Assessing Ecological Risk in a Dynamic Riverbed


Assessing Ecological Risk in a Dynamic Riverbed

Russell Johnson and Tony Rodolakis (Amec Foster Wheeler)
Robert Cleary and Peter LaGoy (NiSource Corporate Services)


Russell Johnson

Amec Foster Wheeler

Speaker Bio:

Mr. Johnson is an environmental consultant with over 30 years of experience directing site assessment and remediation projects, primarily for industrial clients in the Northeast. He has an undergraduate degree from the University of Rhode Island, with a Master’s Degree from the Yale School of Forestry and Environmental Studies. Mr. Johnson is also a Licensed Environmental Professional in Connecticut. His technical and practical experience is based on groundwater flow, contaminant transport, and natural attenuation modeling, the sampling and remediation of light and dense, non-aqueous phase liquids (LNAPL/DNAPL), and groundwater remediation design and construction. Mr. Johnson’s early work focused on the characterization of Superfund sites, many involving chlorinated solvents. For the past 20 years Mr. Johnson has been focusing on the assessment and cleanup of former manufactured gas plant (MGP) sites. Mr. Johnson has incorporated forensic chemistry methods to distinguish sources of MGP residuals and to establish a framework for understanding potential toxic effects to receptors, especially in aquatic environments. He has developed sediment sampling and analysis plans to determine the petrogenic/pyrogenic origin of polynuclear aromatic hydrocarbons, to establish background conditions, and to assess possible routes of tar migration to river environments by comparing chemical fingerprints of soil and sediment samples. Remedial actions have been limited in scope because risk-based closures have been designed for consistency with current and future property use.


Tars were discharged to the Connecticut River near a former manufactured gas plant (MGP) in Springfield, Massachusetts and flowed downstream affecting the riverbed over a 2-mile stretch. A risk-based approach was developed that reduced the remediation footprint in the river by approximately 80 percent. The tar-infused sediment is weathered at the surface and resists erosion, but has been demonstrated to be toxic through laboratory testing. By regulatory definition tar in the upper foot of sediment represents a Substantial Hazard Condition in Massachusetts. This “condition” must be eliminated or mitigated to achieve site closure. As part of the Sediment Contaminant Bioavailability Alliance (SCBA), samples were analyzed for polynuclear aromatic hydrocarbons (PAHs) in bulk sediment and porewater, and submitted for toxicity testing. Based on those data, a total PAH benchmark of 305 mg/kg was established for the upstream portion of the site. The SCBA study included multiple MGPs and it was determined that a toxic unit (TU) of 8 for porewater was a protective threshold for tars. The Connecticut is a bedload river characterized by fairly-uniform sand. Bathymetric surveys indicate that upper sediments are in dynamic equilibrium. Consequently, sediment containing background concentrations of PAHs surround and sometimes bury tar-infused sediment. Fifty samples were collected in the downstream portion of the site and analyzed for total PAH, soot and organic carbon, and grain-size. All but one sample (352 mg/kg) had concentrations below the total PAH threshold, and most were less than 10 mg/kg. Only the maximum concentration had a porewater TU > 8. Because the sediment is a uniform quartz sand, grain size and carbon were not reliable factors in predicting total PAHs. Microscopic analysis revealed that the primary PAH contributors were asphalt particulates, not tar.


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