Recent Developments in Sediment Toxicity Testing

Sediment is an important environmental compartment inhabited by many different aquatic species, including benthic plants, vertebrates, and invertebrates. These species rely on the sediment for rooting, shelter, and/or feeding. Unfortunately, sediments also act as an important sink of certain anthropogenic contaminants, such as heavy metals and hydrophobic organic chemicals, which tend to accumulate in this aquatic compartment. Because many of these contaminants are toxic, they can potentially impact natural processes or habitat quality and thereby be harmful to aquatic ecosystems. Sediment toxicity tests have been developed to investigate potential toxicity to benthic species; these assays typically test the survival (or other response) of organisms after exposure to sediments collected from the field or contaminated in the laboratory. The array of sediment toxicity tests originally developed has gone through several iterations, resulting in the current use of standardized assays that employ standard test organisms and simple, easily quantifiable responses (e.g., death or reproduction). However, the field of sediment toxicity testing continues to evolve through new assays, test organisms, and dosing strategies.

In early 2022, a (virtual) workshop was organized as part of a European Chemical Industry Council–funded project (Cefic-LRI ECO43). Although the ECO43 project specifically focused on improving sediment toxicity testing with difficult-to-test, highly hydrophobic organic substances, the scope of the workshop reached beyond this focus. Approximately 40 people from academia, industry, and the regulatory field discussed the current status and future challenges and needs of sediment toxicity testing for plant protection products, substances of unknown or variable composition, complex reaction products and biological materials (UVCBs), offshore chemicals, and field sediments, as well as modeling and regulatory aspects of sediment toxicity. After the workshop, the idea arose to write critical reviews on the topics discussed during the workshop, combine these with manuscripts resulting from the Cefic-LRI ECO43 project, stimulate colleagues to write other manuscripts on sediment toxicity testing, and publish the collection of manuscripts in a special series in Environmental Toxicology and Chemistry. To identify colleagues who would be interested in jointly submitting manuscripts on sediment toxicity testing, forces were joined with the Society of Environmental Toxicology and Chemistry Sediment Interest Group, which has been active in the field of sediment research for many years, with sediment toxicity being one of its core topics. Despite the busy schedule of all our sediment toxicity colleagues, we managed to put together a collection of eight interesting scientific papers, all dealing with sediment toxicity testing, and particularly focusing on recent developments in this important field of environmental research.

In the first paper, Leppanen et al. (2024) provide an overview of where (in which countries), why (with what purpose), and how sediment toxicity tests should be performed. Moreover, in their critical review, they list several current needs and challenges and identify possible improvements in terms of regulations and actual testing. One of the challenges identified by the authors is the use of standard tests for difficult-to-test chemicals, such as nanomaterials, very hydrophobic substances and UVCBs. Generally, sediment toxicity tests have been developed using easy-to-test chemicals. By straightforwardly applying the tests to chemicals with potentially more complex behavior and not acknowledging that these chemicals may require alternative ways of handling or data interpretation, biased data could result. Along this line of reasoning, Jonker and Diepens (2024a) critically evaluate the performance of standard sediment toxicity assays for very hydrophobic organic chemicals (VHOCs) and show that although the assays perform well in terms of certain specific aspects, they do not perform well in others. 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 false-positive 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).

Spiking is also the subject of two other studies (Valenti et al., 2023; Fischer et al., 2022). Although Jonker and Diepens (2024a) demonstrate that spiking sediments according to an alternative approach based on passive dosing is not successful for liquid substances, Fischer et al. (2022) show that the use of this technique can actually produce stable concentrations of solid chemicals in porewater of sediments. Moreover, by using this approach and subsequently performing additional measurements, the role of dissolved organic carbon in sediment toxicity testing is illustrated. Valenti et al. (2023), on the other hand, focus on the traditional spiking method, that is, with the use of a spiking solvent. Generally, solvents are being used to be able to dose sediments with poorly soluble compounds. This approach calls for additional control systems, to evaluate any effects of residual solvent. Valenti et al. (2023) demonstrate that residual solvent is rare and when properly controlled, the use of both a solvent and a negative control can be eliminated, thereby reducing the use of test organisms and systems without jeopardizing test quality.

An important choice to make when one is designing a sediment toxicity test is whether to use artificial or natural sediment. As also mentioned by Leppanen et al. (2024), the use of natural sediment could make a test more ecologically relevant. This consideration prompted Grønlund et al. (2023) to critically evaluate the pros and cons of applying natural field-collected sediments and to formulate a list of criteria to fulfill when using such sediments in actual toxicity testing. Subsequently, Grønlund et al. (2024) used actual field-collected sediments to perform sediment toxicity tests with the aim of answering the (mechanistic) questions of whether mixture toxicity occurs when multiple contaminants are tested simultaneously and whether any observed toxicity can be described and predicted with models. The last question seamlessly links to one of the recommendations by Leppanen et al. (2024), that is, to explore the potential use of models in sediment toxicity testing in more detail. For this reason and because models are not being used to their fullest potential, Burgess et al. (2023) finally provide a critical review of the different types of models that can be used for this purpose. They conclude with a statement of critical needs, with the aim of broadening the application of mechanistic models in sediment toxicity testing.

All in all, the present collection of studies covers several important steps and aspects at play within the field of sediment toxicity testing, from regulatory aspects to detailed experimental and toxicological issues and modeling. We hope that regulators, academics, people from industry, and consultants will be informed and inspired to improve the current testing by implementing new developments and continuing to improve sediment toxicity testing in the future.

The authors declare no conflicts of interest.

Michiel T. O. Jonker: Writing—original draft. Alan J. Jones: Writing—review & editing.