UST: Residential Heating Oil UST Decommissioning, Petroleum Contaminated Soil Cleanup and Risk-Based Assessment and Closure

Project Description

The project site is a residence. BB&A Environmental was initially retained to decommission a heating oil underground storage tank (UST) in conjunction with a real estate transaction. During the decommissioning a release of heating oil was observed in the soil materials beneath the UST. Petroleum contaminated soil (PCS) accessible to excavation was removed from the subsurface and transported off-site for permanent disposal. Confirmation soil samples collected from remaining PCS in the excavation zone contained diesel-range total petroleum hydrocarbon (TPH) concentrations as high as 20,000 mg/Kg. Based on analytical results of additional soil samples, PCS extended laterally beneath the garage of the residence and off-site. BB&A Environmental recommended a risk-based approach to closing the site. To this end, BB&A Environmental undertook the following investigative actions:

• Delineation of the vertical and lateral extent of soil impact through the completion of push probes on- and off-site to allow the collection of soil samples
• Development of a conceptual side model (CSM) which identified contaminants of concern (COC) present at the site, and selection of potential exposure pathways and potential receptors at the site
• Comparison of exposure pathways and potential receptors selected by the CSM to COC identified benzene at levels above the generic risk-based concentrations (RBCs) for both leaching to groundwater and vapor intrusion into buildings exposure pathways
• Further evaluation of the leaching to groundwater exposure pathway was undertaken by recalculating the equation used to determine the generic RBC using a smaller area of soil impact. The recalculated RBC resulted in a site specific benzene RBC above that of the level of benzene present in soils at the site. Because the highest concentration of benzene detected in soils either on- or off-site was less than the recalculated site specific RBC, the remaining concentration of benzene was deemed protective of groundwater
• Further evaluation of the vapor intrusion into buildings exposure pathway was performed by calculating the 90% upper confidence limit (UCL) of the mean for benzene from confirmation soil samples collected from the excavation zone. Using the 90% UCL of the mean, a 30-year mass averaging calculation for benzene vapor was performed, resulting in a benzene level below the residential indoor air inhalation RBC. Therefore, remaining benzene in soils beneath the site which may volatilize into the indoor air space of the residence was determined not to present a risk to human health
• Reporting all findings of the investigation in a Risk-Based Cleanup Report and certification by BB&A Environmental that the work performed met the appropriate requirements of heating oil tank rules in Oregon. After receiving the contractor certification statement, the Oregon Department of Environmental Quality (DEQ) issued a “Heating Oil Tank File Closure Letter” indicating that the DEQ accepted the certification and closed its file for the site