Post-depositional Cs-137 mobility in the sediments of three shallow coastal lagoons, SW England

We present Cs-137 profiles for three low lying coastal lagoons in Southwest England that show a decline in activity with sediment depth. Cs-137 inventories are lower than expected by comparison with local reference inventories despite the fact that sampling was undertaken in the deep-water zone of each lake where sediment and Cs-137 focusing would be expected. At all three locations, lake sediment Be-7 and unsupported Pb-210 (Pb-210(un)) inventories are not significantly lower than the local reference inventory. Cs-137 inventories in the study cores range from 38 to 95% of local reference inventories. The standing water level and mud: water interface at two sites are below maximum tide level and, at all three sites, salinity increases significantly in the water columns between low and high tide and in the pore waters of the underlying sediments. We suggest that the difference in hydrostatic pressure between sea level and standing water levels in the lagoons forces salt water up through the sediment column and that monovalent cations (especially Na+ and K+) replace Cs-137 on exchange sites leading to the upward migration and loss of Cs-137. Rising sea levels may therefore contribute to remobilisation and release of Cs-137 to the aquatic environment from the sediments of coastal lagoons.