The sediment-water interface of spiked-sediment toxicity tests is a complex exposure system, where multiple uptake pathways exist for benthic organisms. The freely dissolved concentration (C free ) in sediment pore water has been proposed as a relevant exposure metric to hydrophobic organic contaminants (HOCs) in this system. C free , however, has rarely been measured in spiked-sediment toxicity tests. In this study, we first developed a direct immersion SPME method for measuring C free in overlying and pore water in a sediment test using polydimethylsiloxane (PDMS)-coated glass fibers, resulting in sensitive and repeatable in situ measurements of HOCs. Then, we measured This article is protected by copyright. All rights reserved. Accepted ArticleC free and total dissolved concentrations (C diss ) in the sediment test systems with the freshwater amphipod Hyalella azteca, thoroughly evaluated the temporal and spatial profiles of four HOCs (phenanthrene, pyrene, benzo[a]pyrene, and chlorpyrifos). Furthermore, we examined the relationship between the measured concentrations and the lethality of H. azteca. We found that the test system was far from an equilibrium state for all four chemicals tested, where C diss in overlying water changed over the test duration and a vertical C free gradient existed at the sediment-water interface. C diss was larger than C free by a factor of 170-220 in pore water for benzo[a]pyrene due to the strong binding to dissolved organic carbon. The comparison of the median lethal concentrations (LC50) of chlorpyrifos in the sediment test and those in water-only tests indicates that C free in pore water was the most representative indicator for toxicity of this chemical. The method and findings presented in this work warrant further research on the chemical transport mechanisms and the actual exposure in sediment tests using different chemicals, sediments, and test species.

Understanding the changes in the temporal and spatial concentrations of chemical substances in sediment toxicity tests facilitates interpretation of their toxicity and accumulation in benthic organisms because benthic organisms are affected by chemicals via multiple exposure pathways. However, such investigations using chronic sediment toxicity tests have rarely been performed. To examine the concentration profiles of a hydrophobic organic chemical using chronic spiked-sediment toxicity tests, we performed 28-day sediment toxicity tests of fluoranthene with a freshwater amphipod, Hyalella azteca, using a semiflow-through system and compared the results with those of 10-day tests. In these experiments, we measured various types of fluoranthene concentrations over the test periods: total dissolved (C diss ) and freely dissolved (C free ) concentrations in overlying water and porewater as well as sediment concentrations. We also examined which concentration correlated with the amphipod bioconcentration factor (BCF). We found that both overlying water and porewater C free did not differ significantly on days 10 and 28. Sediment concentrations remained almost stable for 28 days, whereas C diss in overlying water varied temporally. These results suggest that the 28-day test provides almost constant concentrations of fluoranthene, particularly in porewater, even in a semi-flow-through system. In addition, the comparison of BCF of fluoranthene on day 10 in the present study with that obtained from water-only tests reported in the literature suggested that C free in pore water was the most representative indicator of bioaccumulation in H. azteca. Our findings support the possible use of a water-exchange system in chronic spiked-sediment toxicity tests of hydrophobic organic chemicals. However, further studies using sediments and chemicals with different properties are warranted to generalize the findings of the present study.

In standardized sediment toxicity tests, the applied water exchange methods range from static to flow-through conditions and vary between protocols and laboratories even for the same test species. This variation...