PCBs in the Detroit River water column

Water samples (60 to 190 L) were taken from the Trenton Channel portion of the Detroit River during eight sampling events from March to October 1995, and in May 1996. Samples were filtered on-site or at the field-lab to retain the suspended particulates. Some of the samples were passed through XAD-2 columns to extract dissolved PCBs, depending on total sample volume and logistical considerations. Concentrations of polychlorinated biphenyls (PCBs) were measured in both the particulate and dissolved fractions. Concentrations ranged from < 5 to 22 ng/L (average = 10 ng/L) in the particulate, and from < 5 to 13 ng/L in the dissolved fractions. The average ratio of dissolved PCB concentration to particle-bound PCB concentration was 0.6; the calculated average total PCB concentration was 17 ng/L. Annual mass transport of PCBs through the Trenton Channel in 1995 was estimated to be 600 kg, with potential point sources along the west shore of the channel contributing less than 1.5% of the total mass of PCBs. A general seasonal trend was observed, with lowest PCB concentrations in March and greatest in July. Concentrations following a significant rainfall increased within the first 24 hr and decreased to approximate ambient levels after 96 hr. In general, PCB concentrations appear to be greater downstream than upstream, however, this trend was reversed for Day-1 of the rainfall events, perhaps due to greater particulate loading from upstream of the channel. Potential point-sources from industrial or municipal waste-water treatment outfalls showed PCB concentrations up to a factor of 4 greater than those from the transects in the channel. However, the annual loading of PCBs to the channel from these sources was small (< 2%) compared to the mass transported through the channel. Dissolved concentrations of PCBs determined by C₁₈ Empore disks were 4.7-fold greater than those determined by XAD-2 (s.d. = 1.5, n = 4). Further investigations validating this finding are necessary. For this reason, comparisons among different studies must be done with caution.