Following a site assessment by soil sampling, an aggressive and expensive pump and treat program was implemented to treat 40 billion gallons of contaminated groundwater. After several years of treatment, the plume remained stable suggesting that significant sources had not been identified accurately in the initial site assessment. Due to difficult site conditions and the costs and variability associated with active soil gas sampling, a large GORE® Survey was implemented in a tiered sampling program. The survey identified a major source not previously located – a buried oil/water separator containing chlorinated solvents.