SIMILAR RATES OF DECREASE OF PERSISTENT, HYDROPHOBIC AND PARTICLE-REACTIVE CONTAMINANTS IN RIVERINE SYSTEMS

Although it is well-known that concentrations of anthropogenic radionu clides and organochlorine compounds in aquatic systems have decreased since their widespread release has stopped in the United States, the m agnitude and variability of rates of decrease are not well-known. Pale olimnological studies of reservoirs provide a tool for evaluating thes e long-term trends in riverine systems. Rates of decrease from the 196 0s to the 1990s of Cs-137, PCBs, and total DDT in dated sediment cores from 11 reservoirs in the eastern and central United States were mode led using first;order rate models. Mean half-times of 10.0 (+/-2.5), 9 .5 (+/-2.2), and 13 (+/-5.8) yr for decay-corrected Cs-137, PCBs, and total DDT, respectively, are surprisingly similar. Similar rates of de crease in a few reservoirs are also demonstrated for chlordane and lea d. Conceptual and simple mathematical models relating two soil distrib utions of Cs-137 to trends in the cores provide insight into differenc es in trends between watersheds with different land uses and suggest t hat trends are controlled by erosion, transport, mixing, and depositio n of sediments. These results, supported by similar trends reported fo r other settings and environmental media, could provide an estimate of the decadal response time of riverine systems to changes in the regul ation of other persistent hydrophobic or particle-reactive contaminant s.