The atmospheric transport and deposition of smelter emissions: evidence from the multi-element geochemistry of snow, Quebec, Canada

Abstract

Abundances of 35 elements and ions were determined by inductively coupled plasma mass spectrometry, inductively coupled plasma atomic emission spectroscopy, and ion chromatography, in samples of snowpack obtained using ultraclean procedures, along three 50-km radial transects emanating from the Horne base metal smelter in Rouyn-Noranda, Quebec. This was done to constrain: what is emitted; regional background concentrations of elements in snow; the rate of anthropogenic deposition on the surrounding landscape; how far emissions are transported; and what processes control their deposition. The approach used is at least as effective as methods based on isotopic ratios but has the added advantage of providing direct constraint on a large suite of elements, and of being independent of changes in the isotopic composition of emissions. Reproducibility of the method was determined using field duplicates and blanks. This accounts for snow heterogeneity, analytical precision, reagent quality, and sample handling and was determined to be ±20%. Concentrations of Cu, Pb, and Zn near the smelter are 525, 353, and 149 μg/L meltwater, respectively, and drop to 2.1, 3.5, and 3.1 μg/L by 45 km distance. The level of enrichment of these elements, relative to upper crustal abundances, similarly drops from about 10,000 to 100 from proximal through to distal sites, forming an “enrichment gradient.” This is the result of a binary mix of smelter emissions and regional background concentrations. Elements that display a significant gradient must therefore be emitted by the smelter. This method shows that Cu, Ag, In, Sb, Pb, As, Tl, Mo, Zn, Cd, Co, Be, Ni, Na, Ba, Fe, Cr, V, Ti, Y, Al, U, Ce, Li, S, La, and Sr are deposited on the landscape by the smelter. For many elements, the slope of this binary mixing line is still non-zero at a distance of 50 km, indicating that the impact of the smelter is still detectable, and that regional background levels have not yet been reached. Data from more distant sites are used to estimate regional background concentrations to be 1.1, 1.7, and 1.6 μg/L meltwater for Cu, Pb, and Zn, respectively. We quantify the partitioning of elements between the dissolved phase (meltwater) and the solid phase (particulates) in melted snow. It varies with distance from the smelter but is element specific. Representative end-members are: Tl, Cu, Pb, Zn, and Sb with 78, 68, 67, 55, and 20% in the dissolved phase near the smelter, respectively, vs. 4, 60, 73, 80, 58% at distances greater than 35 km. This is due to changes in mineralogy and particle size. The significance of this is, that for those elements with high variation such as Tl and Sb, only the total load (meltwater + particulate) can reliably reflect atmospheric deposition. For those elements that vary little such as Cu and Pb, measurement of just the dissolved load (meltwater) can be used as an adequate proxy for the total deposition. Observed patterns of deposition suggest that inside 15 km radius, it occurs by wet+dry mechanisms whereas beyond 15 km wet deposition dominates. Emissions transported beyond 15 km are therefore available for long-range transport during dry weather and likely have an atmospheric residence time similar to that of water vapour. A well fitting mathematical model supports this interpretation. Using modelled background concentrations, the model can account for only 49, 15, 23, and 9% of reported emissions of Cu, Pb, Zn, and Cd within 50 km of the smelter, respectively. Using lower (minimal) background concentrations based on observations at more distant sites, these numbers increase to 78, 19, and 43%, respectively (Cd excluded), and can be considered maximums. The rest of the emissions are therefore transported further distances during dry weather.