Trace metals as indicators of tree rooting in bituminous soils

Abstract

In some reclamation practices, following bitumen mining in the Canadian Boreal forest, overburden containing low concentrations of hydrocarbons (<8% of bulk soil volume) is buried under suitable soils. Residual hydrocarbons may influence the growth of trees used in afforestation of reclaimed sites, thus determining whether roots interact with buried bitumen is a key to predicting reclamation success. As the organic fraction of bitumen is enriched in vanadium, nickel, molybdenum, and rhenium, dendrochemistry may be a method to determine whether tree roots are interacting with bitumen without disturbing soils. We analyzed trace concentrations of these metals in soil, soil pore water, and tree cores of Pinus banksiana growing on natural bitumen deposits and sites free of bitumen. If roots are present within bitumen and passively uptake trace metals during water transport and wood growth, metals enriched in bitumen should be present in higher concentrations in the woody tissue of trees growing in bituminous soils compared to trees growing in bitumen-free soils. Concentrations of nickel in trees growing on shallow bitumen deposits were approximately 3× higher than those growing in bitumen-free soils. The concentration of vanadium (1.33 μg kg −1 in younger wood and 3.21 μg kg −1 in older wood) was also elevated in trees on bituminous sites, though not significantly. Molybdenum concentrations decreased by 25% in trees growing on bituminous sites, likely an outcome of soil pH. This research supports the use of dendrochemistry to investigate, and potentially monitor, tree rooting at depth in substrates with signature metals.

Environmental significance of trace elements in the Athabasca Bituminous Sands: facts and misconceptions

“…Trace elements were determined in tree cores, but also soil and soil solutions. 143 Nickel concentrations in wood from trees growing on shallow bitumen deposits were approximately 3x greater than those growing in bitumen-free soils.…”