Dr. Michiel Jonker
Institute for Risk Assessment Sciences (IRAS)
3584 CM Utrecht
+31 30 253 5338
Dr. Michiel T.O. Jonker, Assistant Professor, Institute for Risk Assessment Sciences (IRAS), Utrecht, NL, firstname.lastname@example.org
Pr. Albert A. Koelmans, Dept. of Aquatic Ecology and Water Quality Management Wageningen University, Wageningen, NK, email@example.com
Joy A. McGrath, Senior Managing Scientist, Exponent, New York, NY, USA, firstname.lastname@example.org
Joop L.M. Hermens, IRAS, Utrecht, NL
David R. Mount, US-EPA, Deluth, MN, USA
Sediment toxicity tests are frequently performed on contaminated field sediments in weight-of-evidence approaches for the purpose of ecological risk assessment (ERA). Also, the tests are often required within the framework of international chemical regulations (e.g. REACH) as part of product risk assessments. Several test methods with benthic organisms are available, including (standardized) assays using Lumbriculus variegatus (survival), Hyallela azteca (survival/growth/reproduction), and Chironomus spec. (survival/time to emergence). Although the toxicity assays generally perform well and deliver useful information on product safety or ecological risks, application of the assays to very hydrophobic organic chemicals (VHOCs), i.e., chemicals with an octanol-water partition coefficient (logKow) above about 6, often results in biased and therefore unreliable data. Main reasons for this include poor test design and increased susceptibility of these substances to artifacts or confounding factors, i.e., physical, chemical, or biological factors that may alter the outcome of a toxicity test. Due to the very hydrophobic properties of VHOCs, testing these substances is particularly challenging and several artifacts potentially can occur. These predominantly relate to the facts that (i) VHOCs have very low aqueous solubility and slow dissolution kinetics (at high concentrations, the compounds may form separate phases of pure compound – crystals or droplets – which only very slowly dissolve and may cause fouling of organisms); (ii) VHOC-sediment equilibration kinetics are slow (obtaining an environmentally-relevant and homogeneous test matrix is challenging); (iii) uptake kinetics of VHOCs in organisms are slow.
Another challenge when working with VHOCs is quantifying the actual exposure of benthic organisms. The traditional way of exposure and risk assessment of contaminated sediments is based on total, solvent-extractable concentrations of sediment-associated chemicals. However, within the scientific environmental community it is generally accepted that this approach does not lead to realistic assessments. Instead, the freely dissolved concentration (Cfree) of a chemical in sediment pore water is considered the driving force behind accumulation and toxicological effects in organisms. When trying to understand, explain, or model sediment toxicity tests with VHOCs, information on Cfree is therefore crucial. Methods to determine Cfree (i.e., passive sampling methods) for moderately hydrophobic chemicals are well-developed and have successfully been applied in many cases; however, passive sampling is more challenging for VHOCs, due to kinetic issues and difficulties in determining the required passive sampler-water partition coefficients.
Finally, modeling sediment toxicity of VHOCs is also challenging. A simple, but for many nonpolar organic chemicals effective way of explaining and modeling sediment toxicity data across sediments and organisms is provided by a modelling framework combining the Equilibrium Partitioning (EqP) Theory and the Target Lipid Model (TLM). The EqP Theory assumes that chemicals present in sediment, porewater, and organisms are in a thermodynamic equilibrium, characterized by equilibrium (sediment-water, organism-water) constants and that toxicity is related to Cfree. The TLM assumes that organism lipid is the target site for toxic effects, as characterized by a critical target lipid body burden. Unfortunately, reliable sediment toxicity data for VHOCs are scarce and validation of the approach has been challenging. High-quality data based on intelligent testing design are therefore needed to investigate the validity of the EqP-TLM approach for this class of substances.
Although sediment toxicity testing has a long tradition, currently there are several gaps in our knowledge on the behavior and bioavailability of VHOCs in sediment toxicity assays and in sediments in general. Moreover, there is a clear lack of high-quality methods and protocols for spiking, handling, and exposure assessment of VHOCs. This hampers realistic product and ecological assessments and may lead to improper or even unwanted management of contaminated sediments and products. Therefore, there is an urgent need for intelligent testing design, practical guidance, and standard protocols for VHOC testing. The general objective of the proposed research is to improve sediment toxicity testing design, performance, and data interpretation for VHOCs and to develop guidance in order to maximize realism and value of future testing and thereby to support product and ecological risk assessment. To satisfy this objective we will:
- Perform a critical literature review on VHOC sediment toxicity data; and design, based on the review results, tiered experimental work;
- Develop, test, and compare different VHOC spiking methods and develop standardized protocols for the preferred approach(es);
- Set up exposure quantification methods (passive sampling) for the VHOCs and develop standardized experimental protocols;
- Evaluate the impacts of VHOC-specific test design parameters/confounding factors on results of sediment toxicity assays and demonstrate means to circumvent or deal with these. Factors that will be investigated include upper limit test concentrations, sediment-VHOC equilibration time before test initiation, exposure duration, and possibility of organism fouling;
- Evaluate the applicability of the EqP-TLM modelling approach to VHOCs and propose modifications or alternatives if necessary;
- Provide recommendations and develop guidance for sediment toxicity tests with VHOCs based on the project results and international expert opinions.