Abstract
Historical gold mining operations between the 1860s and 1940s have left substantial quantities of arsenic- and mercury-rich tailings near abandoned mines in remote and urban areas of Nova Scotia, Canada. Large amounts of materials from the tailings have entered the surface waters of downstream aquatic ecosystems at concentrations that present a risk to benthos. We used paleolimnological approaches to examine long-term trends in sedimentary metal(loid) concentrations, assess potential sediment toxicity, and determine if geochemical recovery has occurred at four lakes located downstream of three productive gold-mining districts. During the historical mining era, sedimentary total arsenic and mercury concentrations and enrichment factors increased substantially at all downstream lakes that received inputs from tailings. Similarly, chromium, lead, and zinc concentrations increased in the sediments after mining activities began and the urbanization that followed. The calculated probable effects of concentration quotients (PEC-Qs) for sediments exceeded the probable biological effects threshold (PEC-Q > 2) during the mining era. Although sedimentary metal(loid) concentrations have decreased for most elements in recent sediments, relatively higher PEC-Q and continued exceedance of Canadian Interim Sediment Quality Guidelines suggest that complete geochemical recovery has not occurred. It is likely that surface runoff from tailing fields, urbanization, and climate-mediated changes are impacting geochemical recovery trajectories.
Introduction
Historical gold-mining operations have increased metal(loid) concentrations in freshwater sediments from Canada (Azcue et al. 1995; Galloway et al. 2018; Tenkouano et al. 2019; Cheney et al. 2020) and around the world (Odumo et al. 2014; Pinedo-Hernández et al. 2015; Moreno-Brush et al. 2016; Hillman et al. 2017; Yaraghi et al. 2020). The mineralization of gold-bearing ores and various ore-processing approaches (e.g., roasting, crushing, amalgamation) used to extract gold produce finely-ground tailings elevated in toxic metal(loid)s, which can be transported to waterbodies via surface runoff or aerial deposition (Azcue et al. 1995; Pelletier et al. 2020; Perrett et al. 2021). Materials released from gold-mine tailings and mineral-processing by-products can accumulate in aquatic sediments, thus polluting benthic habitats. Direct exposure to mining wastes elevated in metal(loid)s is greatest for sediment-dwelling organisms, and thus aquatic ecosystems can be altered over time (Nasser et al. 2016; Thienpont et al. 2016; Stewart et al. 2018; Little et al. 2020).
Some of the earliest and largest gold mining operations in Canada began in the province of Nova Scotia around the mid-19th century (Cranstone 2002). Historical mining operations played a key role in the economic development of Nova Scotia, which spanned eight decades, with three major gold mining rushes occurring between the 1860s and 1940s (Bates 1987). Historical mining activities were decentralized and occurred in 64 historical gold districts across Nova Scotia. Gold-bearing ores in Nova Scotia are often associated with arsenopyrite, and gold was extracted predominantly by the crushing of ores followed by mercury amalgamation. This approach produced massive quantities of tailings elevated in arsenic (As) and mercury (Hg), among other mining-associated contaminants (Parsons et al. 2012). In some Nova Scotia gold districts, cyanidation was also used after the 1890s to enhance gold recovery (Parsons et al. 2012). Collectively, the 64 gold districts produced more than 3 million tonnes of tailings that were disposed of without treatment into the local environment (Drage 2015).
