Wet and dry deposition of Chernobyl releases.

The passage of the Chernobyl plume over the United Kingdom in May 1986 led to the deposition of radionuclides on the ground by wet and dry deposition processes. Here we analyse rainfall during the passage of the plume and the published monitoring data obtained afterwards, and show that levels of deposited 137Cs can be closely related to rainfall intercepting the plume. 137Cs was present in the atmosphere mostly as particulate species with wet deposition mechanisms dominating. In contrast, 131I was present as particulate and vapour phase material, and reported levels on grass and in cow's milk show that both wet and dry deposition mechanisms were important. 131I on grass and in cow's milk therefore shows a different geographic pattern to 137Cs, and is not so closely related to rainfall. We have calculated washout factors for locations where there are data on deposition, rainfall and air concentrations during the passage of the Chernobyl plume. From these factors and interpolated concentrations in the atmosphere, the total deposition of 137Cs has been estimated at each of 4,000 rain gauge stations in the United Kingdom. The results are presented as deposition contours and have been compared with measurements in parts of the country. Estimates of the total deposition of 131I and 137Cs show that less than or equal to 1% of the estimated total releases from Chernobyl were deposited on the United Kingdom.