Oil production began in the 2,700 acre Guadalupe oil field in the late 1940s. In the 1950s, diluent, a kerosene-like hydrocarbon, was placed into wells to help the heavy crude oil flow better. Over the years, the diluent apparently leaked from pipelines in the field, causing contamination of certain surface and subsurface areas. The use of diluent was discontinued in 1990. Unocal ceased all oil production from the field in 1994.
In September 1998, the San Luis Obispo County Board of Supervisors approved a plan to clean up the spilled diluent.
The Guadalupe Restoration Project Team, includes university and private consultants, and public and regulatory agencies.