Traditionally, cyanide, mercury, and acid mine drainage are considered critical environmental hazards associated with gold mining. To our knowledge, this is the first study of hazardous concentrations of soluble boron (B) in a gold mine impoundment tailings dam. We suggest that the B anomaly is a consequence of disposal of gold pyrometallurgical waste (slag). Borax is a common flux used during fire assaying and refining of precious metals. Vast amounts of B-rich slag may have been discarded by mining operations and precious metal refineries worldwide, but the extent and effects of B contamination have not yet been addressed. Anomalous concentrations of soluble B found in the McIntyre mine tailings, Timmins, Ontario highly exceed recommended thresholds for groundwater, freshwater, and soil leachates, and cannot be explained by natural sources alone. Boron is distributed heterogeneously within the tailings dam and correlates positively with the percentage of B38 lm grain-size fraction, indicating that adsorption onto silt-clay particles results in B build-up.Ephemeral, efflorescent Mg-borate found along the dam embankment suggests an outflow of B along paths of high permeability. Leachability tests indicate that slags from gold-ore refineries and fire assaying labs release B12 wt% soluble B in only 24 h. This high leachability suggests that slag discarded on the tailings dam is the dominant source of observed B anomalies. Altered metabasalts with \3,000 mg/kg B may be a minor B source. Gangue minerals such as tourmaline, anhydrite, calcite, and siderite cannot account for the amount of soluble B found in the tailings.
Polychlorinated biphenyls (PCBs) are an environmental concern because of their adverse effects on humans and wildlife, and understanding the contribution of various matrices (i.e., sediment and water) to PCB exposure on aquatic communities is critical for successful remediation of impacted sites. The present study examined the toxicity of different routes of PCB exposure in aquatic invertebrates. In complementary laboratory and field experiments, the authors compared the effects of aqueous versus sedimentary exposure of PCBs on invertebrates. In laboratory bioassays, the planktonic Daphnia pulex and benthic Chironomus dilutus exhibited significant mortality when exposed to PCB-contaminated (1100 µg/g) sediment (p = 0.03 and p < 0.01, respectively). In field experiments, adult Dreissena bugensis were placed in cages along a depth gradient at a reference site and a PCB-impacted site in St. Clair Shores, Michigan, USA. Mussels experienced significantly greater mortality (p < 0.001) when placed in close proximity to impacted-site sediments compared with reference sediments. After 7 d, 94% of D. bugensis survived at the reference site compared with only 57% surviving at the impacted site. In addition, D. bugensis at the impacted site experienced significantly less mortality at the water surface (p < 0.001) compared to those near the sediment. The present study demonstrates the importance of evaluating toxicity at the sediment-water interface.
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