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.

Introduction

The St. Clair River was designated as an area of concern (AOC) under the Great Lakes Water Quality Agreement in 1985 because of contaminated fish, degraded benthic communities, beach closures, and other impairments due to poor water and sediment quality (JGLR, 1985). The river, which borders Canada (Ontario) and the United States (Michigan), flows south 64 km from Lake Huron to Lake St. Clair and is used by both countries for drinking water, recreational boating, angling and commercial shipping. Canadian and American industrial and municipal point sources that historically included petroleum refineries, organic and inorganic chemical manufacturers, paper companies, salt producers and thermal electric generating facilities discharged contaminants to the river directly or through tributaries (St. Clair River RAP, 1991). Additionally, non-point sources such as combined sewer overflows, contaminated sediment and discharges related to agricultural practices contributed contaminant loadings to the river (Kauss and Hamdy, 1985).

The Canadian Federal and Ontario Provincial governments classified the upper reaches of the St. Clair River into three zones based on extensive benthic community impairment; Zone 1, Zone 2 and Zone 3 (Figures 1 and S1). These areas have been intensely studied due to the presence of industrial activity which resulted in sediment contaminated with compounds that greatly exceeded concentrations upstream in Lake Huron and/or Ontario Provincial Sediment Quality Guidelines; these contaminants included mercury (Hg), cadmium, hexachlorobenzene (HCB), octachlorostyrene (OCS), polychlorinated biphenyls (PCBs) and hexachlorobutadiene (HCBD). Additionally, concentrations of these compounds in water exceeded Canadian Environmental Quality Guidelines, Great Lakes Water Quality Agreement Objectives and/or Provincial Water Quality Guidelines (St. Clair River RAP, 1991).

Due to river flow patterns (UGLCCS, 1988), contaminated plumes from shore-based industrial and municipal sources tend to hug the shoreline along the Canadian side of the river. Water samples collected from the upper and lower sections of the river showed this plume was confined to within 300 m from shore (Chan et al., 1986). Historically, degraded benthic communities were identified on the Canadian side for at least 50 km downstream from Sarnia, Ontario (Thornley, 1985). Through efforts of local industries, stakeholders and government agencies, environmental conditions within the river have improved (Marvin et al., 2004). Monitoring showed that by 1990 the zone of benthic community degradation had been reduced to a roughly 9 km area extending downstream from Sarnia, and by 2004 benthic community structure along the Canadian side was considered unimpaired at most sites, compared to upstream reference stations and stations on the U.S. side of the river (Pope, 1993; Milani et al., 2007).

However, historically-contaminated bottom sediment continues to be a source of Hg and industrial chlorinated compounds to the water column, biota and downstream areas (Gewurtz et al., 2010; Jia et al., 2010; Richman and Milani, 2010). Total Hg and methylmercury concentrations in sediment and invertebrates collected from sites exposed to historical industrial discharges were elevated, compared to upstream reference sites, and relationships between total Hg and methylmercury concentrations in sediment and benthic invertebrates were significant (Milani et al., 2007; Richman, 2011a,b). An evaluation of fish tissue concentrations of Hg, HCB, OCS and PCBs in 13 species of fish collected from Lake St. Clair and the St. Clair River showed that concentrations of these compounds decreased from the 1970's to 1980's and 1990's, after which concentrations stabilized or the rate of decline slowed (Gewurtz et al., 2010). These data suggested these trends were due to ambient conditions and exposure, rather than food web processes since the stabilization for each chemical occurred in multiple species of different trophic levels with a variety of feeding habits. Higher concentrations of contaminants in juvenile fish from the St. Clair River, compared with fish collected from Lake Huron, suggested non-atmospheric sources and implicated sediment contamination as the source (Gewurtz et al., 2010). A comparison of suspended sediment Hg concentrations in the St. Clair River to the Detroit River from 2000 to 2004 showed Hg concentrations were elevated throughout the corridor due to re-suspension and transport of Hg-contaminated sediment resulting from historical chlor-alkali plants in the Detroit River and Upper St. Clair River (Jia et al., 2010). Total organic carbon (TOC)-normalized Hg concentrations in suspended sediments from the Upper St. Clair River were roughly 15- to 25-fold higher than concentrations measured upstream in southern Lake Huron (Jia et al., 2010).

In 1994/1995, investigations along roughly 2 km of shoreline within Zone 1 identified the presence of contaminated and toxic sediment (volatile and semi volatile organic compounds and mercury), severely degraded benthic communities and evidence of bottom sediment remobilization (Bedard and Petro, 1997; Farara and Burt, 1997; Moran et al., 1997). Subsequently, in 1996, an area of contaminated sediment (375 m2) around the Cole Drain was remediated, and from 2002 to 2004, a much greater volume (34,000 m3) of additional contaminated sediment was removed from an area impacted by industrial discharge from four outfalls, one of which (1st Street sewer) historically received waste from the operation of a chlor-alkali plant that discharged roughly 91 metric tonnes of Hg to the river between 1949 and 1970 (Hartig, 1983) (Figure 1).

The success and environmental benefits due to the sediment remedial actions were expected to include improvements to the local habitat for benthic invertebrate re-colonization, and removal of highly-contaminated sediment acting as a chronic source of contaminants to downstream areas through re-suspension and subsequent transport. For the Dow waterfront project, the measure of success was based on percent mass removal of contaminants of concern (COCs); they reported >90% removal averaged over the three phases of the project (Dow, 2005). However, there was no way to translate this percentage into a measureable positive impact on the environment since the remedial action objectives were not risk-based or linked backed to achieving a site-specific target concentration as observed in other sediment remediation projects (Nelson and Bergen, 2012, Weston et al., 2002). Given the capital expenditures required for remediation of contaminated sediment, it is essential to obtain understanding of remedial efficacy (Bridges et al., 2010). Although a highly variable metric, we have evaluated the effectiveness of remediation along the Dow waterfront through collection of suspended sediment using sediment traps during pre- and post-remediation periods. Along the combined long axis of all three zones, a series of sediment trap transects were established (D1 – D6, Figures S1 and 1); these transects were sampled in 2001 and from 2006–2011. An upstream reference station (Figures S1 and 1) was sampled in 1994, 1995, 2009 and 2011. Upstream of transect D1; three additional sub-transects (CD1-CD3) provided targeted monitoring pre- (1994/1995) and post- (2010) remediation downstream of the Cole Drain (Figure 1). Post-remediation suspended sediment data was used to assess movement of residual contamination from remediated areas and overall changes in quality of suspended sediments. No specific target concentrations were established, but rather any changes in COC concentrations pre- and post-remediation were assessed. Data were also used to measure quantitative and qualitative differences in suspended sediments transported along the near-shore, including seasonal trends.

Methodology

Deployment of sediment traps

Sediment traps were deployed using divers in Zone 1 at two transects in 1994 and three transects in 1995 to measure movement of contaminated suspended sediment downstream of the Cole Drain and along the Dow waterfront (identified as CD1 to CD3, Figure 1). This methodology has previously been used for the study of contaminants associated with suspended sediments in the Great Lakes (Oliver et al., 1984; Eadie et al., 1984). A reference site transect was also established in 1994 and resampled in 1995 about 750 m upstream of the Cole Drain to reflect any upstream inputs to the study area (Figure 1). One transect (CD3) was resampled in 2010. In 2001 and 2006–2011, sediment traps were deployed at six transects located throughout Zones 2 and 3 (D1 to D6, Figure 1) downstream of Dow to monitor the movement of contaminants from the Dow property pre- and post-remediation. The 1994/1995 upstream reference site was re-sampled in 2009 and 2011.

Typically at each transect, 3 traps were deployed by divers at each of three stations at increasing distances from shore at water depths ranging from 3 m to 6.5 m. There were two deployments within a sampling year (spring and fall) with traps remaining in the water for about two and a half months to ensure sufficient accumulation of sediment for analysis and to capture potential seasonal differences. The spring deployment was from mid-May through July, and the fall deployment from August through October. Exceptions included 1994 and 2007 when traps were only deployed in the fall and successful retrieval of the traps was only possible from three transects (Table 1).

Chemical Analysis

Upon retrieval, the trap tubes were transported to the Ministry of Environment and Climate Change (MOECC) Etobicoke, Ontario laboratory and stored upright in a walk-in refrigerator for a minimum of one week to allow the sediment to re-settle. The surface water was decanted and sediment was removed from the settling tubes and submitted for analysis of TOC, Hg, HCB, HCBD, OCS, particle size and percent moisture. Wet weight of sediment collected from the tubes was recorded in order to calculate suspended sediment and contaminant down-flux rates.

Samples were analysed for COCs following standard methodologies described in Richman and Milani (2010). Suspended sediment samples for all survey years and bottom sediment collected in 1994/1995 and 2001, 2010 and 2011 were analyzed for HCBD, OCS and HCB using MOE method 3270 (MOE, 2008a) using gas chromatography with electron-capture detection (GC-ECD). Samples were analyzed for Hg using cold-vapour atomic absorption following MOE Method 3059 (MOE, 2010a). Sediment samples were analyzed for TOC using MOE method 3142 (MOE, 2010b). Details for the historical collection of bottom sediment, and chemical analysis are provided in Online Supplementary Information.

Data Analysis

Sediment collected in each tube was analysed separately for all contaminants. The Kolmogorov-Smirnov Test was used to test suspended sediment contaminant data for normality, and variances were tested for homogeneity for data collected from 2001–2011. Log10 transformed data within each year were compared to assess spatial and seasonal (spring and fall) trends using the Kruskal Wallis One Way Analysis of Variance (ANOVA) on Ranks followed by the Dunn's Multiple Range Test because the data, following transformation, were either still heterogeneous or not normally distributed depending on the COC. Significant differences in COC concentrations between spring and fall suspended sediment collections were inconsistent among stations and between years so seasonal patterns were not evident. Accordingly, data collected in the spring and fall deployments were combined to provide annual average concentrations for each transect. Annual average contaminant concentrations for a transect were calculated based on the overall mean concentration from the two deployments, which, depending on the sample year and the success of trap retrieval, would typically range from 6–18 data points (i.e. in 1994 and 2007 there were 3 traps per transect; in 2008–2011 there were 9 traps per transect for each of the spring and fall deployments) (Table 1). The Mann-Kendell Test was then used to assess trends through time for each chemical. The Pearson Product Moment Correlation Analysis was used for each year of study to assess relationships between contaminant concentrations and particle size and TOC. The analysis was performed using both transformed and non-transformed data yielding the same results. Comparisons of concentrations for each COC between 1994/1995 and 2010 were made using the t-test at p < 0.001. All statistical procedures were performed using SigmaStat, from Systat Software, Inc. (San Jose, CA, USA).

Down-flux at each transect and for each year was estimated by calculating the dry weight of sediment collected in each trap based on total wet weight of sediment and percent moisture. The dry weight per trap was then multiplied by the number of traps estimated to represent a square metre (i.e. 127 traps based on a 10 cm diameter trap tube), which provided an estimate of areal deposition (g m−2). The deposition value was then divided by the number of days of deployment and multiplied by the individual chemical concentrations to afford contaminant down-flux estimates. A mean for each transect was calculated using data from all traps within a transect combining the spring and fall deployments. The annual mean suspended sediment down-flux and corresponding COC down-flux rates for the entire study area (Zone 2 and Zone 3) was calculated by combining annual data from each transect within a year (Table 1). This data was used to assess if there had been a decrease in contaminants associated with suspended sediment moving downstream through the St. Clair River corridor post remediation.

Results and discussion

Pre-remediation bottom sediment and suspended sediment quality in zone 1 downstream of the Cole Drain

Concentrations of COCs in bottom sediment (0–5 cm) collected in 1994/1995 from the area designated for remediation near the Cole Drain and from an upstream reference site are provided in Table 2. Sediment contamination for HCB, HCBD and OCS prior to remediation was extensive, with concentrations exceeding those at the upstream reference site by up to six orders of magnitude (Farara and Burt, 1997; Kauss and Nettleton, 1999).

Concentrations of COCs in suspended sediment collected from traps in 1994 and 1995 from transects CD1, CD2 and CD3 (Figures 2a and b), located roughly 120 m, 450 m, and 1 km, respectively, downstream of the area designated for remediation (Figure 1) reflected the contribution of contaminated bottom sediment downstream of the Cole Drain. Transect CD1 was established in 1995 in part to measure potential run-off from the former Bayer facility (now LANXESS Specialty Chemicals, Figure 1). Concentrations of COCs at CD1 were lower relative to CD2 (Figure 2a) as the traps at CD1 were deployed behind the Bayer Docks where flow was more restricted, but nevertheless indicated resuspension of contaminated sediment when compared with the upstream stations where concentrations of organic COCs in sediment traps in 1994 and 1995 were <10 ng g −1. The mean HCB, HCBD and OCS concentrations from traps at CD2 and at transect CD3 (located about 500 m farther downstream from CD2, and established to investigate potential resuspension of contaminated sediment along the Dow waterfront downstream of the Dow 1st Street Sewer outfall), indicated significant contamination since bottom sediment along the Dow waterfront was also highly contaminated (Figures 1 and 2b). However, mean Hg concentrations at CD3 (4700 ng g −1, SE 1000 ng g −1) were seven times higher than concentrations at CD2 (638 ng g −1 (SE 78 ng g −1) and three orders of magnitude greater than the upstream reference station (<60 ng g −1). The high Hg concentrations at CD3 reflected known historical discharges of Hg contaminated waste from the chlor-alkali plant to the 1st Street outfall (URS, 2001).

Pre-remediation bottom sediment and suspended sediment quality in Zone 1 at the Dow waterfront

A 2001 sediment survey of the waterfront from the 1st Street Sewer to the downstream property line confirmed high concentrations of contaminants identified in the 1994/1995 surveys. Bottom sediment (0–5 cm) concentrations of COCs were as for Hg, 82,000 ng g −1 for HCB, 43,000 ng g −1 for HCBD and 2 600 ng g −1 for OCS (URS, 2001) (Table 2). Suspended sediment data from the deployment of sediment traps in 2001 downstream of the Dow property validated the concern that contaminated bottom sediment was mobile. Concentrations of COCs in 2001 were highest in suspended sediment collected from transects D1 and D2, located roughly 250 m and 2 km downstream of the Dow property, respectively, relative to other far-field locations within Zone 3 (transects D3 to D6) (Figure 3). The one-way ANOVA using log10-transformed data showed significant differences among transects for all COCs; HCBD (F = 8.5; p < 0.001), OCS (F = 5.9; p < 0.003), HCB (F = 3.9; p < 0.007) and Hg (F = 16.9; p < 0.001). The Holm-Sidak Multiple Comparisons test showed that concentrations of HCBD and Hg were significantly higher (p < 0.007) at the near-field stations (D1 and D2), compared to transects downstream (Figures 3a and b). Concentrations of OCS were significantly higher at D2 than all downstream transects (p < 0.006), and D1 was significantly greater than concentrations at transects D4 and D6 (p < 0.005) (Figure 3c). For HCB, concentrations at D1 were significantly higher (p < 0.004) than D5 and D6 (Figure 3d).

Given that prior to remediation the highest concentrations of contaminants in bottom sediments were present along the waterfront immediately upstream of D1 (URS, 2001; Thornburn et al., 2003; Table 2), and the 1994/1995 sediment trap data indicated that sediment in this area was mobile, the 2001 downstream trap data indicated the contaminated area along the Dow waterfront was the primary source of the re-suspended contaminated sediment to near-field transects (D1 and D2), and that impacts could be measured at least 9 km downstream at D6.

Post-remediation bottom sediment and suspended sediment quality in Zone 1

Cole Drain

Suspended sediments collected in 2010 from traps redeployed at CD3, 14 years after the remediation of the Cole Drain and 5 years after the remediation of the Dow site, had significantly lower concentrations of COCs (Figure 2b). Comparisons were made between pre-remediation years (1994/1995) and 2010 for each COC using the t-test; the t-value ranged from 4.6 to 14.1 (p < 0.001). Depending on the COC, suspended sediment contaminant concentrations were 92%–98% lower post-remediation (Figure 2b). Interestingly, a review of the physical characteristics of the sediment collected in the traps showed a change in the size of particles being accumulated. In 1994/1995, the suspended sediment at CD3 was 53% silt/clay while in 2010 the sediment particle size was 95% silt/clay (Table 1). The higher percent silt/clay in the traps at CD3 in 2010 was likely a reflection of higher percent silt/clay moving through the river in general. Although no upstream reference station was sampled in 2010, a review of the particle size data for all stations downstream of CD3 in 2010 (transects D1 – D6) showed percent silt/clay to be greater than 80% (Figure S2b). Figure S2b highlights the year-to-year variability in particle size with percent silt/clay at times similar to percentages measured in 1994/1995. Resuspension and transport of bottom sediment from Lake Huron and along the St Clair River corridor is influenced by both current regimes and meteorological events, e.g., storms and high winds. According to the International Upper Great Lakes Study, physical forces contributing to water level fluctuations, and by association suspended sediment transport, include changes in conveyance of the river, isostatic adjustments and changes in climactic patterns (IJC, 2009). The episodic nature of significant changes in influencing factors affects the size of particles that can be resuspended and transported, which in turn may contribute to the year-to-year variability in particle sizes collected from the traps. We are unable to speculate as to the potential influence of dreissenid mussels on particle size distribution, nutrients or TOC associated with suspended sediments in the St. Clair River.

Although the traps at CD3 exhibited lower contamination in 2010 than in the 1990's, the concentrations of COCs compared to upstream reference stations indicated the area downstream of the Cole Drain was still a source of contaminated sediment, which was likely due to a combination of residual contamination at the dredge site and un-remediated shoreline. Concentrations of Hg, HCBD, HCB and OCS in bottom (0–5 cm) sediment collected in 2008 from transects in Zone 1 upstream of CD3 were lower than concentrations prior to remediation, but were variable depending on the distance from the Cole Drain and distance from shore (Table 2). At several stations, bottom sediment COC concentrations ranged from 10-fold to more than 100-fold higher than values measured in suspended sediment in 2010. For example, OCS concentrations in bottom sediment ranged from non-detect to as high as 2623 ng g−1, with a mean concentration of 427 ng g−1 (Table 2; Richman, 2008). Suspended sediment in 2010 at CD3 had a maximum OCS concentration of 40 ng g−1. For HCB, bottom sediment concentrations were as high as 1 000 ng/g with a mean concentration of 173 ng g−1 (Table 2), while the suspended sediment concentration was lower at 96 ng g−1 (SD 61 ng g−1). Although contaminated bottom sediment was being re-suspended and collected in the traps, less-contaminated sediment from upstream was also likely having a dilution effect on suspended sediment quality.

Dow waterfront and Zones 2 and 3

Annual suspended sediment data from all transects downstream of the Dow waterfront remediation (D1 to D6) indicated concentrations of COCs were significantly higher (p < 0.05) in 2001 than in any year post-remediation (2006–2011) for HCBD (Q > 3.4), OCS (Q > 4.2) and Hg (Q > 4.3). The Mann-Kendell Test for trend analysis identified a significant decrease in concentration from 2001–2011 for Hg and OCS, but only at transects D1 and D2 (S value: -13). For all COCs at the remaining transects there were no definitive temporal trends, with the exception of D5 which had a significant decrease in HCBD concentrations (S value: -17). These data indicated that following completion of sediment remediation in 2004, there was an initial decrease in suspended sediment contaminant concentrations in 2006 for Hg, OCS and HCBD, with the greatest decreases in COC concentrations at transects closest to the remediation site. This improvement in suspended sediment quality has remained consistent over time at the near field transects up to 2 km downstream of the dredging site (D1 and D2), but additional improvements have not been apparent, particularly since there were increases and decreases in concentrations among the transects for 2010 and 2011 which varied dependent on COC and transect. Although there was a large decrease in suspended sediment concentration for HCB between 1994/1995 and 2010 at CD3 downstream of the Cole Drain (Figure 2b), variability in HCB concentrations in traps downstream of the Dow site (D1 to D6) precluded determination of definitive temporal trends (Figure 3d; note standard error bars). Bottom sediment collected in 2010–2012 from behind industrial docks and along shoreline extending roughly 750 m downstream of the Dow property line also showed high concentrations of COCs deposited from the historical upstream source (Table 2). The resuspension of this bottom sediment within Zone 2 could possibly explain the variability in contaminant concentrations measured in the traps at D2 and D3, since COC concentrations in the traps were within the range of concentrations measured in the bottom sediment. Resuspension of contaminated bottom sediment along the nearshore has likely limited the improvement in suspended sediment quality with increasing distance downstream from the remediation site. The only exception was total Hg; suspended sediment in the traps had lower Hg concentrations than the bottom sediment. We are unable to offer a plausible explanation for this observation, other than Hg having different physical/chemical characteristics than the other COCs that fall into the classification of semi-volatile organic compounds. Other factors could include a failure of the sediment traps to collect contaminated fine particles because of the dimensions of the sediment trap tubes, their placement in the river, current regimes, or a combination of these factors. Only one previous survey in 2006 included determination of methyl-mercury (MeHg). The MeHg concentrations ranged from the low ng g −1 level to ∼100 ng g −1, but there was no definitive correlation with total Hg concentrations.

The similarity in physical characteristics of the suspended sediment including TOC and particle size on an annual basis precluded the need to normalize the sediment for particle size in order to compare contaminant concentrations between transects (Table 1, Figure S2b). A correlation analysis (Pearson Product Moment Correlation) between COC concentrations in sediment traps and TOC and particle size on an annual basis was variable. For some years, individual COCs showed a significant negative correlation with TOC, but r values were low (typically <0.3) and correlations between COC concentrations and percent silt/clay were infrequent. Accordingly, it is unlikely that particle size and/or TOC influenced the spatial patterns of contamination.

A review of bottom sediment data from 2001–2012 for Zones 2 and 3 indicated sediment contamination was dispersed throughout the area downstream of Dow with the highest concentrations in Zone 2 (Table 2); however, due to high variability within samples collected along transects and throughout the zones, there was no statistically significant difference in concentrations between the two zones for any COC. Richman and Milani (2010) compared bottom sediment (0–5 cm) concentrations of Hg, HCBD, OCS and HCB collected in 1990, 2001 and 2004 (Thorburn et al., 2003; Milani et al., 2007) with sediment collected in 2006 and 2008 in Zones 2 and 3 to assess any temporal changes in sediment quality with emphasis on the post-remediation period. To assess changes in contaminant concentrations, it was necessary to account for variability in deposition patterns, particle size, TOC or other physical features. Accordingly, the difference in the contaminant concentrations between duplicate samples collected at 7 stations in 2006 was determined for each station and a 95% confidence interval was calculated on the mean of these differences for the 7 stations. When contaminant data from 2006 and 2008 were compared with data from earlier studies at co-located stations, a change in concentration was confirmed if the difference in concentration was outside the confidence interval established for that particular compound. In general, Richman and Milani (2010) concluded that contaminant concentrations decreased post-remediation for HCBD and OCS and to a lesser degree for HCB, in Zone 2 at locations closest to the remediation site (Table 2), which was consistent with the pattern for the suspended sediment data for OCS and HCBD in 2006. The Hg concentrations did not change over time based on comparison of bottom sediment; this conclusion was corroborated by assessing Hg concentrations in the upper sections of several sediment cores. However, it is noteworthy that depending on sampling location throughout the entire survey area, sediment core data indicated the highest Hg concentrations to be at depth and not available for resuspension and/or transport.

Contaminant down-flux rates

Average daily contaminant down-flux rates were calculated for the annual data sets. The mean contaminant down-flux rates (based on both the spring and fall deployments) for each transect within a survey (i.e. CD1 to CD3 for 1994/1995 and D1 to D6 for 2001–2011) were also calculated. The 1994/1995 contaminant down-flux rates were reported separately for each transect, but the 2001–2011 rates were reported as over-all means using all transects for that survey year to represent general conditions in the upper St. Clair River (Table 3). The data describing the contamination of suspended sediment moving through the Upper St. Clair River was compared to the upstream reference station to assess the suspended sediment quality moving into the head of the river upstream of historical sources. Additionally, annual data from 2006–2011 were compared with the 2001 pre-remediation data to determine any overall decrease in contaminants moving through the system following remediation. Similar to the 2001 data, the 1994 and 1995 contaminant down-flux rates can also be used as historical confirmation of the movement of highly-contaminated sediment prior to remediation.

With the exception of HCB, contaminant down-flux rates, represented by the calculated mean values from Zones 2 and 3, were consistently higher in 2001 compared with succeeding years 2007–2011 (down-flux data was not available for 2006) (Table 3). For Hg, the daily mean down-flux rate in 2001 was 721 µg m−2 day−1, while corresponding rates for 2007–2011 ranged from 181–364 µg m−2 day−1. The decreases in calculated contaminant down-flux rates suggested that the remediation was successful at reducing a significant source of contamination. However, data from 2011 also showed that the Hg down-flux remained 5-to-17 times higher (mean down-flux 9.5 times greater) than at the reference station (range of 9 – 21 µg m−2 day−1) which suggested that the resuspension of contaminated bottom sediments along the St. Clair River remains a source of Hg to downstream areas and potentially the aquatic food chain. Organic contaminants were below the detection limit at the reference station, thus enrichment of these compounds downstream of the historical sources was also evident, but movement and deposition of Hg was the highest among the COCs.

Suspended sediment quality as an overall assessment tool

The current study has highlighted both some of the challenges, and advantages, of using sediment traps as a sampling methodology and suspended sediment quality as a metric for determining the efficacy of remediation of contaminated sediment in geographically expansive and dynamic riverine environments. In the case of the Upper St. Clair River, there is inherent variability in the quality of both bottom and suspended sediments at previously remediated areas, as resuspension of bottom sediment can be highly episodic in nature. The validity of measured suspended sediment contaminant concentrations, and the associated down-flux rates, is heavily influenced by the relative contributions of accumulated material of differing origins and processes, including particulate material from shoreline erosion, material in transport from upstream lakes and rivers, and resuspended bottom sediments derived locally; these relative contributions can be highly variable over time. The trajectory of suspended particulate in the water column is also influenced by current regimes and water flow patterns; therefore, placement of sediment traps in some cases may not have maximized the accumulation of sediment resuspended and transported from remediated areas we targeted in our surveys. In terms of areal calculations of sediment and/or contaminant down-fluxes, estimates derived from sediment traps are far more reliable in open-lake environs, compared to more dynamic riverine systems. Sediment traps are typically employed in low energy environments for estimating epilimnetic sedimentation rates where the influence of resuspended bottom sediments is minimal (Eadie et al., 1984; Oliver et al., 1984). We have previously used TOC and loss on ignition (LOI) to assess the contribution of resuspended bottom sediments to material accumulating in the traps, as bottom sediments are typically much lower in organic content (Marvin et al., 2007); however, in the current study our primary focus is simply on contaminated suspended sediments in the water column that can be transported downstream.

As can be the case for field-based studies, loss of equipment due to factors including meteorology, boat traffic and vandalism, in some cases resulted in reductions in numbers of samples collected during some survey periods, which in turn impacted our statistical analyses. As evidenced by the results of the current study, the influence of resuspended contaminated bottom sediments to material being transported downstream is more definitively assessed in proximity to the source. Despite the shortcomings of the methodology, sediment traps provide time-integrated samples of material in the water column that has the potential to move considerable distances downstream. Collection of suspended sediments by centrifugation and/or whole water sampling provides only a “snap shot” of the quality of suspended material in the water column at the time of sampling. Diver-installed sediment trap tubes and/or single-mount moorings installed using large vessels are robust sampling techniques well-suited to dynamic aquatic environments for long-term monitoring. All of these factors are important to consider given that suspended sediment quality is being used as an important assessment tool in a number of Great Lakes AOCs, including the Detroit River and Niagara River.

Conclusions

Concentrations of Hg, HCBD, OCS and HCB in suspended sediment collected in 1994/1995 and 2001 in the Upper St. Clair River indicated contaminated bottom sediment downstream of the Cole Drain and from the Dow waterfront (Zone 1) was mobile and acting as a source of contamination to downstream areas. Remedial actions were completed downstream of the Cole Drain in 1996, and along the Dow waterfront by December 2004 with placement of fish mix over the remediated site. In 2011, concentrations of COCs in suspended sediments throughout Zones 2 and 3 were still elevated, compared to concentrations measured at an upstream reference site. This was likely due to residual contamination and movement of contaminated bottom sediment along the St. Clair River corridor within Zones 2 and 3. However, data from sediment traps deployed in 2006–2011 indicated that COC concentrations associated with suspended sediments in the nearfield decreased following sediment remediation and have remained consistently lower than pre-remediation concentrations within 2 km of the dredge site, suggesting that the remediation was successful in reducing a localized in-place contaminant source. Contaminant down-flux rates also showed a large decrease post-remediation. Despite the inherent variability in our field measurements due to factors identified in the discussion of suspended sediment quality as an assessment tool, the contaminant trends we have reported are statistically unequivocal. While less definitive a metric in terms of establishing temporal trends, bottom sediment contaminant concentrations generally decreased post-remediation for HCBD, OCS and HCB in Zone 2 closest to the remediation site.

Acknowledgements

The authors thank Mr. Bruce Gray and members of the Technical Operations Services Dive Team, Operational Analytical Laboratories and Research Support Division, Environment and Climate Change Canada, Wendy Page, John Thibeau and Greg Hobson of the Ontario Ministry of the Environment and Climate Change, and the Sediment Unit of the Ontario Ministry of the Environment and Climate Change Laboratory Services Branch.

Funding

Funding was provided by the Ontario Government and Environment and Climate Change Canada through the Great Lakes Action Plan.

Supplemental material

Supplemental materials for this article are available on the publisher's website.

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Supplementary data