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commercial enterprises. Throughout this period and continuing to the present, many of the original bunkers and roads continued to be used for commercial/industrial purposes, with pre- existing RABSP roadways and munitions bunkers subsequently used for the manufacture, storage, transport, and disposal of rocket fuels, explosives, fireworks, and other potentially hazardous substances. Recognizing the association between historical RABSP activities and existing soil and groundwater contamination, the U.S. Environmental Protection Agency (USEPA), California Department of Toxic Substance Control (DTSC) and the California Regional Water Quality Control Board (RWQCB) have issued orders to potentially responsible parties (PRPs) to better characterize and remediate contamination on properties within, and adjacent to, the former RABSP. The PRPs represent more than 20 entities including: the Department of Defense, defense contractors, fireworks companies, a hazardous waste management facility, and other entities. For the purposes of this White Paper, these two source areas are referred to as the 'Bunker Area' (located near the central and southwestern portion of the RABSP) and the "160 -Acre Parcel" (located near the northeastern portion of the RABSP). While some perchlorate impacts that originate near the 'Bunker Area" portion of the RABSP have been well-documented in studies completed by the County, characterization of groundwater impacts from the "160 -Acre Parcel' and within the remainder of the RCB are in their initial stages. As detailed herein, interception and control of impacted groundwater that originates in the former RABSP is considered a critical element for cleaning up the RCB. 3 PROJECT OVERVIEW The water supply restoration and plume containment approach generally includes the following: Water Supply Restoration — A wellhead treatment program is necessary to: 1) restore water supplies; 2) reduce the mass of perchlorate within the purveyors' service areas; and, 3) minimize the treatment times and costs for remediation. Wellhead treatment will include: 1) construction and five years of annual costs for new systems at wells which currently have detections of perchlorate and need treatment (where possible, treatment has been grouped for wells to minimize costs); and, 2) funding non -recovered capital costs, operations and maintenance (O&M) costs for existing treatment systems to date, and five years of annual costs for those systems. Plume Containment— High concentrations of perchlorate have been identified in groundwater downgradient of the former RABSP. The County is currently implementing a plume interception and remediation program to partially address 'Bunker Area" impacts. Based on data developed during preparation of the conceptual model and follow-on field characterization studies, a second plume interception program is needed to address impacts from the "160 -Acre Parcel'. Rialto -Colton Basin Technical Committee— White Paper Revised 2/22/2006 2