Monitoring effects of remediation on natural sediment recovery in Sydney Harbour, Nova Scotia

Abstract

Chemical contaminants were assessed in Sydney Harbour, Nova Scotia during pre-remediation (baseline) and 3 years of remediation of a former coking and steel facility after nearly a century of operation and historical pollution into the Sydney Tar Ponds (STP). Concentrations of polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls, metals, and inorganic parameters measured in sediments and total suspended solids in seawater indicate that the overall spatial distribution pattern of historical contaminants remains unchanged, although at much lower concentrations than previously reported due to natural sediment recovery, despite remediation activities. Measured sediment deposition rates in bottom-moored traps during baseline were low (0.4-0.8 cm year(-1)), but during dredging operations required for construction of new port facilities in the inner Sydney Harbour, sedimentation rates were equivalent to 26-128 cm year(-1). Measurements of sediment chemical contaminants confirmed that natural recovery rates of Sydney Harbour sediments were in broad agreement with predicted concentrations, or in some cases, lower than originally predicted despite remediation activities at the STP site. Overall, most measured contaminants in sediments showed little temporal variability (4 years), except for the detection of significant increases in total PAH concentrations during the onset of remediation monitoring compared to baseline. This slight increase represents only a short-term interruption in the overall natural recovery of sediments in Sydney Harbour, which were enhanced due to the positive impacts of large-scale dredging of less contaminated outer harbor sediments which were discharged into a confined disposal area located in the inner harbor.

In Situ Localization and Quantification of Sediment Deposits after Dredging and Disposal Interventions in Sydney Harbour, Canada, Using a Dynamic Penetrometer

“…The seafloor images confirmed sediment type distributions as described by Jacques Whitford Ltd. (2009) and Walker et al (2013), with mud and silt dominating in the inner harbor inside of the South Bar, and sand in the outer harbor (Figs. 4 A and B).…”

“…Sediment grab samples were taken with a 0.0625m 2 Ponar grab at chosen positions to confirm grain sizes as expected following previous studies (Stewart et al, 2001;Walker et al, 2013). Digital video and still images were taken to characterize different sediment types at various sites using towed and diver-operated underwater cameras.…”

“…The distribution of sediments shows a marked coarsening to seaward, where there is 78-96% sand fractions in the Outer Harbour, and a minor abundance (~12%) of the fine fractions. Total suspended solid (TSS) concentrations were found to be less than 10 mg/L one meter above the bottom prior to dredging in 2011 and 2012 (Walker et al, 2013). After dredging, a significant increase in TSS was observed only at two of eleven locations.…”

“…After dredging, a significant increase in TSS was observed only at two of eleven locations. However, during the dredging activity (Oct 2011 to January 2012) localized TSS concentrations of up to 73 mg/L were measured (Walker et al, 2013). Sediment deposition rates in Sydney harbour are known to be low (0.4-0.8 cm/year), reaching 0.2-2 cm/year in the South Arm near Muggah Creek and Sydney River (Stewart et al, 2001;Lee et al, 2002).…”

“…Sediment deposition rates in Sydney harbour are known to be low (0.4-0.8 cm/year), reaching 0.2-2 cm/year in the South Arm near Muggah Creek and Sydney River (Stewart et al, 2001;Lee et al, 2002). Deposition rates increased significantly (26-128 cm/year) during dredging activities in the northern part of the South Arm (Walker et al, 2013).…”