Characterisation of mineral forms of arsenic in garden soils from a historic gold mining region

Soils in the Yellowknife region were contaminated with arsenic by >50 years of atmospheric mining emissions from ore roasting operations. Persistent community concern regarding contamination of local garden soils prompted this investigation. One hundred and fifteen soil samples were collected from 110 resident gardens and were analysed by inductively coupled plasma mass spectrometry for elemental analysis. Arsenic concentrations were below local remediation soil quality guidelines for residential areas (160 mg kg⁻¹) in all soil samples, but 62 % of samples exceeded national Canadian soil quality guidelines (12 mg kg⁻¹) for the protection of environmental and human health. Ten samples, with relatively high arsenic concentrations, were analysed by scanning electron microscope with automated mineralogy to identify solid phase arsenic hosts; four samples were further analysed by synchrotron-based microanalysis to identify crystal structure of target mineral grains. The predominant mineral host of arsenic in the garden soils was identified as arsenopyrite, which could be geogenic, anthropogenic (e.g., repurposed mine waste), or both. Arsenic trioxide from ore roaster stack emissions was identified in five garden soils mineralogically analysed. Arsenic-bearing iron oxides were detected in nine of the soils mineralogically analysed; in three of these soils, roaster-generated iron oxides generated by ore roasting were identified. Garden soil arsenic concentrations (14 mg kg⁻¹ median) were substantially lower than values determined by a previous study for undisturbed, Public Health Layer soils in the region (390 mg kg⁻¹ median); likely, mixing of surface soils with soil beneath, and use of purchased soils in gardening has dispersed the surficial arsenic enrichment consequent of ore roasting.