

•Gas fluxes at three Fennoscandian sites have been estimated using Fick's law.•Data have been reinterpreted to account for in situ gas production.•A new analytical solution allows different sources of gas to be discriminated.•The new solution has been used to estimate the groundwater residence time at Olkiluoto.

SummaryThe origin and transport of dissolved gases in the geosphere is of interest for assessment studies of nuclear waste repositories. In this paper, we analyse available field measurements of helium, methane and hydrogen at three Fennoscandian sites: Forsmark and Laxemar in Sweden and Olkiluoto in Finland. The field data are interpreted using different analytical models all based on the one-dimensional diffusion equation. The results of the different models provide estimates about the amount of deep gas flux, the in situ production and the groundwater residence time of the considered sites. The computed helium fluxes, which fall within the lower range of crustal degassing fluxes reported by Torgersen (2010), are strictly related with the high tightness of the considered fracture media. The very high estimates of groundwater residence time indicate that, at the considered depths, there are only very few flowing fractures while in the rest of the fractured domain groundwater has been almost motionless during a whole glacial cycle.