Oil exploration and production began in the area with the Sand Dune Oil Company in October 1947. Their efforts were limited to the southwest portion of the Former Guadalupe Oil Field (GOF), as it is known today. The Oil Company was purchased in 1948 by the Continental Oil Company, which completed the first commercial well. Continental completed five additional wells and shut down the field in October 1949. By July 1950, a 28 well field expansion had begun. Unocal Corporation acquired a 49 percent interest in the field in 1951. By March 1953, field production was up to 2,000 barrels per day (bpd) from 34 wells. Unocal purchased the remaining 51 percent of ownership of the field in June 1953. Diluent, a crude oil thinner, was introduced in the 1950's to assist in the production and transportation of the heavy crude. Unocal oil production operations continued until March 1990, with gradual field expansion up to 215 potential producing wells in 1988 and field-wide oil production rates of approximately 4,500 bpd2. As many as 23 wells remained in operation until April 1994 using steam injection for enhanced recovery. In addition to the oil wells and associated pipelines, typical oil field infrastructure was constructed over the site. The infrastructure included tank batteries, surface impoundments, steam generators, weigh meter stations, roads, and electric power distribution equipment. The use of most the crude oil produced from the site was extremely viscous, with a density that causes the crude oil to behave like asphalt at ambient conditions. To enhance the flow characteristics of the crude, two main methods were used; diluent mixing and steam injection. Diluent is similar to kerosene/diesel mixture and contains low levels of volatile compounds (e.g., benzene, toluene, ethyl benzene, and total xylenes [BTEX]) that are frequently associated with petroleum products. Unocal records indicate that diluent was used at the oil field from about 1955 to 1990. During the time period that diluent was utilized, numerous leaks occurred throughout the oilfield.