USING METAL CONCENTRATIONS AND LEAD ISOTOPE RATIOS FROM LICHENS TO TRACK EMISSION AND DEPOSITION PROCESSES NEAR THE WORLD'S LARGEST OIL SANDS DEVELOPMENT PROJECT

Epiphytic lichens obtain a significant portion of their nutrients from the atmosphere and are potential bio-monitors of air pollution. A pilot study on the use of lichens to assess impacts from oil sands processing was conducted on samples collected near the world-class Athabasca deposits in Alberta, Canada during 2002. The lichen samples were collected on N-S and E-W transects at distances from as little as 5 km to as much as 120 km from the mining and processing sites. Analysis of the Hypogymnia physodes samples for a suite of trace elements and lead isotope ratios using sector field ICP-MS indicated that elements incorporated into the lichen tissue reflected source differences, proximity to processing sites, as well as the inherent biogeochemistry of the lichens.

Preliminary source apportionment analysis indicate three main element groupings including a geogenic wind-blown dust factor (Al, Ce, Cr, Fe, La, Nd, Sc and Ti), an oil processing signature (S, N, Mo, Ni and V) and an element association related to the nutrient needs of lichens (Ba, Cd, K, Mn, P, Rb and Zn) that is likely related to wildfires and stemflow. 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 approaches the mining operations indicates other Pb sources, likely related to the oil sands processing, are contributing to the Pb isotope signature. An exponential decrease in concentration of the geogenic grouping of metals versus distance from the mining sites, suggests that most of the wind-blown dust was emitted as coarse particles from abrasion processes. Near source concentrations of metals with an oil signature are less enhanced than the geogenic materials, reflecting emission and deposition of smaller diameter particles 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. Based on proximal versus distal differences in lichen tissue geochemistry, anthropogenic impacts may be influencing nutrient processing by lichens.