SPECIATION AND MOBILITY OF ARSENIC AND ANTIMONY IN THE BAKER CREEK TAILINGS AND PORE WATER, GIANT GOLD MINE, NWT

Mining and processing gold ore at Giant Mine near Yellowknife, Canada, has produced sediments and pore waters with elevated As and Sb concentrations in Baker Creek, which flows through the mine site into Great Slave Lake. Three sites were sampled: 1) Baker Pond, an upstream, unvegetated site underlain by the finest fraction of the tailings, 2) the mouth of the creek, and 3) a densely vegetated section underlain by older tailings. Pore waters were collected using in-situ dialysis arrays that allow for sampling at the centimeter scale. Pore waters were analyzed for As(III/V) and Sb(III/V) redox species, Fe(II/total), anions and cations. Sediment cores were extracted adjacent to peeper sites and extruded at 0.5-1 cm intervals, providing high resolution vertical profiles of solids to accompany those of pore water. As and Sb concentrations range from 0.12-87 mg/L and 0.07-2.8 mg/L, respectively, in pore water, and 332-4834 mg/kg and 40-889 mg/kg, respectively, in sediment. As(III) concentrations are significantly higher than As(V) concentrations in the deeper, more reduced zones (identified as such by increases in Fe(II) and decreases in SO4) at all three sites. Unexpectedly, Sb(V) concentrations are higher than Sb(III) in these reduced zones. Different factors affect the attenuation and release of As and Sb at each site. In Baker Pond, the pore-water profile for Sb indicates a peak concentration (2.77 mg/L) near the sediment-water interface (SWI). The Sb concentration in sediment is also high near the interface (462 mg/kg) but decreases slightly just below the SWI. In contrast, the peak concentration for As is at 17cm below the SWI (75-87 mg/L) and the sediment concentration of As shows a continual increase from 3cm depth to the SWI (1943 mg/kg), corresponding to a continual decrease in pore-water As concentration towards the SWI. In Baker Creek the sediment profiles of As and Sb, when plotted against the immobile elements (Pb, Zn, Cu), show no indication of chemical remobilization post-deposition. This is inconsistent with the pore-water profiles which indicate peak concentrations of Sb(0.14 mg/L) and As(6.4 mg/L) just below the SWI. In the vegetated section of Baker Creek, As and Sb appear to be effectively attenuated at 22cm below the SWI, and is probably related to the presence of organic matter.