Trace elements in peat bog surface waters as indicators of the dissolution of atmospheric dusts from open-pit bitumen mines

Open-pit mining for bitumen extraction generates considerable volumes of dust in northern Alberta. This area contains abundant peatlands, some of which are ombrotrophic bogs that are exclusively fed by atmospheric inputs. The most reactive mineral phases of dust deposited on these bogs can potentially dissolve in their surface waters because of the low pH and abundance of organic acids. Thus, peat bog surface waters could be used as unique monitors, to determine the chemical reactivity and dissolution characteristics of atmospheric dusts in industrial areas. The main goal of this study is to determine whether the elevated rates of dust deposition to peat bogs near bitumen mines have led to greater concentrations of trace elements (TEs) in the surface waters. To achieve this goal, it is essential to ensure that the TEs being measured in surface waters represent dust dissolution only and are not influenced by other element sources such as groundwaters or surface runoff.
Peat bog surface waters were collected in the autumn of 2019 from four peatlands near industry and a control site located more than 260 km upwind. Concentrations of TEs were determined in the dissolved fraction (i.e. filtered through a 0.45 μm membrane) using an ICP-MS. Surface waters near industry have elevated concentrations of Li, Fe, Mn, Ni, Y, selected REE (Tm, Dy, Yb, Sm) and Pb at 3 out of 4 sites, relative to the control location (UTK). Most of the elements are enriched 2x when compared to control site (UTK), but only Li, Mn, and Rb show enrichments >1Ox. At JPH4, the site closest to industry (12 km from the mid-point between the two central bitumen upgraders), a vegetation survey indicates that this peatland includes both ombrotrophic and minerotrophic zones, and this was confirmed by the pH and concentrations of major ions in the surface waters. At McK, the site next closest to industry (25 km), electrical conductivity as well as concentrations of chloride, Na, and K, all increase with distance toward the highway which is evidence of road salt runoff. Thus, at these two sites nearest industry, the TE concentrations in surface waters are supplied not only from airborne dusts, but contributions from groundwaters and road salt must also be considered. In contrast, the surface waters from the McM (49 km) and ANZ (69 km) sites are ombrotrophic, and elevated concentration of TEs in these samples can be attributed exclusively to dust dissolution.
The elevated concentrations of lithophile TEs in the dissolved fraction includes those which are
mobile in surficial environments (e.g. Li and Sr), but also those which are immobile (e.g. Y and the lanthanides). Elevated concentrations of the former are not surprising, but elevated concentrations of the latter are puzzling, given that they tend to be hosted within stable mineral phases that are resistant to chemical weathering. Size-resolved TE analyses of the dissolved fraction using AF4-ICP-MS will be used to distinguish between colloidal forms and ionic species, to differentiate inputs of nano-dusts from mineral dissolution in bog waters.