Nearshore Sedimentary Mercury Concentrations Reflect Legacy Point Sources and Variable Sedimentation Patterns Under a Natural Recovery Strategy

Abstract

The St. Lawrence River at Cornwall, Ontario, Canada, received substantial inputs of mercury from local, shorelinebased industries through much of the 20th century. Although emission controls were implemented in the late 20th century to reduce the influx of mercury and other metals entering the river, legacy contamination of riverine sediments continues to be a concern. Monitored natural recovery was prescribed in 2005 to remediate contaminated sediments; however, few surveys have been undertaken to examine its effectiveness on shallow, nearshore sediments in contaminated areas. Surface sediments were collected at shallow, nearshore sites in contaminated zones and upstream reference areas to evaluate the current state of sedimentary contamination of mercury and other metals. A Getis-Ord Gi* "hot spot" analysis was employed to assess the spatial distribution of contaminants. In addition, 3 sediment cores were collected from contaminated zones and dated using radioisotopes ( 210 Pb) to assess sedimentation patterns over time. Results indicated that surface sediments from contaminated zones remained elevated in mercury relative to reference sites but spatial distribution of contaminants was highly heterogeneous. Dated sediment cores suggested that sedimentation was not occurring consistently across all areas; variable sedimentation and resuspension patterns over small spatial scales were likely factors driving heterogeneous sedimentary contamination. Such patterns complicate remediation strategies because unburied sediments may serve as continuing sources of contaminants to the ecosystem.

Historical gold mining increased metal(loid) concentrations in lake sediments from Nova Scotia, Canada

“…For example, peak Hg levels of ∼20 000 ng/g were reported from highly-contaminated Phantom Lake near Flin Flon, Manitoba, smelters (Ma et al 2013). Similarly, exceptionally high Hg levels (>17 000 ng/g) were reported in St. Lawrence River sediments impacted by effluent from a mercury cell chlor-alkali plant and pulp paper factory near Cornwall, Ontario, where industrial activities led to severe sediment pollution (Razavi et al 2013;Moir et al 2021). However, broadscale assessments of sedimentary Hg concentrations from other lakes in Canada typically report much lower values because most sites are primarily affected by long-range atmospheric deposition (Muir et al 2009;Drevnick et al 2016;Roberts et al 2019;Galloway et al 2023).…”

Subfossil Chironomid Assemblages as Indicators of Remedial Efficacy in the Historically Contaminated St. Lawrence River at Cornwall, Ontario

“…1a). Zone 1 is 1.4 km downstream of the former pulp-and-paper mill (Domtar, c. 1881-2006) and chlor-alkali (ICI, c. 1935-1995) discharges and is characterized by abundant macrophytes, organic deposits, and high sedimentary methylmercury concentrations (Ridal et al 2010;Moir et al 2021). Prior to 1970, Zone 1 received up to 590 kg/year of Hg from ICI, and an unknown amount of Hg from Domtar, as e uent was discharged directly from the shoreline (MOE 1979).…”

“…Historic Zone 2 sedimentary Zn concentrations were as high as 3800 µg/g and were associated with impaired benthos (Gri ths 1988), and modern-day Zn concentrations remain elevated (up to 1300 µg /g; Windle and Ridal 2019). Modern-day Zone 2 Hg concentrations are variable and range from as low as 0.09 µg/g to as high as 8.0 µg/g (Moir et al 2021). Both Zone 1 and Zone 2 may have been affected by hydrologic change during construction of the St. Lawrence Seaway from 1954 to 1958.…”

“…Over the course of the twentieth century, e uent from waterfront industries loaded organic material and metal(loids), including neurotoxic mercury (Hg), into nearshore depositional basins, and contributed to the region's designation as an Area of Concern (Dreier 2000). Despite industrial improvements beginning in the 1970s, the creation of a remedial action plan in 1992, implementation of a sediment strategy in 2005, and the closure of point-source polluters by 2006 (deBarros and Anderson 2010), legacy contaminants remain in waterfront sediment in concentrations exceeding provincial sediment quality guidelines and elevated compared to upstream reference sites (Moir et al 2021). Additionally, waterfront sh continue to display elevated Hg concentrations (Choy et al 2008; Lapointe and Ridal 2019), re ecting modern-day Hg inputs from local sources (Ramboll Environ 2016).…”

“…Interpreting benthic invertebrate recovery is challenging as the two main depositional basins along the Cornwall waterfront (Zone 1 and Zone 2) were exposed to unique pollution sources and combinations of contaminants. Zone 1 primarily experienced elevated Hg concentrations and organic matter, and still receives modern-day Hg inputs (Moir et al 2021), while Zone 2 primarily experienced heavy metal loading (Zn, Cu, Pb, and relatively lower amounts of Hg; Dreier 2000).…”