SITE INFORMATION AND CORRESPONDENCE

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Californi- •gional Water Quality Co' 1.1 Board : l\- Central Valley Region , v Arnold Linda S.Adams Secretaryfor Sacramento Main Office Schwaaenegger Environmental 11020 Sun Center Drive#200,Rancho Cordova,California 95670-6114 Governor Protection Phone(916)464-3291 •FAX(916)4644645 hup://www.waterbowds.ca.gov/centralval ley 15 December 2006 , Ms. Margaret Lagorio Supervising Registered Environmental Health Specialist San Joaquin County Environmental Health Department 304 East Weber Avenue, Third Floor Stockton, CA 95202 NON-CONCURRENCE TO SAN JOAQUIN COUNTY CASE CLOSURE REQUEST, SHELL STATION NO. 204-7524-4404, 3011 W. BENJAMIN HOLT DRIVE, STOCKTON, CALIFORNIA, SAN JOAQUIN COUNTY On 30 August 2006, Regional Board staff received the San Joaquin County Environmental Health Department's (SJCEHD) Case Closure Summary (attached) for the Shell Station No. 204-7524-4404 (Site) with a request for closure concurrence from the Executive Officer of the Central Valley Regional Water Quality Control Board (Regional Board). Regional Board staff reviewed the Case Closure summary and the Site file, and do not concur with SJCEHD's closure recommendation. The SCEHD closure recommendation is not appropriate at this time (see enclosed memo and Appendix A checklist) for the following: • The lack of data beyond the current monitoring well network (downgradient wells are impacted by MtBE) shows the extent of the MtBE is not defined laterally in the apparent downgradient direction, which is a requirement for closure. • While MtBE concentrations onsite have declined 3 orders of magnitude since 1997, residual MtBE concentrations continue to fluctuate in the downgradient monitoring wells to levels above Water Quality Objectives (5 ug/L), which are not protective of beneficial uses of groundwater and may indicate an unstable groundwater plume with continued leaching of MtBE contamination from soil to groundwater (See Graphs in Memo). • The data show that the MtBE groundwater plume has moved under Interstate 5 and may be continuing towards sensitive receptors (irrigation wells) on the opposite (east) side of Interstate 5. Shell must show declining concentration trends for all constituents, predict when Water Quality Objectives will be met, and determine the downgradient extent of the petroleum hydrocarbons in groundwater to evaluate threats to sensitive receptors. Until this work is completed, a closure recommendation is inappropriate. California Environmental Protection Agency Qa Recycled Paper