Northwest Territories Geological Survey

With recent analytical advances, metal stable isotopes are increasingly being used as tools to trace the sources, transport and cycling of metal contamination in the environment. Coal, bitumen and crude oil are enriched in vanadium, and anthropogenic activities associated with fossil fuel production and use can release vanadium to the atmosphere, soil, and water. The objective of this study was to investigate vanadium as a potential isotopic tracer of metal contamination associated with fossil fuels. The first measurements of vanadium isotope ratios in geological materials by multi-collector inductively coupled mass spectrometer (MC-ICP-MS) were published less than a decade ago, and environmental variation of vanadium isotope ratios remains poorly characterized.

Vanadium isotope ratios were measured in environmental samples from the Oil Sands Region in northern Alberta and from Great Slave Lake in the Northwest Territories. Samples of soil, plant roots, lichen, and American marten liver (Martes Americana) were selected from monitoring programs in northern Alberta to cover a gradient in vanadium contamination (with varying distance from Oil Sands developments). A sediment core from a nearfield lake was examined for variation of vanadium isotope ratios among layers in the sediment depth profile. Additionally, samples of lake sediment and aquatic biota (benthic algae, plankton, and fish) were selected from a monitoring study in Great Slave Lake to track isotopic fractionation of vanadium associated with dietary transfer through an aquatic food web. Vanadium isotope ratios were measured on over 50 environmental samples following chemical isolation of vanadium and detection on a MC-ICP-MS at the Woods Hole Oceanographic Institute (Massachusetts, USA) using published methods.

Distinct vanadium isotope ratios (d51V, reported relative to an Alfa Aesar standard) were measured in lichen (-0.89±0.12‰), lake sediment (-0.50±0.22), soil (-0.68±0.11‰), and martin liver (-1.73±0.17‰) from northern Alberta. The vanadium isotope ratios of lichen, soil, and marten liver did not vary with their concentrations of vanadium or proximity to Oil Sands emissions. The d51V profile in the lake sediment core was relatively homogenous (-0.52 to -0.69‰) between sediment layers of 2 to 14.5 cm depth. However, the surface layer (0-1 cm) had an enriched d51V value of -0.12‰. Plant roots showed heavier d51V values than the sediment they were growing in at four sites. The results of food web samples from Great Slave Lake (currently in progress) will also be presented. These observations indicate considerable variation of vanadium isotopes ratios in soil, lake sediment, and biota (d51V spanning ~2‰) that reflect complex processes of stable isotope fractionation in the environment.