Arsenic from Legacy Roaster Emissions in Soils in the Yellowknife AreaTuesday, November 20, 2018 - 09:40 to 09:59 Theatre 2
The objective of this research was to investigate concentrations and speciation of arsenic in soils within 30 km of Yellowknife to identify whether arsenic in soils is from a natural geologic source or is derived from past industrial activities. Over 400 samples were collected as soil cores from locations not on mine property and undisturbed by buildings, roads, mining or other visible human activities. This was done to minimize the influence of recent post-mining activities and examine the effect of natural processes and the legacy of airborne emissions from former ore roasting. Most of the analyses have been conducted on the Public Health Layer (top 5 cm). The concentration of arsenic in these near-surface soils measured up to 4700 mg/kg with some of the highest concentrations found near the new Giant Mine bypass road west of the former Giant Mine roaster. Forty soil samples collected from below the Public Health Layer (approximately 10 cm to 40 cm below the surface) were analysed and are lower in total concentration of arsenic than the corresponding samples from the Public Health Layer, with the exception of two samples.
Mineralogical analysis was conducted on almost 100 soil samples and, of these, 80% contained arsenic trioxide, indicative of roaster stack emissions. The common occurrence of arsenic hosted as arsenic trioxide also explains why arsenic concentrations are highly variable at the local scale, and even between field duplicates, likely because of the uneven distribution of very arsenic-rich compounds in the soil samples. Another roaster-generated compound, iron oxide, was also recognized in the soils. Arsenic content in iron oxides from samples collected near Con Mine was significantly less than arsenic content in iron oxides from Giant Mine, offering a potential avenue for distinguishing contamination between the two mine sites.
Remediation guidelines for arsenic in Yellowknife area soils were established based on an estimated natural background concentration of 150 mg/kg with a reasonable upper limit (90th percentile) of 300 mg/kg. The current guidelines of 160 mg/kg (residential) and 340 mg/kg (industrial) were largely based on this background estimate and the assumptions that: 1) there was limited public access to arsenic-bearing soils; and 2) that exposure would be limited by the cold climate. These new results indicate many areas within and near Yellowknife where soil concentrations in the most accessible part of the soil profile are much higher than current guidelines and that arsenic is hosted primarily as arsenic trioxide, which is considered one of the most bioaccessible forms of solid phase arsenic. In light of this new information, it is suggested that the remediation criteria for soils in the Yellowknife area be revisited to reflect potential increased public exposure to high arsenic soil in the region.