Trends in suspended sediment quality in the upper St. Clair River: Assessment of large-scale remediation of contaminated sediments in a dynamic riverine environment

Abstract

Sediment downstream of industrial sources in the Upper St. Clair River was historically contaminated with mercury, hexachlorobutadiene, hexachlorobenzene and octachlorostyrene. Concentrations of contaminants of concern in suspended sediment collected from traps in 1994/1995 and 2001 suggested bottom sediment was mobile and a source of contamination to downstream areas. Sediment was dredged in 1996 and in 2002–2004 from two contaminated areas (Cole Drain and Dow waterfront, respectively). Post remediation concentrations of contaminants of concern in bottom sediment and suspended sediment throughout downstream areas were high, relative to concentrations measured at the upstream reference sites; however, data from sediment traps deployed in 2006–2011 indicated that concentrations of contaminants of concern were trending downward since the remediation efforts. There was a significant (p ≤ 0.05) decrease in concentrations of mercury, hexachlorobutadiene and octachlorosytrene for suspended sediment in 2006 (post-remediation of the Dow waterfront) compared to 2001 (pre-remediation) and contaminants of concern suspended sediment concentrations have remained consistent between 2006 and 2011. Contaminant down-flux rates for the upper St. Clair River have also decreased since remediation; however, data for hexachlorobenzene were variable with no apparent trend through time. Bottom sediment data for hexachlorobenzene, octachlorostyrene and hexachlorobutadiene collected in 2006 and 2008 downstream of the remediation areas have also shown a downward trend when compared to concentration reported for 1990–2001; however, declines in mercury were not apparent.