Trihydro Corporation, a prime contractor to a major integrated petroleum refining company with historic operations across the United States, has responsibility for the remediation of groundwater impacted by hydrocarbons at a site in Ohio. One of the key issues to be resolved as the groundwater remediation program moved forward was the potential for subsurface vapor intrusion into indoor air in a residential neighborhood adjacent to the site. Light, non-aqueous phase liquid (LNAPL) hydrocarbons from the former refinery operations were present at one time beneath a portion of the community. The U.S. EPA Region 5 required an assessment of the potential for vapor intrusion, and Trihydro wished to ensure that the sampling and analysis program would provide the best practicable data quality. Background concentrations of some gasoline constituents (such as benzene) can exceed risk-based target concentrations for inhalation exposures, so it was clear from the beginning of the project that a forensic analysis could be required to ascertain the source of certain vapors. Resolution of any off-site vapor intrusion issues is part of long-term strategy for redevelopment of the original refinery site for recreational, commercial, and/or manufacturing uses.
Geosyntec's Scope of Services
Trihydro selected Geosyntec to design soil gas sampling protocols with several quality assurance/quality control (QA/QC) measures to ensure the integrity of the data, and to develop a scope of work that provided a statistically valid comparison of sub-slab to near-slab soil gas chemistry, as well as a comparison between areas underlain by hydrocarbons from the former refinery versus those remote from the refinery impacts. Our approach called for the collection of vertical profiles of soil gas samples to assess the downward flux of oxygen from the atmosphere, and the production of carbon dioxide resulting from biodegradation of the hydrocarbon vapors. In implementing the sampling program, Geosyntec provided a field team of a dozen experienced staff, and trained TriHydro staff in the detailed sampling protocols to create a team capable of collecting the requisite number of samples within the time requirements. Together, Geosyntec and TriHydro installed 86 sub-slab probes and 100 near-slab soil gas probes within or immediately adjacent to 43 residences. The team collected over 300 soil gas samples from these and seven nests of soil gas probes at depths of 5, 10, 15, 20, 30, 40 and 50 feet below ground to provide sufficient data to assess the potential for vapor intrusion risks, all within a period of only 6 weeks.
Geosyntec calibrated a mathematical model of vapor transport and biodegradation to the vertical profiles of hydrocarbon vapors, oxygen and carbon dioxide, and demonstrated that intrinsic biodegradation reduces concentrations of hydrocarbons to levels that pose no risk via vapor intrusion. Geosyntec worked closely with TriHydro to generate a report of findings. U.S. EPA Region 5 concurred with the report finding that vapor intrusion posed no unacceptable human health risk. Throughout the assignment, Geosyntec provided support to TriHydro for regulatory meetings and negotiation. As a subcontractor, Geosyntec worked with TriHydro on a number of fronts (regulatory, strategy development, and technology transfer) to ensure that their mission as the prime contractor was met in a timely cost-effective manner.