Quantitative Thermodynamic Exposure Assessment (QTEA) Supporting Resilient Contaminated Sediment Site Restoration
Creators: name: Bridges, Todd S.
name: Gidley, Philip T.
name: Lotufo, Guilherme R.
name: Kennedy, Alan J.
name: Ruiz, Carlos E.
name: Melby, Nicolas L.
name: Ballentine, Mark L.
name: Wooley, Allyson
name: Laber, Charles
name: Ghosh, Upal
name: Burgess, Robert M.
name: Fernandez, Loretta A.
name: Wang, Alice
name: Mayer, Philip
name: Schmidt, Stine
Publisher: name: Maryland Shared Open Access Repository
Year: 2022

description: Objective The primary objective of this study was to develop an integrated, comprehensive capability for evaluating the effect of chemical influx on the performance of in situ remedies, specifically the Quantitative Thermodynamic Exposure Assessment (QTEA). Using both ex situ equilibrium sampling and in situ passive samplers with performance reference compounds (PRC), exposure processes were assessed using experimental mesocosms. Using polymer sampling, tissue analysis, and modeling, QTEA was developed to guide and monitor site restoration through in situ remediation. Experiments were performed to reveal how activated carbon amendments can be most effectively applied to enhance the resilience of in situ sediment remedies challenged by ongoing sources of impact. In addition, the RECOVERY modeling framework was improved to better simulate the addition of amendments, in particular activated carbon. Technical Approach The overarching hypothesis for the project was that measured gradients in chemical activity can provide new information about the history and chemodynamics of impacted sediments. A series of mesocosm experiments were conducted in which field-collected sediments impacted with polychlorinated biphenyls (PCBs) were employed. Mesocosm experiments ranged from simple sediment beds and no biota to complex systems with biota (worms, clams, and fish) and activated carbon. Both ex situ equilibrium samplers and in situ passive samplers were used to evaluate changes in exposure produced by different forms of in situ remediation (e.g., thin-layer caps, amendment with activated carbon) under different chemical influx conditions (e.g., in the presence of ongoing inputs of clean or impacted sediment). Polymer sampler data were used to quantify chemical processes and exposures to fish and benthic invertebrates included in the mesocosm experiments. Results Results from this project include: No remedy using activated carbon (AC) or thin-layer capping was resilient (e.g. significant reduction &gt;50%) to ongoing inputs of impact. Therefore, the in situ remedies evaluated may only partially succeed under conditions of ongoing inputs at impacted sites. Ongoing inputs must be controlled or at least minimized. Overall, thin sand capping outperformed AC mix with regard to native bedded congeners and input PCB 173. Sand cap was particularly effective in reducing superhydrophobic PCB accumulation in fish. Different polymer methods (in situ passive sampling with PRCs and ex situ equilibrium sampling with multiple polymer thicknesses) provided similar results. Comparisons of biota uptake and polymer equilibrium concentrations showed that AC caused reductions in polymer concentrations that were not always observed in tissue concentrations. The thermodynamic level of the sediment polymers was greater than the biota. Overall, polymers were successful to monitor remediation as an important augmentation to bioaccumulation measurements. A fully successful remedy by AC was likely not achieved in the present study because of temporal factors. Sorption kinetics were slow from the sediment to the AC, especially for the most hydrophobic, highly chlorinated congeners (hepta- and higher). Within the three-month timeframe of the experiment, di-, tri-, tetra-, penta-, and hexa-chlorinated PCB congeners were treated effectively with AC, while the hept-, octa-, nona-chlorinated congeners were treated more effectively by the ?layering effect? of the thin-layer caps. Reductions in bioavailability by AC (aside from continued inputs) required a dose equal to about half the natural total organic carbon (TOC) of the sediment. It is well established that in order to ensure effective treatment, the fundamental nature of the sediment TOC in the bioactive zone must be altered from labile (i.e., amorphous) to black carbon (e.g., AC). Although some sediments may be effectively treated at lower doses, the experiments indicated that effective reductions of PCB bioavailability occurred at 50% of the TOC. The results of this work showed AC and thin-layer capping can provide value separately and in combination beyond what may be achievable by monitored natural recovery (MNR) alone. Thin-layer capping with AC should be considered as an early, first-step intervention at sites requiring enhanced MNR. Polymers can provide useful information for monitoring remedy performance. While freely dissolved PCB concentration in sediment porewater provides one extreme boundary of thermodynamic potential for bioaccumulation in aquatic organisms, the actual steady state bioaccumulation is greatly impacted by the specific exposure conditions (e.g. differing contributions of sediment and water column exposures) and organism-specific biological attributes. Therefore, polymer sampling may not be able to completely replace the need to measure uptake by biota. However, in this study, uptake in mesocosm organisms was accurately predicted using a novel simplified model that captures the complexity of the aquatic food web, which may result in streamlined monitoring approaches. Benefits In situ remedies for impacted sediments offer a number of potential advantages over remedies that predominately rely upon removal of impacted sediments, including risk reduction and reduced costs. However, in order for these advantages to be fully realized, in situ remedies must provide effective risk reduction over the long-term. Achieving this objective requires the development of remedies that are resilient with respect to processes that can compromise or degrade the remedy?s ability to provide risk reduction benefits, such as through rec-impact of surface sediments. This study provided insights needed to design resilient remedies while also producing the tools to evaluate management of in situ remedies to directly support remedial project managers and other potential beneficiaries at impacted sediment sites. (Project Completion - 2022)