Annual cycle of radionuclide contamination on tide-washed pasture in the Mersey Estuary, NW England

Abstract

The radionuclide burden of vegetation comprising a tide-washed pasture at Ince Marsh in the Mersey Estuary, U.K., derives mainly from adhered external particulates originating as suspended sediments in estuarine water. Radionuclide concentrations are dominated by the growth cycle of the vegetation, with the highest winter levels of contamination activity an order of magnitude greater than the lowest levels in mid-summer. A secondary effect due to sediment transfer during periods of severe flooding produces subsidiary features on this dominant seasonal profile. Radionuclide concentrations on vegetation are in the range137Cs=8–191,134Cs=0.3–0.9,241Am=0.6–46,238Pu=0.1–1.5, and239/240Pu=0.8–44 Bq kg−1. These ranges reflect the relative concentrations of radionuclides in estuarine sediment (137Cs=615,241Am=202, and239/240Pu=104 Bq kg−1) rather than the values in filtered estuary water (137Cs=0.4,241Am=0.001, and239/240Pu=0.001 Bq 1−1). Median Kd values for these radionuclide species are Cs=1,400, Am=200,000, and Pu=80,000 1 kg−1).