A regional survey of deposition of Cs-137, based on precipitation

A method to determine the amount of deposition of 137Cs based on precipitation measurements has been tested in the province of Skaene (an area of about 100 x 100 km2) in the southern part of Sweden. By measuring the activity concentration of the precipitation at a single location the deposition over a larger region has been calculated by multiplying the activity concentration with the amount of precipitation obtained from a network of between 113 and 143 precipitation stations. The deposition of 137Cs in the investigated area amounts to about 1-3 kBq/m2 and is almost equally divided between fallout from nuclear weapons tests and from the Chernobyl nuclear accident. Both the pre-Chernobyl and post-Chernobyl depositions have been calculated from the precipitation measurements. The method was validated by measuring the activity concentration of 134Cs and 137CS in soil samples, by comparisons with in situ measurements and by aerial surveys. The soil samples were collected at 16 sites distributed in a grid pattern over the whole of the investigated area and 16 sites distributed in a grid pattern over the city of Lund. Yearly samplings have also been performed between 1988 and 1998 at Blentarp. The agreement between calculated and measured deposition is good for the pre-Chernobyl, as well as, for the post-Chernobyl deposition. It may then be concluded that this method allows a good and detailed knowledge in retrospect of the deposition, as well as, a way to make a good first prognosis about fresh fallout. (au)