SITE INFORMATION AND CORRESPONDENCE

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X Case Closure Summary Page 2 4. r Iing Underground Fuel Storage Tank Proe4-0-11 � 1 111. Release and Site Characterization Information continued Maximum Documented Contaminant Concentrations--Before and After Cleanup Soil Wate Contaminant Soil Soil Water Water Contaminant Before After Before Water After Before After Before After (PPm) (ppm) (ppm) (Ppm) TPH (Gas) 3,800 <1.0 93 <0.050 MTBE NA 0.095 0 0.080 <1.0 32 <0.050 TBA NA <0.025 NAA <0.005 TPH(Diesel) 45 <0.0005 EDB NA <0.005 NA <0.0005 Benzene 5.9 <0.005 6.5 <p_005 NA <0.0005 Toluene 46 <0.005 35 <0.0005 1,2-DCA NA Ethylbenzene 23 <0.005 4,5 <0.0005 Oxygenates NA <0.005 NA <0.005 Xylenes 150 <0.005 25 <0.0005 Water Before results are from SB5 on 12112196. Soil Before results are from over-excavation sampling 8129195. Water After results reflected the latest round of MW sampling in April 2003 and CPT investigation in July 2003. Soil samples reflected the last time soil samples were collected during installation of MW 6, 7, and 8 in November 2000. The presence of MTBE in the soil samples most likely comes from cross-contamination of dissolved MTBE. Concentrations of all contaminants in groundwater in the latest sampling were not detect except MTBE. The MTBE plume is mobile in a down gradient direction, but the concentration has declined since 1996. The extent of the plume has been defined,with the vertical extent at less than 100 ft bsg (from CPT samples). The dissolved MTBE appears to be in a sand layer at approximately 60 feet to 85 feet bsg, as consistently indicated by the results of the deepest well on site MW-9 which was screened 70 to 75 feet bsg. The consultant calculated an attenuation by dispersion time of 1260 days for the plume, which translates to a transport distance of less than 100 feet beyond the furthest down-gradient well MW-10. They also plotted a Non-Steady State F Spreadsheet Analytical Model,which estimated a period of 4.3 years for the MTBE plume to reach a concentration of 4.96 ugll of MTBE at a projected distance of 1,700 feet and model velocity of 0.07 ft/day. With the exception of one 53.7-foot monitoring well located 1,600 feet southeast of the site, no other wells were identified in a 2,000-foot well survey. The remaining mass of dissolved MTBE was calculated at 1.4 pounds. Beneficial uses of groundwater are protected, now and in the future. Iv. Closure rFDoEescom:pleted corrective action protect existing beneficial uses per the Regional Board Basin Plan? Yes pleted corrective action protect potential beneficial uses per the Regional Board Basin Plan? Yes I� Does corrective action protect public health for current land use? Yes Site management requirements: none Should corrective action be reviewed if land use changes?Yes Monitoring wells decommissioned: Yes Number Decommissioned: 10 Number retained: 0 i List enforcement actions taken: NOR sent List enforcement actions rescinded: None V. Local Agency Representative Data Name: Donn eran Title: Director of Environmental Health Signature: V V\� ri.w v .1-- Date: 1 f— ��—0 t f VI. Central Valley Regional Water Quality Control Board RB Notification Date Submitted to RB: Response Date: March 8, 2004 RB Staff: James L. Barton Response: No Further Action Required Occurrence Title:Assoc Eng Geologist VII. Additional Comments, Data, etc. 4 MW were destroyed by overdrilling, 6 MW were destroyed by pressure grout as witnessed on April 7 to 8, 2004 This document and the related CASE CLOSURE LETTER,Shall be retained by the lead agency as part of the official site file. (08124193) Exhibit N Page 2 of 2