Cesium kinetics in a montane lake ecosystem.

Abstract

This study was initiated to determine the kinetics of cesium transport in East Twin Lake, a 5-ha, natural semi-drainage lake which lies at an elevation of 2880 m in the north-central Colorado Rockies. One kilogram of 133Cs (as 133CsCl) was introduced into the water of East Twin Lake on 15 September 1970. Samples of water, seston, sediment, amphipods, zooplankton, trout and two species of vegetation were obtained over a 393-day period following the dosing event. Stable cesium was measured by neutron activation analysis. The loss of 133Cs from water occurred in two phases. The rapid phase accounted for the loss of 60% of the cesium from water and had a loss half time of 0.5 days while the slow phase had a loss half time of 130 days. The seston fraction of each water sample contained from 25 to 80% of the 133Cs present in each liter of unfiltered water. Bottom sediments were identified as the major site of deposition of the 133Cs dose. The sediment, at 393 days post-dosing, accounted for 82% of the 133Cs inventory in the lake whereas water and seston accounted for only 3 and 14%, respectively. Amphipods and zooplankton reached equilibrium with the water within about 3 weeks after the 133Cs administration and achieved concentration factors (wet weight) of about 700 and 150, respectively. During mid-winter, however, these organisms showed increased concentrations of 133Cs with concentration factors ranging up to 3700 and 350 for amphipods and zooplankton, respectively. Trout accumulated the 133Cs more slowly than the invertebrates and reached a maximum concentration factor of about 5600 some 260 days following the dosing event. Absolute quantities as small as 10−8 g of 133Cs were detectable. Concentrations of 133Cs and fallout 137Cs in trout muscle were significantly correlated which indicated that the kinetic behavior of the isotopes in this component of the lake was similar. Furthermore, the stable tracer obviates the radiation protection problems that would arise from contaminating a public water with a radiosotope of cesium.