Assessing and reducing risks at the Montague and Goldenville gold districts in Nova Scotia

Recent studies at historical gold mines in Nova Scotia have identified several areas where exposure to mine wastes may pose a risk to both ecosystem and human health. Arsenopyrite (FeAsS) occurs naturally in the ore and surrounding bedrock in these gold deposits, and was concentrated in the tailings during milling operations. The concentration of arsenic (As) in tailings at these sites is generally two to four orders of magnitude higher than the 12 mg/kg Canadian Soil Quality Guideline for As in residential and parkland soils. Two sites, Montague and Goldenville, are of particular concern, as the tailings are located close to residential properties and are occasionally used for racing off-road vehicles. In 2005, the Province of Nova Scotia established the Historic Gold Mines Advisory Committee to examine these risks in more detai (hyyp://www.gov. ns.ca/nse/contaminatedsites/goldmines.asp). Since that time, research has been carried out to examine the concentration, solid-phase speciation and bioaccessibility of As in tailings, airborne particulates and forest soils near these sites to clarify the spatial extent of mine tailings, the mineral hosts for As, and the fate of windblown tailings dusts. Environmental Site Assessments have also been completed at Montague and Goldenville to examine the concentrations of As and Hg in tailings and soils near residential areas. Delineation of the area impacted by tailings requires an understanding of the naturally occurring concentrations of As and mercury (Hg) in soils overlying the variably mineralized bedrock within these gold districts. In 2007, Natural Resources Canada (NRCan) collected samples of the top 0-5 cm of surface soil (the Public Health layer) from 46 sites near Montague, and 39 sites near Goldenville. Samples of individual soil horizons (H, Ae, B, and C) were also collected from 10 sites in Montague and 6 sites in Goldenville to evaluate the vertical distribution of elements in the soil profile. All samples were air dried, sieved to various grain size fractions (<2 mm, <150 µm, <63 µm), and digested and analyzed for metal(loids) and organic carbon using protocols commonly employed during environmental assessments (e.g. EPA Method 3050B). Results from these surveys show that the concentrations of As and Hg in all soil horizons are generally higher down-ice (south) of the ore zones in both districts, reflecting glacial erosion and transport of mineralized bedrock containing arsenopyrite and other sulphide minerals. Analysis of the top 0-5 cm of soils shows the following ranges in As and Hg concentrations (<2 mm, HNO3-H2O2 digestion): Montague: As, 2-273 mg/kg (median 40 mg/kg); Hg, 72-490 ?g/ kg (median 164 ug/kg); Goldenville: As, 2-140 mg/kg (median 13 mg/kg); Hg, 60-312 ug/kg (median 123 ug/kg). In general, the concentrations of As are highest in the B and C horizon soils, whereas Hg concentrations are highest in the organic-rich humus (H) layer. Data for As and Hg in soils from Montague are in close agreement with results from previous soil surveys in nine gold districts conducted by the Nova Scotia Department of Natural Resources in 2003-2005. However, as compared to Montague, the concentrations of both As and Hg are significantly lower in most soil horizons at Goldenville. To better understand the solubility of As in mine tailings and soils at these sites, NRCan partnered with Queen's University and the Royal Military College to characterize the mineralogy and bioaccessibility of arsenic in a suite of 29 samples. The solid-phase hosts of As were determined using a micro-analytical method designed to characterize complex samples at the micron scale combining petrography, electron microprobe, and synchrotron-based grain-by-grain microXANES (X-ray absorption near-edge structure) and microXRD (X-ray diffraction). Mineralogical analyses of the tailings and windblown dusts show that As is hosted in arsenopyrite and a variety of weathering-related phases including scorodite (FeAsO4·2H2O), Ca-Fe arsenates, and As bound to Fe oxides. Bulk XANES shows arsenic in these near-surface samples is mainly in the pentavalent form (As5+), indicating that most of the arsenopyrite (As1-) originally present in the tailings and soils has been oxidized during weathering reactions. The in vitro bioaccessibility of As in the samples was measured using a physiologically-based extraction test (PBET) and ranges from 0.5 to 49% of total As. A weak negative correlation was observed between total and bioaccessible As concentrations, and the As bioaccessibility was not correlated with other elements. The highest As bioaccessibility is associated with the presence of Ca-Fe arsenate minerals. Samples containing As predominantly as arsenopyrite or scorodite have the lowest bioaccessibility (<1%). Other As species identified (predominantly amorphous iron arsenates and arsenic-bearing iron(oxy)hydroxides) are associated with intermediate bioaccessibility (1 to 10%). The presence of a more soluble As phase, even at low concentrations, results in increased As bioaccessibility from the mixed As phases associated with tailings and mine-impacted soils. Remediation strategies for high-As mine wastes at publicly accessible sites like those in Nova Scotia typically employ clean soil covers to reduce human exposure and dust generation. However, burying the tailings under soil may trigger dissolution of the As-bearing minerals and lead to accelerated release of As to local streams and groundwater. Other conventional tailings remediation designs such as flooding, removal or fencing are also problematic because of the high solubility of some As minerals, dust hazards, expenses associated with removal, and community desire to maintain site access. Our ongoing research at Montague and Goldenville uses laboratory experiments and field tests to investigate the biogeochemical stability of different tailings types to design the best plan to protect downstream surface and ground waters and reduce risks to human health. This research will provide experimentally tested recommendations applicable to many of the thousands of active and abandoned mine sites across Canada.

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