Submarine groundwater discharge (SGD) to the Gulf of Bothnia, Baltic Sea, has been believed to be insignificant from hydrological models, yet geochemical investigations of SGD in this basin are limited. In this study, 223 Ra, 224 Ra, 226 Ra, and 228 Ra have been complemented by stable δ 18 O and δ 2 Η isotopes to characterize and quantify SGD rates from the coast of Forsmark, Sweden to Öregrundsgrepen Strait (Gulf of Bothnia). In shallow bays, SGD has been traced using relations between Ra, δ 2 Η and δ 18 Ο isotopes and salinity. Zonal SGD from deformation zones, expressed as hydraulically conductive fractures, overlain by the Börstilåsen esker, a strip of conductive gravel sediment, have been traced through offshore 224 Ra xs and 226 Ra trends. On the basis of a 224 Ra xs mass balance, the SGD flow rate ranges from (5.5 ± 3.0) • 10 3 m 3 d-1 to (950 ± 520) • 10 3 m 3 d-1. These rates are up to two orders of magnitude higher than those determined from local hydrological models, which consider only the fresh component of SGD. From the divergence between the hydrological and 224 Ra xs models, it is inferred that the site is influenced by a component of recirculated seawater.

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