SPECIATION, TRANSPORT, AND FATE OF ARSENIC IN THE SEAL HARBOUR GOLD DISTRICTS, NOVA SCOTIA, CANADA

Arsenopyrite occurs naturally in Cambro-Ordovician lode gold deposits in Nova Scotia, and is present at high concentrations in the tailings at past-producing mines. This study is a multi-disciplinary investigation of the dispersion, speciation and fate of metal(loid)s in the environment surrounding the Upper and Lower Seal Harbour mining districts. Early gold recovery efforts (1893-1927) employed stamp mills and Hg amalgamation, whereas later operations (1936-1941) used mainly cyanidation. More than 500,000 tonnes of tailings from these mills were slurried into local watercourses, and have been transported more than 2 km to the ocean where they form an intertidal tailings flat. From 2003 to 2005, samples of tailings, soil, till, rock, sediment, water, and biota were collected throughout the Seal Harbour districts. Chemical analyses of 200 tailings and stream sediment samples show high concentrations of both As (39 ppm to 31 wt.%; mean 1 wt.%) and Hg (12 ppb to 120 ppm; mean 3 ppm). The speciation of As in these samples was examined using electron microprobe, conventional and synchrotron-based X-ray diffraction, X-ray absorption near edge structure, and sequential extractions. In addition to As-bearing sulfides, As is hosted by a variety of secondary phases such as scorodite (FeAsO₄·2H₂O), amorphous Fe arsenates, Ca-Fe arsenates, and Fe oxyhydroxides. Dissolved As concentrations in surface waters and tailings pore waters indicate that the tailings continue to release significant quantities of As. In the Summer, dissolved As concentrations in surface waters range from 20 to 6200 ppb, as compared to background values of <2 ppb. In the Spring and Fall, dissolved As concentrations are generally 2-3 times lower than in the Summer. Measurements of As(III) and As(V) show that As(V) constitutes more than 90% of the dissolved As in most surface waters. Analyses of sediments and water in the adjacent marine environment indicate that Seal Harbour is an effective trap for tailings, and As-enriched floc formed upon mixing of river water. Biological sampling by project partners has demonstrated that both As and Hg have bioaccumulated to various degrees in terrestrial and marine biota, including eels, clams, and mussels. A bivalve shellfish closure is now in effect for Seal Harbour to prevent harvesting of As-contaminated clams.