Selenium speciation and localization in sediment and benthic invertebrates from lakes receiving treated metal mine effluent

Abstract

The objective of this research project was to establish a better understanding of the mechanism(s) and route(s) by which selenium (Se) may enter an aquatic ecosystem that has been receiving treated metal mine effluent from an upstream uranium milling operation. Synchrotron based X-ray absorption spectroscopy (XAS) and X-ray fluorescence (XRF) imaging, which require little sample pre-treatment, were employed to study the speciation and distribution of Se in complex sediment and benthic invertebrates samples collected from the field. Laboratory based inductively coupled plasma mass spectrometry (ICP-MS) provided quantitative Se concentrations. Samples were taken from Fox Lake and Unknown Lakes, downstream of the mill, and Yeoung Lake as a control. The variation in Se speciation as a function of depth in intact sediment cores may provide insight into the species of Se available to the sediment dwelling benthic invertebrate communities. Therefore, a custom sample holder was designed to facilitate analysis of intact sediment cores at cryogenic temperatures. Additionally, laboratory reared chironomids were water-exposed to various Se species, to compare their Se speciation and localization to chironomids collected in the field.

The successful demonstration of the custom sample holder and viable use of synchrotron XAS and XRF in studying sediment and chironomid samples have revealed that biologically relevant Se forms were present in sediment at depths accessible by the benthic invertebrate community. These Se forms included selenomethionine-like and selenite species, and to a lesser degree elemental Se; an increased proportion of reduced Se species was observed as depth increased. Other elements measured concurrently with Se included As, Zn, Cu, Ni, Fe, and Mn, providing an estimation of the redox boundary found both in Fox and Unknown Lake, as well as suggesting the presence of iron species that could aid in the reduction of Se. Field and laboratory reared chironomids showed similar Se species, and XRF imaging revealed the localization of Se in 4 distinct regions: head capsule, brain, salivary glands, and gut lining. Overall, the project has provided important insights into the interactions of Se with this aquatic ecosystem, which may have future applications in cold water systems with elevated Se concentrations.