Lake water and sediment chemistry responses to reductions in mercury, other heavy metals, and sulfur dioxide emissions from the Flin Flon smelter, Manitoba, Canada, with historic comparisons to other emitters.

For many decades, the copper-zinc (Cu-Zn) smelter at Flin Flon, Manitoba was North America's largest single-source mercury (Hg) emitter, 4th largest Canadian sulfur dioxide (SO2) emitter, and a major heavy metal emitter. Various improvements were made in Cu smelter and Zn refinery operations over the years, and the Cu-smelter was closed in 2010 because it could not meet SO2 reduction targets while the Zn refinery continued to operate. From 2008-2017, we investigated water and sediment responses to emission decreases, comparing our data with that generated by others during studies conducted in the 1970s and early 1980s. Sedimentary metal concentrations remain highly elevated in near-field lakes (≤ 8 km of the smelter) and elevated in mid-field lakes (> 20km and <45 km from the smelter), reflecting the continued entry of metals from the large reservoirs deposited on lake watersheds over the ∼80 years of smelter operations. Similar spatial patterns were observed in water although Hg exhibited less spatial variability. Hg, Pb, and Zn exceeded standard Canadian Probable Effects Level (PEL) guidelines in near-field lake sediments whereas mid-field lakes exceeded Interim Sediment Quality Guidelines (ISQGs). SO4 concentrations in lake waters began declining in the late 1970s, a probable combined response to a 1974 increase in stack height and decreases in long-range S02 sources due to US-Canadian acid-rain controls; concentrations continued to decline over 2011-2017 with the closure of the Cu smelter and reduced SO2 emissions by the smelter and elsewhere. Near- and mid-field lakes remained alkaline while lakes with lower buffering capacity located 68-74 km to the northwest, that were reportedly acidic in 1982, were alkaline in 2008 with pH increasing through to 2016. We compare our findings with observations from four other Canadian smelters, two coal-fired power plants complexes, and the Alberta oil sands, and thereby explore how emitter history, distance from the source, and geological/watershed settings profoundly impact emission effects on sediment and lake chemistry.