Modelling the fate of PCBs and MIREX in aquatic ecosystems using the TOXFATE model

Predictive modelling of the fate of two persistent toxic organic chemicals, PCBs and Mirex, is discussed in light of the results from oceanographic scale investigations from the Niagara River to Lake Ontario, to the St. Lawrence River Estuary aquatic ecosystem. A mathematical model, TOXFATE, is used to run simulations of the fate of Mirex in Lake Ontario, a relatively small part of the total system, using a mass balance approach. TOXFATE features simulations of the fate of Mirex in lake water, plankton, benthos, suspended and bottom sediments, small and large fish (sculpins and salmonids). A friendly user interface (TOXSHELL) facilitates running the program on microcomputers. Concentrations of “dissolved” (the fraction not removed by high-speed centrifugation) persistent toxic organic chemicals in the Niagara River are in the ng L −1 or ng m −3 range, yet, the total load transported into Lake Ontario can be considerable given the high discharge of some 6000 m 3 sec −1. The river draining Lake Ontario is the St. Lawrence, and PCB loads actually double due to the various sources along the river. The insecticide and flame retardant, Mirex was essentially introduced from only two point sources, the Niagara and Oswego Rivers. The chemical is still detectable some 1000 km downstream of the main site near Niagara Falls of its original introduction to this river-lake-estuary system. The system's recovery from Mirex pollution is related to the major natural aquatic processes of the system and is not compounded by continuing point and non-point source inputs. Simulations show the fast response of Mirex concentration in water following a reduction in loadings in the early 1960s and a much slower reaction of bottom sediments and fish to the same loadings reduction.