Sediment properties influencing the bioavailability of uranium to Chironomus dilutus larvae in spiked field sediments

• Examined modifying effects of 18 freshwater field sediments on U bioavailability.
• Total U concentrations in sediment are poor predictors of U bioavailability.
• U bioaccumulation significantly correlated with U in overlying and pore water.
• Sediment fine fraction best explained the observed variation in U bioaccumulation.
• Sediment quality guidelines must incorporate the influence of modifying factors.

The partitioning of metals between dissolved and solid phases directly affects metal bioavailability to benthic invertebrates and is influenced by metal-binding properties of sediment phases. Little research has been done examining the effects of sediment properties on the bioavailability of uranium (U) to freshwater benthic invertebrates. In the present study, 18 field sediments with a wide range of properties (total organic carbon, fine fraction, cation exchange capacity, and iron content) were amended with the same concentrations of U to characterize the effects of these sediment properties on U bioavailability to freshwater midge, Chironomus dilutus. Bioaccumulation of U by C. dilutus larvae varied by over an order of magnitude when exposed to sediments spiked with 50 mg U kg−1 d.w. (5–69 mg U kg−1 d.w.) and 500 mg U kg−1 d.w. (20–452 mg U kg−1 d.w.), depending on the type of sediment. Variance in U bioaccumulation was best explained by differences in the cation exchange capacity, fine fraction (≤50 μm particle size), and Fe content of U-spiked sediment, with generated regression equations predicting observed bioaccumulation within a factor of two. The presented regression equations offer an easy-to-apply method for accounting for the influence of sediment properties on U bioavailability in freshwater sediment, with fine fraction being the single most practical variable. This research strongly supports that risk assessments and guidelines for U-contaminated sediments should not ignore the influence of sediment properties that can result in substantial differences in the bioaccumulation of U in benthic invertebrates.