The Mobility of Arsenic in the Watershed of a Small Subarctic Lake Impacted by Mining Pollution: What Does This Mean for the Long-Term Fate of Arsenic in the Yellowknife Area?Tuesday, November 20, 2018 - 11:00 to 11:19 Theatre 2
The early years of historical mining activities in the Yellowknife region resulted in the release of large amounts of arsenic, antimony, and metals to the surrounding area. Sixty years after the bulk of these emissions were deposited large amounts of arsenic and antimony remain in lake sediments and soils in the region, and surface waters of many small lakes continue to exhibit elevated concentrations of these metalloids. Understanding the chemical recovery of small lakes from mining pollution in the region requires the consideration of processes occurring within lakes and their surrounding catchment. Studies that integrate these processes provide important information on the long-term fate of arsenic in impacted subarctic environments.
This presentation will draw on results from year-round sampling in the watershed of a small shallow lake (1.2 km2; < 3 m maximum depth) to discuss the various fluxes of arsenic in a landscape impacted by 50 years of mining pollution. Hydrologic inputs and outputs of arsenic from the lake were measured by combining bi-weekly chemical sampling and continuous flow measurements at the lake inflow and outflow. The flux of arsenic between lake sediments and the overlying water column was measured using a combination of porewater extraction techniques and experimental field incubations of lake sediments. The contribution of arsenic from surface runoff from the surrounding catchment was estimated in a small subcatchment by measuring discharge volume and chemistry. Contemporary atmospheric loading of arsenic to the watershed was measured by collecting rain and snow for chemical analyses.
Seasonality is an important feature of subarctic environments and early results from this study show that the mobility of arsenic varies across landscape units and is seasonally dependent. Lake sediments were a small source of arsenic to overlying water during the open-water season when lake water is well-oxygenated. These sediments became a substantial source of arsenic by mid-winter once anoxic conditions developed at the sediment boundary, and water column arsenic concentrations increased almost three-fold compared with late summer measurements (September: 50 µg/L - April: 141 µg/L). In spring, lake water arsenic concentrations decreased rapidly to less than 40 µg/L once snowmelt entered the lake but prior to the loss of ice cover and peak freshet at the lake outlet. Terrestrial contributions of arsenic to the lake via surface runoff were isolated to the snowmelt period in early May and during record precipitation periods in June and July. Loading estimates during these periods indicate that substantial amounts of arsenic continue to be transported from the terrestrial to aquatic environment.
These observations highlight the importance of considering environmental processes across seasons in evaluating the long-term fate of arsenic in shallow lakes in the region. The annual remobilization of sediment As into overlying waters under ice may be a significant process inhibiting the long-term recovery of mine-impacted lakes since it does not typically coincide with periods of high flow at lake outlets. Large winter increases in lake water arsenic also suggest that winter processes should be considered when evaluating exposure of aquatic life to arsenic.