Improving Sediment Toxicity Testing for Very Hydrophobic Chemicals: Part 2—Exposure Duration, Upper Limit Test Concentrations, and Distinguishing Actual Toxicity from Physical Effects

Abstract

Sediment toxicity testing with very hydrophobic organic chemicals (VHOCs) is challenging because of the chemicals’ low aqueous solubilities and slow kinetics. The present study presents the results of experiments investigating whether the standard exposure duration of 28 days with benthic invertebrates is sufficient for VHOCs; above which concentrations in sediment VHOCs are present as “free phase,” that is, crystals or non‐aqueous‐phase liquids (NAPLs); and whether it is possible to discriminate between actual VHOC toxicity and physical effects caused by NAPLs through fouling of the test organisms. The results suggest that the standard sediment toxicity test duration is sufficient for obtaining steady‐state VHOC concentrations in Hyalella azteca and Lumbriculus variegatus, provided that spiking and equilibration are performed properly (i.e., no free phase present). Under these conditions, transient (days 3–20) peak‐shaped toxicokinetics were observed, with steady‐state concentrations reached at approximately 28 days. The concentration above which NAPLs are present, the so‐called critical separate phase concentration (CSPC), was determined for several VHOCs by modeling and two experimental methods. Modeling resulted in unrealistic and variable data and therefore should be applied with caution. Experimentally determining CSPCs was successful and yielded values of approximately 1000 (400–2000) mg/kg dry weight, depending on the chemical. Finally, it was demonstrated that distinguishing actual toxicity from physical effects is possible by applying a well‐considered test setup, combining toxicity tests with multiple invertebrates (including Lumbriculus, which serves as a negative control for fouling); a broad test concentration range, preferably up to at least 30 000 mg/kg; and passive sampling to localize the CSPC. Applying this setup, false‐positive effects due to fouling, as well as false‐negative results due to testing at too low concentrations (trying to stay below the CSPC), can be avoided. Environ Toxicol Chem 2024;00:1–12. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Sediment Toxicity Tests: A Critical Review of Their use in Environmental Regulations

“…Existing test protocols and OECD guidelines have been designed primarily for relatively "easy"-to-test chemicals which potentially dissolve in water. Their application to other chemical groups, such as ionized chemicals (e.g., PFAS), nanomaterials, very hydrophobic chemicals, and substances of unknown or variable composition, complex reaction products, and biological materials (UVCBs), may result in biased data because of the occurrence of experimental artifacts (Jonker & Diepens, 2024b). There is standard guidance for alleviating this challenge, but this only applies to aqueous-phase testing (OECD, 2019b).…”

“…There is standard guidance for alleviating this challenge, but this only applies to aqueous-phase testing (OECD, 2019b). Not all chemicals have similar fate in the sedimentary compartment, and it would be beneficial to have distinct guidelines in terms of (1) exposure quantification to assess uptake routes and bioavailable or bioaccessible fractions with passive sampling or profile sampling, (2) spiking technique and equilibrium times, and (3) data interpretation (Jonker & Diepens, 2024a, 2024b. For instance, dedicated regulations and protocols need to be developed for nanomaterials and micro-/nanoplastics because their fate and exposure are driven by other chemical and physical processes and not simply dissolution (e.g., agglomeration, precipitation, dissolution, and/or transformation).…”

Recent Developments in Sediment Toxicity Testing

“…In particular, the methods of spiking (i.e., the means of introducing the test chemicals into the sediment phase) and equilibrating appear to be crucial for these chemicals. In a second study from these authors, additional challenges when working with liquid VHOCs are illustrated, but that study also shows that potential artifacts (notably falsepositive responses caused by fouling of test organisms with liquid substances) can be circumvented by applying a well-thought-out test design, provided that spiking is performed appropriately (Jonker & Diepens, 2024b).…”