Evaluation of approaches for the derivation of defensible sediment quality guidelines for application at Saskatchewan uranium operations

Sediment quality guidelines (SQGs) or values (SQVs) are used to assess the potential risk of contaminants in sediment to aquatic organisms, specifically benthic invertebrates. The objectives of this research were to evaluate SQGs derived specifically for use by the uranium industry in Canada, propose alternative SQVs for use at uranium operations in northern Saskatchewan using a novel approach, and investigate the potentially bioavailable fractions of metals in sediment which could become an alternative measurement to the typically used total metal concentrations. The screening-level concentration (SLC) approach has been used in Ontario, Canada, to derive lowest effect levels (LELs) and severe effect levels for use as SQGs. This approach was adopted by the Canadian Nuclear Safety Commission (CNSC) to set guidelines for metals (As, Cr, Cu, Pb, Mo, Ni, Se, U, and V) and radionuclides (Ra-226, Pb-210, and Po-210) in sediment at northern Saskatchewan uranium mining and milling operations. The SLC approach is based on total metal and radionuclide concentrations in sediment, and corresponding benthic invertebrate community composition data for a specific sampling site. In this study, sediment chemistry (total metals and radionuclides) and benthic community data from northern Saskatchewan uranium operations were compiled and examined. Results indicated that the CNSC-derived SQGs had limited relationships to observed effects, or lack thereof, on benthic invertebrate communities near uranium operations in Saskatchewan. On a general basis, the correct alignment of at least one LEL exceedence at an effect site was observed 95% of the time, but on an element-specific basis many of the elements had concentrations at effect sites below their LELs. Furthermore, concentrations of the evaluated elements exceeded at least one LEL at 60% of the no-effect sites. The high number of exceedences of LELs at reference and no-effect sites (false-positives) calls to question the appropriateness of the CNSC-derived SQGs. To address the deficiencies of the SLC approach, a new approach was developed to derive alternative metal SQVs for the uranium industry in northern Saskatchewan that was based on the use of reference and no-effect site field data. Three different sets of NE values were derived using combinations of benthic invertebrate community effect criteria (abundance, richness, evenness, Bray-Curtis index). Additionally, reference (REF) values were derived based solely on sediment metal concentrations from reference sites. In general, NE values derived using abundance, richness, and evenness (NE1 and NE2 values) were found to be higher than the NE values derived using all four metrics (NE3 values). Derived NE values for Cr, Cu, Pb, and V did not change with the incorporation of additional effects criteria due to a lack of influence from the uranium operations on these metal concentrations (i.e., they were generally present at or near background levels). However, a gradient of exposure concentrations was apparent for As, Mo, Ni, Se, and U in sediment which allowed for acceptable exposure levels of these metals in sediment to be defined. The findings from this assessment proposed a set of new, alternate SQVs for use at the uranium industry in northern Saskatchewan. Often, only total metals concentrations are measured in the evaluation of contaminated sediment. However, this measure may over-estimate metal exposure to benthic invertebrates. Using sediment cores collected from near the Rabbit Lake and Key Lake uranium operations, along with a series of mixed-sediments (contaminated and reference sediment), metal bioaccumulation experiments using Chironomus dilutus were conducted in the laboratory. Metal concentrations in extracts from single extractions with either potassium phosphate or hydrochloric acid on wet and dried sediment, pore-water, and whole-sediment were used to evaluate metal bioaccumulation in test organisms. Depending on the metal, pore-water isolated using peepers generally exhibited the best relationship with tissue metal concentrations. Based on this finding, it is suggested that pore-water sampling using peepers (an in-situ dialysis device) be added to environmental sampling programs at Saskatchewan uranium operations so that sediment metal availability to benthic invertebrates can be better assessed. With the eventual development of a larger pore-water metals dataset, SQGs based on pore-water metals concentrations could likely be derived as an alternative to existing SQGs based on total metal concentrations in sediment.

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