Emerging Contaminants Summit

March 10-11, 2020

Westminster, CO
The Westin Westminster
(only 15 miles from downtown Denver)

chakrabartiArnab Chakrabarti
Senior Associate Engineer
Terraphase Engineering 

Arnab Chakrabarti is an engineer at Terraphase Engineering with over 15 years of experience in environmental site characterization, remediation, and project management. He received his B.S. in Chemical Engineering from the University of Michigan and his M.S. in Environmental Engineering from the University of California Berkeley. He lives in Oakland, California with his wife and two children.



Re-Assessment of Ecological Risk at a Mature Near-Bay Petroleum Sites Due to Emerging Polar Degradation Metabolite Contaminants

Polar petroleum degradation metabolites have been identified as an emerging contaminant issue, particularly relevant to the San Francisco Bay region, which contains numerous mature petroleum sites (e.g., refineries, Department of Defense Sites) in proximity to estuarine waters. Select sites with previously established total petroleum hydrocarbon (TPH)-specific point-of-compliance levels for groundwater discharging to the Bay are currently being re-assessed for potential ecological risk attributed to polar compounds. Use of the silica gel cleanup (SGC) method for groundwater analysis was previously approved by the San Francisco Bay Regional Water Quality Control Board (RWQCB) at these sites based on existing guidance at the time. Although non-SGC analysis of TPH in groundwater is now being required by the RWQCB, a regulatory screening level for polar compounds has yet to be established due to (a) difficulty in quantifying the complex mixture of transient petroleum degradation metabolites by commercial analytical laboratories and (b) a paucity of toxicological data available for polar compounds

One near-Bay site that is currently participating in such a site conceptual model re-assessment, is a former 400-acre fuel storage site. A previous offshore ecological risk assessment performed at the site did not find toxicity in off-shore sediments. Per a RWQCB request, groundwater was analyzed without SGC starting in 2014 to qualitatively evaluate the presence of polar compounds in groundwater discharging to the Bay and empirical methods were subsequently selected in 2016 to assess potential ecological receptor risk from polar compounds. The selected method consisted of direct aquatic toxicity testing of representative saltwater species (benthic invertebrate, crustacean, and fish) using groundwater that contained polar compounds only, and that had been adjusted for estuarine conditions.

Results of direct aquatic toxicity testing showed an effect (both acute and chronic growth) from 100%, undiluted, groundwater for the mysid shrimp receptor only. Toxic effects were not observed for the benthic invertebrate (mussel embryo) or the fish (inland silverside) species.  Confounding factors were excluded by confirming no toxicity resulted from background groundwater that had no polar compounds, as well as through use of reference toxicant tests. Based on these results, serial dilution testing is being conducted using the mysid shrimp to generate a dose-response curve for use in establishing a regulatory point-of-compliance threshold. In the absence of polar-compound-specific regulatory screening levels, other near-Bay sites approaching regulatory closure, or even having obtained regulatory closure, may have to adopt a similar site-specific risk-assessment approach until regulatory screening levels are established.

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