Long-term distribution and migration of 137Cs in a small lake ecosystem with organic-rich catchment: A case study of Lake Vorsee (Southern Germany).

Abstract

In the shallow eutrophic Lake Vorsee with an organic-rich catchment the 137Cs activity concentration in water samples (n\u202f=\u202f250) and different fish species (n\u202f=\u202f631) was determined for a period of 30 years starting soon after the Chernobyl fallout. To understand the distribution and migration of 137Cs in the ecosystem of Lake Vorsee, also the 137Cs inventory in soils of the catchment area, in sediments, in suspended matter, in water plants, as well as its vertical distribution in soils and sediments were studied. The time dependency of the 137Cs activity concentration in water and fish was modelled with a compartment model (AQUASCOPE) as well as with the sum of two exponential functions. Results for effective half-lives of 137Cs in water are Teff1 \u202f=\u202f1.2 a and Teff2 \u202f=\u202f7.2 a; for small cyprinidae Teff1 \u202f=\u202f0.6 a and Teff2 \u202f=\u202f8.9 a; and for pike Teff1 \u202f=\u202f1.4 a and Teff2 \u202f=\u202f12.1 a. The vertical distribution of 137Cs activity concentration in soils was described by a partial differential equation (diffusion constant D = (1.05\u202f±\u202f1.16) cm2⋅a-1 and convection speed v = (0.14\u202f±\u202f0.07) cm⋅a-1) and its distribution in sediments by a system of two coupled partial differential equations (fixation rate f\u202f=\u202f2.0 d-1; distribution coefficient Kdex = (543\u202f±\u202f140) L⋅kg-1; sedimentation rate RS = (0.030\u202f±\u202f0.010) g⋅(cm2⋅a)-1; bioturbation constant Dbio, decreasing linearly with depth with Dbioabs = (3.3\u202f±\u202f1.0) cm2⋅a-1 at the surface). According to the extended AQUASCOPE model, the following fractions of the total 137Cs inventory were found in the different compartments of the lake ecosystem in 2016: 9.0⋅10-1 in soils of the catchment area, 1.0⋅10-1 in sediments, 5.5⋅10-5 in water, 1.5⋅10-5 in suspended matter, 1.3⋅10-5 in watermilfoil, and 1.6⋅10-6 in fish.