Desorption and diffusion of episodic pollutants in sediments: a 3-phase model applied to Chernobyl 137Cs

Eight years after the Chernobyl fallout, the distribution of 137Cs observed in the sediment of a small lake (Loppesjön, central Sweden) suggests that most of the 137Cs entering the lake was rapidly deposited in the sediment. However, vertical profiles also showed that 137Cs is not irreversibly bound to particles, but slowly remobilized during the following years. The 137Cs profile at the deepest, usually anoxic site shows both a distinct peak and long tails. This indicates a gradual desorption of 137Cs from the original deposit and a vertical redistribution caused by diffusion in a matrix with different sorption properties. A 3-phase model was developed that describes slowly desorbing binding sites in the initial deposit by an instantaneous equilibration changing slowly over time. The model provides numerical and analytical solutions for complex diffusive decay processes in aquatic sediments.