Determination of radium isotopes and 222Rn in a groundwater affected coastal area of the Baltic Sea and the underlying sub-sea floor aquifer

All four naturally occurring radium isotopes (223Ra, 224Ra, 226Ra, 228Ra) and 222Rn in the groundwater affected Eckernförder Bay (EB) of the Baltic Sea (Germany) were measured using alpha-spectrometry and liquid scintillation (LS) counting. The applied analytical methods are optimally adapted for extensive field surveillance of short-lived radiotracers. Dispersive physical mixing acting over time scales on the order of days is responsible for the distribution of 223Ra, 224Ra and 222Rn in EB. The distribution of these natural tracers is controlled by the strength of the sedimentary source, the influence of direct groundwater input, the dispersive mixing coefficient in the water column and their radioactive decay. Using a simple one-dimensional transport model, the distribution of 224Ra near the sea floor can be described with a horizontal dispersivity in the range between 100 to 5×101 m2 s−1. From the inventory of 222Rn (72.4±7.4 Bq m−2) in the EB, the groundwater discharge rate is estimated to be <1.7 m3 s−1. In order to balance the inventory of 223Ra (0.52±0.22 Bq m−2) and 224Ra (6.46±2.6 B qm−2) a source other than groundwater seepage has to be responsible for almost all of the 224Ra and 223Ra inventory of the EB. Diffusion from sediments seems to be the major source for short-lived Ra isotopes in the lower water column of EB.