Spatiotemporal Distribution of Hydrophobic Organic Contaminants in Spiked‐Sediment Toxicity Tests: Measuring Total and Freely Dissolved Concentrations in Porewater and Overlying Water

Abstract

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.

Mind the Exposure Gaps—Modeling Chemical Transport in Sediment Toxicity Tests

“…This assumption is supported by experiments that showed no concentration gradient between the top and bottom positions of the water phase. 23 The diffusion model without DOC-mediated transport (w/o DOC model) assumes that all chemicals released from the sediment are freely dissolved in each water phase, i.e., chemical transport in the sediment and UWL is solely dependent on the molecular diffusion of the chemical in water. The diffusion model with DOC-mediated transport (w/ DOC model) accounts for binding to and mediated transport by DOC in water, integrating the transport of free and DOC-bound chemicals.…”

“…Chemical properties were adjusted so that the model results agree with experimental data for DOC concentrations in pore water ([DOC] pore ) after the 10-day test and in overlying water ([DOC] over ) over time. 23 See Section S-2 for more details. We note that this modeling was done to obtain time-and position-dependent [DOC] pore and [DOC] over that are consistent with measured values.…”

“…The experimental methods are summarized in Section S-3 and explained in detail in the cited article. 23 Briefly, artificial sediment was prepared following the OECD guideline, 35 spiked with the test chemical(s) and mixed on a roller shaker for homogenization. The spiked wet sediment was distributed to glass beakers, which were placed below a semi-flow-through water exchange system.…”

“…20,21 Recently, in the standardized 10-day sediment toxicity test with the amphipod Hyalella azteca, 22 we experimentally observed C free,pore that were by a factor of up to ∼10 higher than C free,over and a decreasing trend of C diss in the overlying water (C diss,over ) over time. 23 Moreover, a vertical concentration gradient of hydrophilic chemicals in the upper sediment was measured in static sediment toxicity tests with Chironomus riparius in the literature. 24 These observations indicate that the dynamic processes mentioned above lead to variable exposure conditions in space and time.…”

“…In this study, we developed a one-dimensional diffusion model to simulate the chemical transport over time and space in sediment toxicity tests. We applied the model to the standardized 10-day H. azteca sediment toxicity test and compared the model predictions to the experimental data set from Hiki et al, 23 which included C free,over , C free,pore , C diss,over , and dissolved concentrations in pore water (C diss,pore ) of four chemicals with moderate to high hydrophobicity. Following model validation, we performed model simulations for hypothetical chemicals with varying sorption properties to identify the system and chemical properties that drive the chemical transport and determine C free and C diss in the toxicity test.…”