COUPLING LEAD ISOTOPE RATIOS AND ELEMENT CONCENTRATIONS IN EPIPHYTIC LICHENS TO TRACK PROCESSES IN THE ATHABASCA OIL SANDS REGION

A study on the coupled use of element concentrations and lead isotope ratios from lichens to assess impacts from oil sands mining and processing operations was conducted on Hypogymnia physodes samples collected in the Athabasca Oil Sands Region (AOSR) in Alberta, Canada during 2008. The lichens selected for analysis were collected using a stratified grid approach, and included 121 sampling sites as far as 160 km from the mining and processing locations. The concentrations of elements incorporated into the lichen tissue reflected source differences, proximity to mining and oil processing sites, topography, ecosystem differences, and the metabolic biogeochemistry of the lichens. Preliminary source apportionment analysis indicate three main element groupings including a fugitive dust factor (Al, Ce, Fe, La, Nd, Ti), an oil processing factor (S, N, Mo, Ni, V) and a factor associated with the nutrient needs of lichens (K, Mn, P, Rb, Zn).The samples collected beyond 50 km from the mining and processing sites cluster into a lead isotope grouping with a ²⁰⁷ Pb / ²⁰⁶ Pb ratio of 0.8650 and a ²⁰⁸ Pb / ²⁰⁶ Pb ratio near 2.105. This grouping likely reflects the regional background lead isotope ratio signature. The lowering of the ²⁰⁷ Pb / ²⁰⁶ Pb and ²⁰⁸ Pb / ²⁰⁶ Pb ratios as one nears the mining operations indicates other Pb source(s), likely related to the oil sands processing are contributing to the observed Pb isotope shift. An exponential decrease in concentration of the metals associated with fugitive dust versus distance from the mining sites, suggests elevated coarse particle emissions associated with mining operations. Near source concentrations of metals with an oil signature are less enhanced than the metals in the fugitive dust, reflecting emission and deposition of smaller diameter particles and gases at greater distances from processing stack sources. The mining and oil processing impacts are superimposed over the elemental signature that reflects the nutrient needs of the lichens. The lead isotope ratios were found to be a better predictor of the extent of the source impacts than element concentrations because the lead isotope ratios are not affected by either the metabolic processing of elements by the lichens or ecosystem controls on atmospheric deposition processes.