The Tri-County Metropolitan Transportation Authority (Tri-Met, Portland, Oregon) has been operating the Center Street bus storage and maintenance facility since the early 1970s. Streetcar maintenance and other operations occurred at the site since the early 1900s. Releases of diesel, used motor oil, unleaded gasoline and automatic transmission fluid had been reported. Subsurface investigations revealed impacts to soil and groundwater by petroleum hydrocarbons, volatile organic compounds (VOCs), and polynuclear aromatic hydrocarbons (PAHs), and free product was present on the groundwater surface.

A groundwater/product recovery and treatment system operated at the site from January 1998 to April 2001. The system was installed with an autodialer capability to notify maintenance staff of system stoppages. The cumulative product volume recovered by the system totaled approximately 3,076 gallons. Approximately 19 million gallons of impacted groundwater was captured by the recovery system over the system lifetime.

The post-remediation monitoring program includes monthly measurements of groundwater elevation and product thickness, and quarterly analysis of groundwater samples. Product thickness and VOC/PAH concentrations in groundwater decreased significantly following recovery system activation, and have continued to decline following system shutdown.

Testing of groundwater for natural attenuation parameters occurred in May and August of 2001. An anaerobic environment existed in the area of the site with the greatest VOC/PAH impact to groundwater, as indicated by: 1) negative oxidation-reduction potential; 2) depletion of sulfate; and 3) increases in methane, total alkalinity, and ferrous iron concentrations. Anaerobic conditions strengthened between May and August 2001. Bacterial populations that can utilize select hydrocarbons (diesel, gasoline, phenanthrene) as carbon sources were present in groundwater samples. Diesel-degrading bacteria predominated. Between the May and August 2001 events, product thickness, COC concentrations, and bacterial populations all decreased. The data are strongly indicative of effective biological attenuation of the petroleum hydrocarbons. Natural attenuation data to be collected during spring 2002 will also be shared at the conference.