PCB availability assessment of river dredging using caged clams and fish

The effects of dredging to remove sediments contaminated with polychlorinated biphenyls (PCBs) were studied in the South Branch of the Shiawassee River in south-central Michigan. Caged fingernail clams, Sphaerium striatinum (Lamarck), and fathead minnows, Pimephales promelas Rafinesque, were used to monitor the bioavailability of PCBs, and these data were compared with changes in water column concentrations before dredging, during dredging and up to six months after dredging was completed. Monitoring of water, clams and fish during dredging confirmed that significant amounts of PCBs were released from the sediments. At all locations downstream and in the area of the dredging, there were increases in the availability of PCBs for at least six months. PCB concentrations in clams and fish in the dredged zone increased from 64.5 to 87.95 μg/g dry weight and from 13.82 to 18.30 μg/g dry weight, respectively, after dredging. Downstream (11 km), post-dredge uptake was also higher; however, clams showed less uptake than fish. This implied different uptake routes for the two organisms. There was no noticeable change in total PCB concentration in the water after dredging. In situ experiments were run to determine uptake curves and time to uptake equilibrium for both clams and fish. Results of tests for equilibrium uptake by fatheads agreed with literature values. Uptake maxima occurred within 20 to 30 d. For clams, maximum uptake for Aroclor 1242 occurred after 9 d. Log bioconcentration factors for clams ranged from 2.6 to 4.5 for Aroclor 1242 and from 3.0 to 4.4 for Aroclor 1254; for fish, they ranged from 3.0 to 4.4 for Aroclor 1242 and from 4.5 to 5.5 for Aroclor 1254.