VARIABLE VADOSE ZONE SATURATION CONDITIONS AFFECTING ARSENIC MINERALOGY & MOBILITY IN HISTORICAL GOLD MINE TAILINGS

Historical gold mine tailings in Nova Scotia, Canada have been weathering for over 70 years. Two of these sites, Montague and Goldenville, are publicly accessible and pose a threat to local human and ecosystem health due to high As concentrations associated with the tailings (max 25 wt.% As in solids and 100 mg/L As in pore waters). Both sites are presently being considered for remediation in the near future. However, hydrogeological assessment of these sites, an important factor in remediation decisions, has been limited. The objective of this work is to correlate groundwater flow characteristics with tailings pore water geochemistry and mineralogy.

The tailings at Montague and Goldenville have relatively low topography with distinct unsaturated and saturated areas that relate to differences in mineralogy and geochemistry. Three areas of variable vadose zone saturation have been identified. The topographically highest parts of the tailings have the deepest water table (0.5 – 3 m below surface). Periodic dry conditions over the last seven decades have led to intense oxidation of primary sulfides and supersaturation of pore waters with respect to oxidized As phases (e.g. scorodite, FeAsO₄·2H₂O), as well as the formation of hardpans consisting of cemented tailings. These pore waters have an average Eh of 600 mV. Conversely, in areas of fluctuating water table levels tailings display gleyic color patterns, hardpans are poorly cemented, and Fe oxide-coated root casts occur in near-surface unoxidized tailings. These features reflect mixtures of oxidized and reduced As hosts as a result of wetting-drying cycles. The third type of vadose zone conditions are areas of prolonged saturation. This occurs in the topographically lowest areas of the tailings where the water table is at surface for extended periods of time. Tailings here remain saturated long enough for near-surface zones of relatively reducing conditions (average Eh 145 mV) to develop and support the precipitation of reduced As-bearing phases (e.g. secondary Fe sulfides) near surface.

A shallow water table throughout these tailings deposits has contributed to a continuum of saturation and redox conditions, and As hosts. These mineralogical differences, combined with knowledge of seasonality and groundwater flow direction, can inform remediation decisions.