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

• Published in: Journal of paleolimnology Vol. 29; no. 4; pp. 459 - 473
• Main Authors: Appleby, Pg, Haworth, Ey, Michel, H, Short, Db, Laptev, G, Piliposian, Gt
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Nature B.V 01.05.2003
• Subjects: Archives & records; Catchments; Cores; Freshwater; Lake sediments; Lakes; Radioisotopes; Radiometric dating; Spatial distribution; Streams; Transport processes; Water column; British Isles, England, Cumbria, Lake District, Blelham Tarn
• ISSN: 0921-2728; 1573-0417
• DOI: 10.1023/A:1024437426878

Although lake sediment archives are widely used for reconstructing historical records of atmospherically delivered pollutants, the quantitative relationship between fallout levels and their record in the sediments is complex and not well known. The original input signal from the atmosphere can be significantly distorted by mediating transport processes in the catchment, through the water column and within the sediments themselves. Since these processes also affect the fallout radionuclide ^sup 210^Pb commonly used to date sediments, a better understanding of their impact is also important to improving the accuracy and reliability of sediment dating. Blelham Tarn has been the subject of a number of palaeolimnological investigations using radiometric dating techniques since the early 1970s. More recently it was the site of a major study carried out within the EU Transuranics project concerning the long-term fate of fallout radionuclides in catchment/lake systems. This paper reviews the radiometric data from this study and uses the results to determine mass balances for fallout ^sup 210^Pb, ^sup 137^Cs and ^sup 239+240^Pu in Blelham Tarn, and their spatial distribution over the bed of the lake. Atmospheric fluxes were determined by measuring concentrations in rainwater and cumulative inventories in soil cores from non-eroding sites. Sediment records in a grid of 16 cores were used to determine the spatial distribution over the bed of the lake, and net inputs from the catchment. Mass balance calculations indicate that c. 47% of ^sup 210^Pb in the sediments derives from erosive inputs from the catchment. For ^sup 239+240^Pu the figure rises to 61%. Reduced amounts of ^sup 137^Cs in the sediments are attributed to greater losses of this radionuclide from the water column via the outflow due to its greater solubility. Inputs of radionuclides from the catchment are concentrated near one of the major input streams. Away from this part of the lake the sediment record is dominated by direct atmospheric fallout, though the detailed pattern is influenced by sediment focussing. A one parameter catchment/lake transport model is developed that incorporates the assumption that transport rates will decline with time as fallout on the catchment diffuses into the soil and becomes less available for removal. Values of the transport parameter were calculated for ^sup 210^Pb and ^sup 239+240^Pu and found to be comparable. The results suggest that it will take c. 11000 years to remove 50% of ^sup 239+240^Pu from the catchment to the lake.[PUBLICATION ABSTRACT]