Analysis of Rates of Decline of PCBs in Different Lake Superior Media

Abstract

Mathematical models and partitioning theory predict that PCB concentrations in Great Lakes sediments and biota will respond more slowly to reductions in external PCB loading than PCB concentrations in the water column. This prediction was tested by comparing rates of decrease of PCBs, over the last two decades, in lake trout, smelt, gull eggs, bottom sediments, settling particles, and the water column of Lake Superior. Consistent with the model prediction, PCBs in slow responding media, bottom sediments and the biota, decreased from 3% to 8% each year over the last two decades. In contrast, PCB concentrations in media predicted to respond rapidly, the water column and settling particles, decreased much faster, by about 23% each year. Consequently, by 1996 water column PCB concentrations were only about 3% of the 1980 concentration, whereas recent PCB concentrations in gull eggs and lake trout were about 30% to 40% of the 1980 value. This synoptic view of PCB declines in different media has the following implications. First, over the 1980s and 1990s, PCBs in the biota and sediments approached steady-state with external loading much more slowly than did PCBs in the water column or settling particles. Second, if PCBs in biota had declined as fast as those in the water column, biota concentrations would now be an order of magnitude less than those currently observed. Third, based on the second conclusion, previous controls on PCBs have been more effective than is often assum ed. PCB concentrations in biota are still responding to those past remedial efforts and may decrease another 90% or more even if external loading remains constant into the future. However, as in the past, the yearly rate of decline of PCBs in biota and sediments in the future will be 5% to 10% per year.