Mercury Isotope Variations in Lake Sediment Cores in Response to Direct Mercury Emissions from Non-Ferrous Metal Smelters and Legacy Mercury Remobilization

Abstract

Nature archives record atmospheric mercury (Hg) depositions from directly emitted Hg and re-emitted legacy Hg. Tracing the legacy versus newly deposited Hg is still, however, challenging. Here, we measured Hg isotope compositions in three dated sediment cores at different distances from the Flin Flon smelter, the largest Canadian Hg sources to the atmosphere during the 1930s–2000s. During the smelter’s operative period, Hg isotope compositions showed limited variations in the near-field lake (<10 km) sediments but were rather variable in middle- (20–75 km) and far-field lake (∼800 km) sediments. Only the post-2000 sediments in middle/far-field lakes showed significantly negative Hg isotope shifts, while sediments from the 1970s–1990s had Hg isotope values resembling those of near-field lake post-1930 sediments. We suggest that the smelter’s peak Hg emissions during the 1970s–1990s, which coincided with the deployment of a super stack in the mid-1970s, largely increased the long-range dispersion of smelter plumes. For the top post-2000 sediments, the fugitive dust from ore tailings and terrestrial legacy Hg re-emissions dominated Hg deposition in near-field lakes and middle/far-field lakes, respectively. Our study demonstrates that legacy Hg remobilization now exports substantial amounts of Hg to ecosystems, highlighting the need for aggressive remediation measures of Hg-contaminated sites.

Tracing the sources and depositional history of mercury to coastal northeastern U.S. lakes

“…Local emissions from urban centers and smelters typically have less radiogenic 206 Pb /207 Pb ratios similar to leaded gasoline and coal combustion, 90 and the 1940 peak in radiogenic Pb is not found in other lakes in the northeastern United States. 27,28,85 The negative peak in δ 202 Hg is also difficult to tie to a local emission source; Hg isotopes from point sources such as smelter operations can have distinct δ 202 Hg and Δ 199 Hg from those typical of bulk anthropogenic emissions, 49 but there are no known historic smelter operations within the region. The peaks may therefore reflect a local re-release of deposited Pb and Hg.…”

“…Terrestrial re-emission of Hg was recently shown to become the dominant source of deposition to lakes downwind of a point source, long after smelter emissions ceased. 49 Continued Pb deposition at remote sites, has also raised the possibility of watershed recycling of Pb, or of small, unaccounted emissions of Pb in modern times. 28 The continued presence of anthropogenic Hg and Pb, even at remote sites with no landscape disturbance, implies that long recovery times as well as stringent emissions standards are needed to control Hg contamination in remote lakes environments.…”

“…18,34,48 However, regional Hg sources, such as plumes from smelting operations, have recently been shown to have low d 202 Hg values, distinct from coal combustion sources. 49 Positive shis in D 199 Hg and D 200 Hg with increased Hg ux during industrialization have also been frequently observed in sediment archives, but have been attributed to changes in atmospheric and watershed processes as well as anthropogenic Hg inputs. 18,33,34,50 In recent years, decreases in Hg accumulation rates have been observed in several, but not all, northeastern United States and Canadian lake sediment records, 13,15,27,[51][52][53] corresponding to regional and national emissions reductions, and despite an increase in global emissions over this period.…”

“…18,34,48 However, regional Hg sources, such as plumes from smelting operations, have recently been shown to have low δ 202 Hg values, distinct from coal combustion sources. 49 Positive shifts in Δ 199 Hg and Δ 200 Hg with increased Hg flux during industrialization have also been frequently observed in sediment archives, but have been attributed to changes in atmospheric and watershed processes as well as anthropogenic Hg inputs. 18,33,34,50…”

“…2 Re-emission of deposited anthropogenic Hg can also be a substantial source of Hg following the closure or clean-up of an emission source. 49 Together, these sources and delivery pathways complicate the environmental response to decreased atmospheric emissions.…”