Transport mechanisms and rates for long lived Chernobyl deposits in mid-Wales

The deposition and transport of 134Cs and 137Cs derived from Chernobyl within 3 upland catchments in mid-Wales (Upper Severn, Wye and Rheidol) has been examined between 1987-1989. An increase in the pre-existing inventory of 137Cs of 15% was observed. During the study period all fluvial outputs of 134Cs and 137Cs were in association with suspended and bedload sediments with no radiocaesium being detectable in solute form. The major source of 137Cs on suspended sediments was ''old'' 137Cs originating from fallout due to nuclear weapons testing. Mineral magnetic measurements combined with radiometric data suggest that the Chernobyl component in association with suspended sediments originates from top-soil sediment sources whilst the fine particulate material in association with bedload is supplied from subsurface and channel sources. Outputs of Chernobyl-labelled sediments from the catchments indicated by river and lake sampling were low. In 1988 ∼0.08% of the soil inventory of Chernobyl derived radiocaesium was removed from the Wye and Severn catchments via fluvial transport, whilst between January and August 1989 ∼0.04% was removed. Lake sediment inventories in Llyn Llygad Rheidol also suggest that rates of transfer across the upper Rheidol catchment are low. Sampling of the Hafren Forest indicates ∼0.01 GBq of 137Cs and ∼0.006 GBq of 134Cs deposited due to the Chernobyl accident is stored in the canopy of the forest. Losses to the forest floor from a Norway Spruce canopy were 13% and 15% of the activity contained in the overlying canopy in 1988 and 1989 and 16.5% and 9% in 1988 and 1989 under Sitka Spruce. (author)