Refining our understanding of metal bioavailability in sediments using information from porewater: Application of a multimetal biotic ligand model as an extension of the equilibrium partitioning sediment benchmarks

Abstract

The equilibrium partitioning sediment benchmarks (ESBs) derived by the US Environmental Protection Agency (USEPA) in 2005 provide a mechanistic framework for understanding metal bioavailability in sediments by considering equilibrium partitioning (EqP) theory, which predicts that metal bioavailability in sediments is determined largely by partitioning to sediment particles. Factors that favor the partitioning of metals to sediment particles, such as the presence of acid volatile sulfide (AVS) and sediment organic matter, reduce metal bioavailability to benthic organisms. Because ESBs link metal bioavailability to partitioning to particles, they also predict that measuring metals in porewater can lead to a more accurate assessment of bioavailability and toxicity to benthic organisms. At the time of their development, sediment ESBs based on the analysis of porewater metal concentrations were limited to comparison with hardness-dependent metals criteria for the calculation of interstitial water benchmark units (IWBUs). However, the multimetal biotic ligand model (mBLM) provides a more comprehensive assessment of porewater metal concentrations, because it considers factors in addition to hardness, such as pH and dissolved organic carbon, and allows for interactions between metals. To evaluate the utility of the various sediment and porewater ESBs, four Hyalella azteca bioassay studies were identified that included sediment and porewater measurements of metals and porewater bioavailability parameters. Evaluations of excess simultaneously extracted metals, IWBUs, and mBLM toxic units (TUs) were compared among the bioassay studies. For porewater, IWBUs and mBLM TUs were calculated using porewater metal concentrations from samples collected using centrifugation and peepers. The percentage of correct predictions of toxicity was calculated for each benchmark comparison. The mBLM-based assessment using peeper data provided the most accurate predictions for the greatest number of samples among the evaluation methods considered. This evaluation demonstrates the value of porewater-based evaluations in conjunction with sediment chemistry in understanding toxicity observed in bioassay studies.

Leachable Additives of Tire Particles Explain the Shift in Microbial Community Composition and Function in Coastal Sediments

“…The present study first identified the leachable additives of TPs in the pore water of sediment, which could better reflect the bioavailability of the released tire chemicals in the soil or sediments. 37,38 Moreover, previous studies focused only on the leaching process of heavy metals and PAHs and failed to quantify other organic compounds. 19,21,39 Considering that TPs contain a variety of additives, investigating only heavy metals and PAHs may lead to the main toxic compounds in the leachate being overlooked, resulting in the underestimation of the toxicity of leachate to organisms or microorganisms in the soil/sediment.…”

“…The main problem is that the measured additive concentrations may not represent the bioavailable concentrations in solid media due to the limitations of the extraction methods, such as acid-based extraction or high-temperature extraction, which may enhance the leaching process. The present study first identified the leachable additives of TPs in the pore water of sediment, which could better reflect the bioavailability of the released tire chemicals in the soil or sediments. , Moreover, previous studies focused only on the leaching process of heavy metals and PAHs and failed to quantify other organic compounds. ,, Considering that TPs contain a variety of additives, investigating only heavy metals and PAHs may lead to the main toxic compounds in the leachate being overlooked, resulting in the underestimation of the toxicity of leachate to organisms or microorganisms in the soil/sediment. This was confirmed by the positive linear relationships between the levels of six organic compounds (acetophenone, 4-methylaniline, benzothiazole, octadecyl acetate, N-cyclohexylfomamide, and dicyclohexylamine) in pore water and TP amendment in this study ( R = 0.58–0.94, P < 0.01, Table S8).…”

Collection and use of porewater data from sediment bioassay studies for understanding exposure to bioavailable metals

“…Comparisons of calculated toxic units with bioassay results suggest that metal concentration data collected from peepers (in combination with other porewater parameters collected via centrifugation) may more accurately reflect organism exposures than metal concentrations and bioavailability parameter data collected via centrifugation (Santore, Toll, et al, 2021). These results suggest that the collection of metal concentration data via peepers may be very useful for understanding metal toxicity in sediment bioassays, even when other bioavailability parameters must be collected by another method, such as centrifugation.…”

“…One-to-one plots of compiled porewater chemistry data were created to show porewater chemical concentrations by the extraction method for each data source. Figures comparing BLM toxic units calculated using porewater metal concentrations data collected via centrifugation as well as peeper and organism survival from several bioassay studies created for a companion paper (Santore, Toll, et al, 2021) were also examined to address questions about the use of porewater data in an EqP context.…”

“…The second is to collect field sediment samples that are then used to conduct sediment bioassays in the laboratory. Sediment porewater samples are collected during sediment bioassays to characterize the bioavailable metal concentrations to which the test organisms are exposed (Cleveland et al, 2017; Santore, Toll, et al, 2021). Because field‐collected sediments are manipulated prior to their use in sediment bioassays and overlying water in the field is different from that in the laboratory, porewater chemistry will be different in bioassay chambers compared to the field.…”