Arsenic speciation in surface waters and lake sediments in an abandoned mine site and field observations of arsenic eco-toxicity

The region of Cobalt in Ontario, Canada has been affected by arsenic (As) and trace metals contamination since the early twentieth century due to unconfined dumping of high As-containing tailings. The concentrations of total dissolved As in lakes and streams have exceeded Canadian drinking water quality guidelines. The present study was focused on the examination of conditions that could influence chemical transformations of As(III) to As(V) and As speciation in lakes and stream waters and sediment of this area, as toxicity and mobility differ between different forms of As. Laboratory results showed that the stability of As(III) is strongly influenced by temperature, light, the presence of nitrate and chloride and by pH. The study shows that although total As concentrations were relatively high in the water samples collected in the area, As(V) was the predominant species in almost all water samples, and the most toxic form of As (III) was low, often < 1% of total dissolved. The concentrations of total and bioavailable As in sediment/tailing core samples of two studied lakes varied greatly with the depth and sampling sites, but the extracted form was generally in a lower proportion (< 35%). Iron oxides and organic matter likely play an important role in As transformation and mobilization in the sediment. Field observations showed a partial recovery of the two lakes where the ecosystem has adapted to the tailings. The presence of fish communities in both high As-containing lake invites the questions on whether As (III) should be a more appropriate parameter than total As in eco-toxicity and environmental assessments.