Air–surface exchange of mercury in natural and anthropogenically impacted landscapes in Atlantic Canada

Abstract

Understanding the relative importance of various sources of mercury within ecosystems and their subsequent impact on biota requires a thorough understanding of the biogeochemical cycling of mercury. An important component of the biogeochemical mercury cycle involves the exchange of gas-phase mercury between the atmosphere and various landscape surfaces. This study examines air–surface exchange (flux) of gaseous elemental mercury in selected undisturbed and anthropogenically impacted aquatic and terrestrial landscape settings. The objective of this study was to quantify air–surface mercury exchange rates in these contrasting landscape settings, to put constraints on these rates and provide insight into physical processes controlling mercury flux. Mean daily mercury flux was typically low over natural forest soils varying from −0.4 (net deposition) to 2.2 ng m −2 h −1 (net evasion). Minimum and maximum flux rates measured at these sites over the diurnal cycle ranged from a minimum of −1.3 to a maximum of 5.7 ng m −2 h −1 . Low flux rates from forest soils was partially due to poor light penetration through the forest canopy. Average daily mercury flux over undisturbed glacial till soil at an open field site with full sun exposure was also low (0.9 ng m −2 h −1 ) compared to the same soil mixed over a depth of 2 m (8.0 ng m −2 h −1 ). Mean daily mercury flux rates measured over pristine freshwater lake surfaces were also low (0.7–6.5 ng m −2 h −1 ), although the overall range in flux varied more widely (−0.3 to 44 ng m −2 h −1 ). Total mercury concentrations in marine water at a moderately polluted coastal harbour site (mean c. 0.8 ng l −1 ) and mean daily mercury flux rates over salt water (0.7 ng m −2 h −1 ) were similar to those measured over freshwater lakes. The highest daily average and diurnal range in mercury flux rates were measured at two abandoned gold mine tailings sites (130 and 237 ng m −2 h −1 ). The abandoned mine sites comprised tailings high in mercury content due to the past use of the mercury amalgamation process in gold extraction from the ore. Flux rates from tailings were two orders of magnitude higher than those observed over undisturbed native soils of similar parent material. Higher flux rates at the mine tailings sites were accompanied by ambient air concentrations 5–10 times background levels at 20 cm above the tailings. Mercury flux from vegetation has not been widely considered. A preliminary study of mercury flux from a white pine tree ( Pinus strobus ) indicated that actively growing trees might play a role in the atmospheric mercury cycle.

Gaseous mercury flux measurements from two mine tailing sites in the Seal Harbour area of Nova Scotia

“…The mid-day flux-rates for both days at this site were similar, although somewhat higher on the second day of sampling (1292 ng m −2 hr −1 vs 1120 ng m −2 hr −1 ). The flux-rate measured at Site 2 is similar to levels reported by Beauchamp et al (2002) from measurements at another abandoned mine tailings study in Goldenville, Nova Scotia. The more recent studies by Wang et al (2005 and in China and Eckley et al (2011) in Nevada have focused on active mine sites with Hg flux-rates much higher that observed at this abandoned (>90 yrs) mine tailings site.…”

“…The Caribou and Goldenville mine sites have a very similar gold mining history (Beauchamp et al 1998;Sunderland & Chmura 2000;Beauchamp et al 2002;Wong et al 2002) to Goldboro, while the two sites at Kejimkujik are areas characteristic of natural geology reflecting background Hg flux and TGM levels. The flux-rate and TGM data (TGM -measured near the soil surface) at Goldenville and Caribou (TGM ranges from 3.5 to 8.4 ng m −3 and Hg flux values of 130 to 237 ng m −2 hr −1 ) are similar to what we see in this work in Goldboro (2.6 to 11.9 ng m −3 for TGM and flux-rate of 18 to 652 ng m −2 hr −1 ).…”

Gaseous mercury emissions from natural sources in Canadian landscapes

“…Terrestrial locations were chosen to include sites with naturally elevated mercury concentrations in the substrate, and sites with background concentrations (less than 100 ppb) at locations that are remote from industrial point sources, where long‐range atmospheric transport would be the only possible anthropogenic source of mercury. More detailed rationales for site selection are given by Poissant and Casimir [1998], Rasmussen et al [1998b], Poissant et al [2000], and Beauchamp et al [2002]. Freshwater sites were selected based on dissolved organic carbon (DOC) content covering a range from low to high DOC, while the Halifax Harbour site was selected as a marine ecosystem moderately impacted by urban sewage and industrial activity [ Beauchamp et al , 2002].…”

“…More detailed rationales for site selection are given by Poissant and Casimir [1998], Rasmussen et al [1998b], Poissant et al [2000], and Beauchamp et al [2002]. Freshwater sites were selected based on dissolved organic carbon (DOC) content covering a range from low to high DOC, while the Halifax Harbour site was selected as a marine ecosystem moderately impacted by urban sewage and industrial activity [ Beauchamp et al , 2002].…”

Photoreducible Mercury Loss from Arctic Snow Is Influenced by Temperature and Snow Age

“…47 for terrestrial and freshwater Hg(0) flux, and O'Driscoll et al 48 for freshwater−water Hg(0) flux. Prior to use, the chamber was acid washed, triple rinsed with Milli-Q water, and blanked by placing the chamber over an acid washed Milli-Q rinsed Teflon sheet and flushing with zero air, until Hg(0) concentration measured with a Tekran 2537B fell below the instrumental detection limit.…”