Evaluating the functional equivalency of test organism performance in negative and solvent controls during chronic sediment ecotoxicity studies based on US EPA guidance.

The U.S. Environmental Protection Agency (EPA) considers sediment toxicity tests as conditional registration requirements for pesticides with soil Kd ≥50 L/kg-solid, Koc ≥1000 L/kg-organic carbon, or Log Kow ≥3. Hydrophobicity of these compounds often necessitates use of solvents to ensure accurate and homogeneous dosing of spiked-sediment studies. For sediment tests, a volatile solvent (e.g., acetone) is generally used as a transient carrier. Due to low water solubility, test material is dissolved in a volatile solvent to create stock solutions. A measured aliquot of stock solution is then mixed with sand substrate, after which the solvent is evaporated. This spiking process results in negligible solvent exposure to organisms. In 2016, EPA released final ecotoxicity test guidelines for sub-chronic freshwater (850.1735) and marine (850.1740) sediment test. These methods provide an option for conducting experiments with only a solvent control, and no negative control. To adopt this testing strategy, functional equivalency between the negative and solvent control must be demonstrated. These test guidelines describe specific factors that should be considered for evaluating functional equivalency including: (a) concentration of solvent in the test sediment after evaporation, (b) levels of solvent that are known to affect organism health, (c) known impurities in the solvent and their potential impact on organism health, and (d) historical organism performance of solvent vs. negative controls. Our analysis considers these factors and overall supports the elimination of the negative control requirement as this change is unlikely to impact the robustness or interpretability of spiked-sediment toxicity tests. This article is protected by copyright. All rights reserved. Environ Toxicol Chem 2023;00:0-0. © 2023 SETAC.