THE TRANSPORT OF CHERNOBYL-DERIVED RADIOCESIUM THROUGH 2 FRESH-WATER LAKES IN CUMBRIA, UK

The specific activities of Cs-137 and Cs-134 in the waters and sedimen ts of Windermere North Basin (WNB) and Esthwaite Water (EW) in the Eng lish Lake District were determined over an 18-month period, immediatel y following the catastrophic accident at the Chernobyl nuclear reactor . Input of Chernobyl-derived Cs to the lake surfaces (approximately 20 00 Bq Cs-137 m-2) occurred predominantly through direct atmospheric de position over the period 3-20 May, 1986. The initial and highest speci fic activities (C(o)) of Cs-137 were approximately 80 Bq m-3 and appro ximately 390 Bq m-3 in WNB and EW, respectively, reflecting the greate r mean depth of WNB (25.1 m) compared with E W(6.4 m). Of the initial input, an estimated 37-41% and 32% was hydraulically flushed from WNB and EW, respectively, the remainder accumulating in the sediments. Ret ention half-life within the lake waters was 70 days in WNB and 15 days in EW The temporal decline in caesium in surface waters could be mode lled by assuming that either direct adsorption to the sediments or ass ociation with settling particles occurred in conjunction with hydrauli c flushing. Model fits resulted in values of 0.10 +/- 0.05 cm for the boundary layer thickness, and 10(5) litre kg-1 for the partition coeff icients, K(d), in both lakes, indicating that transport by particles m ay be the dominant process. A small fraction (0.25%) of the caesium ac cumulating in the sediment appeared to be remobilized into the overlyi ng waters when they became anoxic. This was the major source of radioc aesium in EW after April 1987 (ca. 1 year after the initial input).