Grain-Scale Analysis of Solid-Phase Organic Matter and Arsenic Mobility in Mining-Impacted 1 Sediment from Sub‐Arctic Lakes, Northwest Territories, Canada

Abstract

Arsenic (As) is commonly sequestered at the sediment-water interface (SWI) in mining-impacted lakes through adsorption and/or co-precipitation with authigenic iron (Fe)-(oxy)hydroxides or sulphides. The results of this study demonstrate that the accumulation of solid-phase organic matter (OM) in near-surface sediments also influences the mobility and fate of As in sub-Arctic lakes. Sediment gravity cores, sediment grab samples, and porewaters were collected from three lakes downstream of the former Tundra gold mine, Northwest Territories. Analysis of sediment using combined micro-X-ray fluorescence/diffraction, K-edge X-ray Absorption Near-Edge Structure (XANES), and organic petrography shows that As is associated with both aquatic (benthic and planktonic alginate) and terrestrially-derived OM (cutinite; funginite). Most As is hosted by fine-grained Fe-(oxy)hydroxides or sulphide minerals ( e.g., goethite, orpiment, lepidocrocite, mackinawite); however, grain-scale synchrotron-based analysis shows that As is also associated with amorphous OM. Mixed As oxidation states in porewater (median = 62 % As (V), 18 % As (III); n = 20) and sediment (median = 80 % As (-I) and (III), 19 % As (V); n = 9) indicate the presence of variable redox conditions in the near-surface sediment and suggest that post-depositional remobilization of As has occurred . Detailed characterization of As-bearing OM at and below the SWI suggests that OM plays an important role in stabilizing redox-sensitive authigenic minerals and associated As. Based on these findings, it is expected that increased concentrations of labile OM will drive post-depositional surface-enrichment of As in mining-impacted lakes and may increase or decrease As flux from sediments to overlying surface waters.

Environmental change in Yellowknife Bay, Northwest Territories, over the last two centuries inferred through diatom paleolimnological analysis

“…Organic matter overtime has gradually increased as well, which may result in more decomposition in the hypolimnion which ultimately could also contribute to decreasing oxygen levels in deep water however overall organic matter levels still remain relatively low. The chemical behaviour of arsenic is sensitive to the presence of oxygen and in the absence of oxygen, mineral-bound arsenic in sediment can dissolve and diffuse upwards to overlying water (Andrade et al, 2010;Martin & Pedersen, 2002;McDonald et al, 2015;Palmer et al, 2020) or become bound to sulphides and this can be an effective means of sequestering arsenic into sediments (Miller et al, 2021). As Andrade et al (2010) This was done by studying changes in the diatom assemblage and productivity of the bay over the last century in order to infer historical environmental conditions.…”