Selenium distribution in a lake system receiving effluent from a metal mining and milling operation in Northern Saskatchewan, Canada.

The release of selenium (Se) at relatively low concentrations into aquatic ecosystems over time can result in the accumulation and, if thresholds are exceeded, subsequent adverse effects in sensitive species, including higher trophic levels (such as fish). A milling operation in Northern Saskatchewan, Canada, releases treated effluent into a small stream system, and Se has accumulated in sediments and aquatic biota over time. The present study evaluated four small lakes downstream of the effluent discharge point, and one lake upstream, in order to describe and understand the distribution of Se in abiotic environmental compartments and the transfer of Se into benthic macroinvertebrates. The concentrations of Se in sampled sediments were highly variable but exceeded proposed thresholds for the protection of fish and aquatic birds in all study lakes downstream of the effluent discharge point. Selenium concentrations in surface water, whole-sediment, and sediment pore water revealed that whole-body Se concentrations in benthic invertebrates (chironomids) are best correlated with Se in pore water. It is proposed that Se accumulates in sediments through an association with the total organic carbon content of sediment and that Se is fixed from the surface water by micro-organisms and primary producers. The relationship between Se in pore water and Se in whole sediments appears to be influenced by the organic carbon content of each medium, and Se bioavailability in sediment and transfer to higher trophic levels via benthic macroinvertebrates is likely speciation dependent.