Temporal changes in the distribution of natural radionuclides along the Cumbrian coast following the reduction of discharges from a phosphoric acid production plant

The concentrations of a range of natural radionuclides (238U, 234U, 232Th, 230Th, 228Th, 210Pb and 210Po) in the marine environment near a phosphoric acid production plant have been monitored over the period 1992–1997. At the beginning of the study period, concentrations of 238U and its daughters were clearly enhanced in mussel, winkle and sediment samples. The highest concentrations of 238U, 230Th, 210Pb and 210Po in the sediment of Whitehaven harbour were 685±17, 1290±35, 140±2 and 7750±200 Bq kg−1, respectively. Since 1992, discharges of these radionuclides have been greatly reduced and levels in the environment have decreased accordingly. The radiation dose to the general public is mainly due to the ingestion of 210Po and to a lesser extent 210Pb in molluscs. The committed effective dose to a group of local seafood consumers via this route has decreased from 1.0 in 1993 to 0.19 mSv yr−1 in 1997, although the latter is still above the expected background level of 0.1 mSv yr−1. The enhanced levels are maintained by the reduced but not insignificant discharges from the plant and, at Whitehaven perhaps, by the continuing presence of phosphate ore in the sediment.