The Role of Background in Sediment Evaluations for MGPs: A Spectrum of Ramifications
The Role of Background in Sediment Evaluations for MGPs:
A Spectrum of Ramifications
Michael Kierski and Charlie Menzie (Exponent)
Sarah Meyer and Jennifer Hagen (Natural Resource Technology, Inc. (NRT) an OBG Company)
Dr. Kierski is an environmental biologist and toxicologist who helps his clients evaluate and solve their complex environmental problems. Over the past 30 years, Dr. Kierski has evaluated human health and ecological risks associated with chemicals in all components of the environment including soil, soil gas, sediment, groundwater, drinking water, air, food, and biota. He has evaluated risk at legacy contaminated sites where chemical contamination is present, and has performed numerous evaluations specifically related to the potential for vapor intrusion into buildings at many of these sites. He has worked on over 50 manufactured gas plant (MGP) sites where he has performed human health and ecological risk assessments.
A Midwest utility working with the U.S. Environmental Protection Agency developed a multi-site approach to manage 20 former manufactured gas plant (MGP) sites in the Superfund Alternatives Site Program. As part of the sediment investigation for each MGP site, background (referred to here as ambient) conditions are characterized for the main MGP group of chemicals of potential concern (COPCs), which are polycyclic aromatic hydrocarbons (PAHs). The magnitude of the ambient PAH conditions upstream of the MGP site affect the determination of which sediments require consideration for remedial action, because remediation of sediments below background or ambient conditions is not required per Superfund policy. There has been a broad spectrum of outcomes regarding ambient PAH concentrations at the MGP sites investigated so far within the program. This presentation provides site-specific case studies using three sites that span the spectrum of ambient PAH concentrations (low, medium, and high); the sites demonstrate the changing role ambient conditions played in characterizing human health and ecological risk from the sediments and selection of the sediment zones requiring consideration for remedial action. At the medium and high end of the ambient PAH concentration range, incorporation of the ambient conditions into the decision-making process can dramatically affect the size of the footprint of sediments requiring consideration for remediation. Lastly, we discuss how the sediment evaluation is modified dependent on the level of ambient PAH contamination. Modifications ranged from extensive use of site-specific sediment toxicity testing to characterize risk-based zones at low ambient PAH concentrations to focusing primarily on forensic chemistry methods at high ambient PAH concentrations to identify sediment zones that depart from ambient conditions.