TRC was tasked with addressing the discovery of Tertiary Butyl Alcohol (TBA)—a degradation product of the gasoline oxygenate MTBE—in a production well completed in the deep Silverado aquifer below the refinery. Since the early 1990s, TRC had been conducting remedial investigations at this hydrogeologically complex site located above a major fault zone, and had developed a conceptual site model for predicting and evaluating control strategies for plume migration, as well as a risk-based Master Plan for addressing site-wide hydrocarbon impacts. Plume control strategies involving air sparging/soil vapor extraction and oxygenation to enhance natural degradation have been successful in controlling shallow plume migration without groundwater pump and treat or extensive product recovery operations. The new challenge was to assess and mitigate TBA in the deeper aquifer, which is a major drinking water aquifer in the basin.