A MODEL FOR THE BEHAVIOR OF CS-137 IN WATERSHED SCENARIOS

A mathematical model of radiocesium transfer in a lake and its catchme nt is presented in an attempt to improve previous approaches. The expe rimental data base of the Esthwaite Water location, U.K., prepared in the international program on Validation of Model Predictions (VAMP) of IAEA/CEC, was used as a test scenario. Radiocesium enters the lake vi a: (1) a quick pulse of atmospheric deposition for 1-2 days; and (2) a lasting but smaller contribution from the catchment carried by the wa ter runoff and the dragged sediments. An auxiliary model of the catchm ent was used to quantify these two source terms. The temporal evolutio n of the Cs-137 concentration in this lake after the contamination by the Chernobyl accident is modelled using a compartmental approach foll owing the Codell model. The desorption/adsorption dynamics between wat er and sediments are considered both in the drainage water and in the lake boundary layer. The most important processes transferring contami nation out of the lake water are adsorption of radioactivity by suspen ded sediments and their sedimentation; these processes are modelled th rough a distribution coefficient and through an aerial removal rate. S ummer stratification is modelled introducing some auxiliary hypotheses . When using a priori probability distributions for the parameters bas ed both on experimental evidence and on information from the literatur e the model simulates the observed radiocesium concentrations with an expected underprediction bias of 30% in the integrated concentrations in water. The concentration in sediments is inside the observed range in most of the runs. After the calibration of the model to the best fi ts observed with parameters inside their expected ranges, the bias dec reases to -6% in the epilimnion, 8% in the hypolimnion and -4.4% in se diments, with an unexplained variance of +3.2%, +3% and +1%, respectiv ely.