Controls on the Spatial Distribution of Arsenic Concentration and Solid-Phase Speciation in Long Lake SedimentsTuesday, November 20, 2018 - 10:40 to 10:59 Theatre 2
Sediment arsenic concentrations in Long Lake, Yellowknife, Canada, which is used for recreational purposes, are elevated relative to Canadian and regional guidelines and exceed 3000 mg/kg in selected samples. This degree of arsenic enrichment can be largely attributed to the aerial deposition of arsenic trioxide from legacy stack emissions at Giant Mine and other former gold-mining operations in the Yellowknife area. Arsenic has persisted in lake sediments as arsenic trioxide for more than 60 years after peak mining emissions, but there is also evidence that arsenic trioxide has geochemically transformed to less bioaccessible arsenic-hosting phases such as arsenic sulphides.
Forty-seven sediment cores were collected in July 2016 as part of a spatial survey to elucidate the physical and geochemical controls on the distribution of arsenic in Long Lake sediments. High-resolution profiles of dissolved arsenic in bottom water and porewater were also collected to determine arsenic remobilization and diffusion rates across the sediment-water interface. Linear regression was used to explore the relationships between arsenic, other relevant geochemical variables, and water depth in all sediment depth intervals.
Arsenic concentrations and solid-phase speciation in Long Lake sediments exhibit considerable lateral and vertical variation. Two distinct types of sediment arsenic concentration profiles were identified and are interpreted to represent areas of sediment erosion and deposition. Multiple linear regression indicates that water depth, as a proxy for sediment texture, is the best predictor of arsenic concentrations in near-surface sediments but is a weaker predictor of arsenic concentrations in deeper sediment layers due to the existence of the two types of arsenic profiles identified in this study. Iron concentration, as an indicator of arsenic, iron, and sulphur co-diagenesis, is a better predictor of arsenic concentration at greater sediment depths.
Sediments are a source of arsenic to surface waters through diffusion-controlled release to bottom water; rates of arsenic release into the water column vary spatially with changes in sediment texture and porosity. In areas of sediment accumulation, rates of solid-phase As burial may exceed diffusion rates. The observed variations in the distribution and mobility of arsenic are interpreted to reflect the interplay between sediment-focusing processes and redox reactions.
The spatial heterogeneity of arsenic distribution observed in this study emphasizes the advantages of a multi-station approach for capturing whole-lake accumulation trends. Understanding the depositional patterns of atmospherically deposited arsenic in sediments is also important in the context of risk assessment, as it highlights areas of lakes where humans may be exposed to high sediment arsenic concentrations and more bioaccessible arsenic-hosting solid phases.