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 Börstilåsen esker, a strip of conductive gravel sediment, have been traced through offshore ²²⁴Ra_xs and ²²⁶Ra trends. On the basis of a ²²⁴Ra_xs 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 ²²⁴Ra_xs models, it is inferred that the site is influenced by a component of recirculated seawater.