Arsenic concentrations and solid phase speciation in soils in the Yellowknife region

Abstract

Roasting of gold-bearing arsenopyrite (FeAsS) ore at mines around Yellowknife resulted in the release of arsenic trioxide (As2O3) via airborne emissions. Previous studies highlighted a persistent elevated arsenic (As) in local lake sediments and surface waters 50 years after the bulk of these emissions were released. The objectives of this research are to: 1) characterize regional concentrations of As in Yellowknife area soils; 2) identify factors that explain regional patterns of distribution; and 3) distinguish between natural and anthropogenic As sources in undisturbed soils. Total element analyses were completed for 125 Public Health Layer (PHL) (top 5 cm of cored soil samples), 43 grab samples and 37 samples taken from greater depths in cores, or down core (DC) samples. Total As concentrations ranged from 1.5 to 4900 mg/kg in PHL samples, 12 to 930 mg/kg in grab samples, and 1.6 to 1200 mg/kg in DC samples. The most significant factors on As concentrations in PHL samples include the distance and direction from former mine roasters, soil depth, and terrain unit. Fifty samples were selected for scanning electron microscopy and automated mineralogy (SEM/AM) to identify the solid species of As present within soil units. Anthropogenic sources of As are characterized by the presence of As2O3 and distinctive As-bearing Fe-oxides derived from roaster stack emissions. The most common As-bearing phases identified in soils throughout the study area included: As-bearing Fe-oxides/Fe-oxide mixes (present in 98% of samples), organic Fe-oxide mixes with As (98% of samples), As2O3 (82%), and As-bearing Mn-oxides (60% of samples). Findings from this thesis suggest that previously estimated background levels for As ranging from 0 to 300 mg/kg are too high; median total As-concentrations decrease from 130 mg/kg to 22.5 mg/kg and finally 8.1 mg/kg for samples collected at distances of 0 – 10 km, 10 – 20 km and 20 – 30 km from the Giant roaster. Additional sampling initiatives need to target soils at further distances to determine an appropriate total As background concentration that can be used to help guide future risk assessment recommendations for the Yellowknife area.