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

In the shallow eutrophic Lake Vorsee with an organic-rich catchment the ¹³⁷ Cs activity concentration in water samples (n = 250) and different fish species (n = 631) was determined for a period of 30 years starting soon after the Chernobyl fallout. To understand the distribution and migration of ¹³⁷ Cs in the ecosystem of Lake Vorsee, also the ¹³⁷ Cs 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 ¹³⁷ Cs 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 ¹³⁷ Cs in water are T _eff1  = 1.2 a and T _eff2  = 7.2 a; for small cyprinidae T _eff1  = 0.6 a and T _eff2  = 8.9 a; and for pike T _eff1  = 1.4 a and T _eff2  = 12.1 a. The vertical distribution of ¹³⁷ Cs activity concentration in soils was described by a partial differential equation (diffusion constant D = (1.05 ± 1.16) cm ² ⋅a ^-1 and convection speed v = (0.14 ± 0.07) cm⋅a ^-1 ) and its distribution in sediments by a system of two coupled partial differential equations (fixation rate f = 2.0 d ^-1 ; distribution coefficient K _d ^ex = (543 ± 140) L⋅kg ^-1 ; sedimentation rate R _S = (0.030 ± 0.010) g⋅(cm ² ⋅a) ^-1 ; bioturbation constant D _bio , decreasing linearly with depth with D _bioabs = (3.3 ± 1.0) cm ² ⋅a ^-1 at the surface). According to the extended AQUASCOPE model, the following fractions of the total ¹³⁷ Cs 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.
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