The transport and mass balance of fallout radionuclides in Brotherswater, Cumbria (UK)

This paper investigates the role of intervening transport processes on lake sediment records of the atmospherically deposited radionuclides Pb-210 and Cs-137. Brotherswater is of particular interest to this issue in that its large catchment/lake area ratio and short water residence time are likely to amplify the influence of these processes, both from the catchment and through the water column. Brotherswater is also unique in being the site of two earlier multicore studies that, together with the present study, span a period of 4 decades. Measurements of fallout radionuclides were made on soil cores, suspended sediments and sediment cores, and the results combined with those from earlier studies to construct mass balances for Pb-210 and Cs-137 in Brotherswater. The results showed that catchment inputs accounted for 63% of Pb-210 entering the lake. Further, just 47% of Pb-210 entering the water column was delivered to the sediment record. For comparison, in an earlier study at nearby Blelham Tarn with a relatively smaller catchment but longer water residence time it was shown that 47% of Pb-210 inputs were delivered via the catchment, 75% of which were delivered to the sediment record. Results from both sites suggest that Pb-210 is predominantly transported on fine particulates with a mean particle size of 3-4 mu m. Their relatively slow removal from the water column allows them to be transported relatively uniformly throughout the lake and may help account for the fact that simple Pb-210 dating models are relatively reliable in spite of the complexities of the transport processes. Mass balance calculations for Cs-137 are more complicated because of the variable fallout record. Measurements of Cs-137 in the input stream and water column showed that catchment inputs are still significant 30 years after the last significant fallout (Chernobyl). Modelled results showed that catchment inputs delayed the date of peak inputs of weapons test fallout to the lake though by no more than 2 years. Although the results presented here are primarily concerned with fallout radionuclides and their reliability for dating, they also have implications for the use of sediment archives in reconstructing historical records of other atmospherically deposited substances such as trace metals or persistent organic pollutants.