Analysis of rates of decline of PCBs in different Lake Superior media

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 p rediction was tested by comparing rates of decrease of PCBs, over the last two decades, in lake trout, smelt, gull eggs, bottom sediments, settling pa rticles, and the water column of Lake Superior. Consistent with the model p rediction, PCBs in slow responding media, bottom sediments and the biota, d ecreased from 3% to 8% each year over. the last two decades. III contrast. PCB concentrations in media predicted to respond rapidly, the water column and settling particles, decreased much faster, by about 23% each year. Cons equently, by 1996 water column PCB concentrations were only about 3% of the 1980 concentration, whereas recent PCB concentrations in gull eggs and lak e 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 w ith external loading much more slowly than did PCBs bz 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 magnitu de less than those currently observed. Third, based on the second conclusio n, previous controls on PCBs have been more effective than is often assumed . PCB concentrations in biota are still responding to those past remedial e fforts and may decrease another 90% or mure even if external loading remain s constant into the future. However, as in the past, the yearly rate of dec line of PCBs in biota and sediments in the future will be 5% to 10% per yea r.