Submarine groundwater discharge at Forsmark, Gulf of Bothnia, provided by Ra isotopes

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, 223Ra, 224Ra, 226Ra, and 228Ra have been complemented by stable δ18O and δ2Η 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, δ2Η and δ18Ο isotopes and salinity. Zonal SGD from deformation zones, expressed as hydraulically conductive fractures, overlain by the BörstilÃ¥sen esker, a strip of conductive gravel sediment, have been traced through offshore 224Raxs and 226Ra trends. On the basis of a 224Raxs mass balance, the SGD flow rate ranges from (5.5 ± 3.0) · 103 m3 d− 1 to (950 ± 520) · 103 m3 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 224Raxs models, it is inferred that the site is influenced by a component of recirculated seawater.