DEPOSITION, MOBILIZATION AND POSSIBLE SOURCES OF POLYCHLORINATED BIPHENYL SPILLS INTO CANALS AND DRAINS IN ST. CLAIR SHORES, MICHIGAN

Chemical analysis of dated sediment core indicates that between 250 and 1,000 L of polychlorinated biphenyls (PCB) were deposited in St. Clair Shores Michigan canals between 1955-1965 and 1983-2003. Most of the PCBs were derived from the 124 hectare Ten Mile Drain (TMD) urban watershed and deposited in a “delta-like” structure at the mouth of the TMD. The PCBs consist of Aroclor 1242>>1254>1260 and were deposited between 1983 and 2003 coincident with >1.0 m drops in the level of the hydraulically connected Lake St. Clair. These lake level drops likely produced cones of depression in the water table around the TMD about 2 km from the canals and helped mobilize the PCB-bearing dense non-aqueous phase liquids into the drain.

The PCBs have been found in fill around the TMD at 3 different locations about 50 m apart. The predominantly Aroclor 1242 composition and concentration indicate that the TMD spill is probably transformer oil. Viable sources include transformers damaged by an F3 tornado that struck St. Clair Shores in 1983 and a power substation. The PCBs probably did not originate by illicit dumping, cutting fluid leaks from a machine shop, ink leaks from a print shop, or heavy industry.

PCBs in the 1955-1965 spill were derived from 40 hectares that are now part of TMD watershed and were deposited in bottomset bed sediments near a “delta-like” structure at the pre-1957 Marsaque Drain mouth. These PCBs have higher Aroclor 1260 concentrations and are weathered. The 20 to 45 L of PCBs deposited in 1955-1965 was derived from a transformer with possible contributions from atmospheric deposition originating in the Detroit industrial center.

Layered canal sediment at the coring site are minimally disturbed because: (1) water depth of >2 m between 1926 and the late 1990s was twice that of powerboats using the canal; (2) shallow water depths at the canal mouth precluded boats from entering them for many years; (3) boat traffic was limited prior to 1960; and (4) semi-consolidation of new canal sediment each winter makes it more resistant to resuspension by propeller wash. The sediment deposition rate at the coring site was 3.2 cm/yr from 1926-1965 and 1.8 cm/yr from 1965-2003. These rates are consistent with a complex history involving the formation of 2 “delta-like” structures by sediment delivered from 2 watersheds to 3 different mouths.