RI/FS: Former Automotive Repair Facility/Oregon Garden Property

Project Description

The subject property is currently a vacant parcel that was acquired by the City of Silverton as part of the Oregon Garden site. The subject site was formerly utilized for vehicle repair and fuel storage using underground storage tanks (USTs). Previous investigations included Phase I and Phase II Environmental Site Assessments (ESAs). Project tasks included: UST decommissioning activities, petroleum contaminated soil (PCS) cleanup, and completion of a Remedial Investigation and Feasibility Study (RI/FS) including a human health and ecological risk assessment.

The RI included the collection of soil and groundwater samples for laboratory analysis from push probes and the installation of groundwater monitoring wells for water quality monitoring. In addition, a former on-site water well was retained for sampling a deeper aquifer given the presence of organic solvents on-site. The results of these investigations indicated petroleum hydrocarbon impact and the presence of the chlorinated solvents (primarily trichloroethene [TCE]) in soil and groundwater. The RI/FS determined that elevated concentrations of TCE in soil posed a long-term threat to groundwater quality and that groundwater had been impacted. The RI/FS identified a localized area of elevated soil impact that was likely the ongoing source of groundwater impact by TCE and related chlorinated compounds. Recommended corrective actions included the removal and off-site disposal of residual soil impact containing elevated concentrations of TCE. This work was completed in 2004 and compliance monitoring was then undertaken for a period of two (2) years.

A human health risk assessment was also completed as part of the RI/FS process. The risk assessment included development of a conceptual site model (CSM) and identification of potential receptors. Based on current and likely future land use and zoning, construction worker (on-site and along Main Street) and excavation worker/landscaper (on-site) were identified as potential receptors. Using these receptors, an evaluation of 10 exposure pathways was performed. Leaching to Groundwater and Ingestion/Inhalation from Tap Water were excluded as applicable exposure pathways. The following exposure pathways were determined to be applicable to the site: for soil - Construction Worker, Excavation Worker, and Volatilization to Outdoor Air and for groundwater - Groundwater in Excavation, Volatilization to Outdoor Air, and Construction Worker. Using the selected receptors and applicable exposure pathways, TCE was the only contaminant of concern (COC) identified in groundwater. No COC were identified in residual soil following soil cleanup activities.

Cumulative and ecological risks were also evaluated. Although no “hot spots” or cumulative risks were identified in soil, cumulative risk associated with dissolved TCE in groundwater slightly exceeded the excess cancer risk of 1x10-6. Non-carcinogenic risk in groundwater was calculated to be below a hazard indices (HI) of 1.0. Ecologically, groundwater to surface water was the only identified ecological pathway of concern. None of the detected compounds exceeded Level II Screening Values when comparing detected compounds in the nearest well to surface water in wetland areas.