The purpose of the Proposed Action is to secure sewage effluent discharge compliance with the Cease and Desist Order issued by the San Diego Regional Water Quality Control Board (RWQCB) at existing Sewage Treatment Plants (STPs) 1, 2, 3, 8, and 13 within the Lower Santa Margarita Basin. Compliance with the Cease and Desist Order would in turn allow for compliance with the San Diego Water Quality Control Plan (Basin Plan), thereby enhancing water quality within the Basin and providing long-term protection of the aquifer at Marine Corps Base (MCB) Camp Pendleton. The portion of the Proposed Action analyzed in the Final Environmental Imp act Statement/Report (FEIS/R) and approved in the Record of Decision involves the construction of new facilities to convey sewage from the STPs downstream through a system of pumps and pipelines to percolation ponds in the Lemon Grove area of MCB Camp Pendleton. The ultimate disposal of the effluent, which may include advanced treatment, is considered in detail in this Draft Supplementary Environmental Impact Statement (SEIS). Alternative 1 would provide facilities for percolation of the total volume of effluent by enhancement of the Lemon Grove ponds with vertical drains of sand and gravel. Alternative 2 would add advanced wastewater treatment of effluent from STPs 1 and 2, with live-stream discharge of that effluent at the Ysidora Flats section of the Santa Margarita River. The effluent from STPs 3, 8 and 13 would be disposed at the Lemon Grove ponds with vertical drains. Alternative 3 would provide tertiary treatment and nutrient removal for STP 13 effluent, blending with secondary effluent from STPs 1, 2, 3, and 8, with discharge at the Ysidora Flats section of the Santa Margarita River. Alternative 4 would provide advanced wastewater treatment facilities for disposal of the total volume of effluent by gravity injection wells at the Lemon Grove ponds.