Model construct and calibration of an integrated water quality model (LM2-Toxic) for the Lake Michigan Mass Balance Project

The Lake Michigan Mass Balance Project (LMMBP) was a part of the Enhanced Monitoring Plan (EMP) for Lake Michigan (McCarty et al., 2006). PCBs (polychlorinated biphenyls) were one of the targeted pollutants studied in the project. As one of the components in the overall LMMBP modeling framework, the LM2-Toxic model was developed to simulate fate and transport of PCB congeners in the Lake Michigan ecosystem. The purpose of the LM2-Toxic model was to address the relationship between sources of PCBs and their resultant concentrations in water and sediments of Lake Michigan and identify key environmental processes that strongly influence mass fluxes of the targeted pollutants in the lake. The model is sophisticated and is an extension of the widely used WASP model in terms of describing physical and biochemical processes within an aquatic system. Three organic carbon sorbents (biotic carbon, particulate detrital carbon, and dissolved organic carbon) and 54 PCB congeners were simulated as individual state variables. Using 1994 and 1995 project-generated field data, the organic carbon solids dynamics were calibrated first. This was followed by the calibration of PCB dynamics. The model construct and calibration results demonstrated significant improvements over historical mass balance models applied to the Lake Michigan ecosystem. The model results indicate that air-water exchange and interaction between the water column and sediment are the most important processes for PCBs in Lake Michigan.