Soil from the Yellowknife, NT Region: Spatial Distribution of Arsenic, Characterization of Solid Phase Arsenic Hosts, and Distinguishing Giant and Con Mine Contamination

Abstract

Historical gold mining in the Yellowknife, Northwest Territories region has led to a legacy of arsenic contamination in the region. Roasting of arsenopyrite hosting gold ore released arsenic trioxide (As2O3) via airborne emissions. Recent studies have highlighted the persistence of As2O3 in local sediments and surface waters. However, questions remain regarding the regional extent and nature of arsenic in soils from the region. The main objective of this research is to report the concentration and speciation of arsenic in 311 near-surface soil samples collected within 30 km of Yellowknife. Soil samples were cored from locations that were undisturbed by recent human activities to minimize the influence of recent post-mining activities and to examine the effect of natural processes and the legacy of airborne emissions from former ore roasting. Analyses in this study focused on the Public Health Layer (PHL), which is defined as the top 5 cm of material. The arsenic concentrations for the region varied widely, ranging from 1.0 to 4,700 mg/kg. Statistical analysis indicates the distance from former ore roasters, soil horizon depth, terrain unit, and the relative direction the sample was collected from Giant Mine are the most significant factors on arsenic concentrations in the PHL. The dominant arsenic species in the soil samples are roaster-derived iron oxides containing arsenic, As2O3, and natural iron oxides. Of the samples completed for detailed mineralogy, 57% (n = 44) of samples analyzed contain 80% or greater anthropogenic arsenic (i.e. a combination of roaster-derived iron oxides and arsenic trioxide). These data suggest the current background arsenic value of 150 mg/kg (GNWT, 2003), which is over 12 times the Canadian Environmental Quality Guideline of 12 mg/kg for soil (CCME, 2015), should be revisited. Additionally, the remediation guidelines of 160 and 340 mg/kg for residential and industrial areas, respectively, (GNWT, 2003) should also be revisited. This study provides data that can support future risk assessments to human and ecological health from arsenic-derived stack emissions.