Strontium 90 in the Great Lakes: Concentration-time model

Concentration-time models for ⁹⁰Sr in the Great Lakes deal with transient concentrations of a radionuclide in a chain of mixed lakes and in a chain of periodically stratified lakes. In the model, the lakes receive time-dependent input of ⁹⁰Sr from atmosphere and land (0–10% of annual deposit in the land drainage basins). Removal of ⁹⁰Sr from lake water is by outflow and decay. In the five Great Lakes computed concentrations of ⁹⁰Sr rise from nil (assumed) in 1953 to a maximum in 1964–1965, followed by a gradual decline until 1969. Computed concentrations are comparable to the range of reported values. However, a closer agreement between the model and reported concentrations requires the rates of input in 1962–1964 to be higher than the mean, as well as the rates of removal to be faster (by 6–8%) than the removal by decay and mean outflow. Seasonal stratification of the lakes has virtually no effect on mean annual concentrations of ⁹⁰Sr.