Formatted Title:

Combining Passive Sampling Data with Toxicity Testing for Benthic Risk Evaluation at a Contaminated Sediment Site

Background/Objectives:

A client confidential former chemical manufacturing plant (Site) located in southeastern Massachusetts has been undergoing environmental investigations under the Resource Conservation and Recovery Act (RCRA) since the 1990s. This presentation focuses on an area of concern (AOC) at the site that is a 9-acre freshwater wetland supporting aquatic life, benthic communities and wildlife. A screening-level ecological risk assessment (SLERA) was completed for the Site in 2021 which relied on bulk sediment chemistry data collected between the early 2000s and 2021. While the foodweb modeling performed in the SLERA indicated a lack of risks to wildlife, screening levels for contaminants of concern (COCs; PCBs and mercury) were exceeded in sediment and a baseline ecological risk assessment (BERA) was required to evaluate potential adverse effects to invertebrates from exposure to COCs in sediment.

Approach/Activities:

Chemistry in AOC sediment indicated significant exceedances of benthic invertebrate screening levels, with concentrations of total PCBs and mercury in bulk sediment measured at over 100x their respective screening levels, therefore, a focused evaluation of bioavailability of COCs was undertaken in the BERA. Passive sampling of porewater was performed using SP3 polyethylene-based samplers and diffusive samplers to measure freely dissolved concentrations of PCBs and mercury, respectively. Concentrations in porewater were then compared to benchmarks protective of aquatic life, including 0.54 µg/L for total PCBs and 1.4 µg/L for mercury. Toxicity tests included survival and growth tests with 28-day amphipod (Hyalella azteca) and 10-day larval midge (Chironomus dilutus) using sediment samples collected from the wetland and two site-specific reference sites. Growth and survival endpoints were compared against respective reference station endpoints, and regressions (linear, exponential, and logarithmic) were explored to evaluate relationships between toxicity test endpoints and concentrations of COPECs in sediment.

Results/Lessons Learned:

The results of the investigation provided multiple strong lines of evidence that exposure to COPECs in sediment and surface water samples in the wetland is not resulting in adverse impacts to aquatic and benthic life. Passive sampling results confirmed freely dissolved and bioavailable concentrations do not exceed screening values protective of aquatic organisms; mercury total PCBs concentrations fell below both preliminary and refined screening values, with the exception of one sample exceeding the conservative preliminary screening value for PCBs. Evaluation of toxicity tests in the wetland revealed high survival for both C. dilutus and H. azteca. Comparisons of dry weight and biomass to respective reference sites showed no statistically significant and biologically relevant (>20%) differences between one reference site (SS-SS2), however there were significant decreases in dry weight and biomass for H. azteca when compared to the other reference (SD-601C). While slight decreases in growth of H. azteca were observed relative to the SD-601C reference site, no significant relationships with R² > 0.5 between growth and biomass of organisms and concentrations of COPECs in sediment were observed. Results of this investigation highlight the importance of evaluating bioavailability of sediment and show that reduced bioavailability in sediments limits impacts to benthic communities at this freshwater wetland.

Jennifer Huha

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(Group 1, Poster Board #26) Combining Passive Sampling Data with Toxicity Testing for Benthic Risk Evaluation at a Contaminated Sediment Site