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a _ Clayton ENVRO\t1ENTAL CO%SLtTANT5 (SJC/EHD) noted that the SJC/EHD had determined that all appropriate response action, or monitoring required with regard to the underground storage tank formerly located at this site had been performed. She recommended site certification and requested that the RWQCB concur. This letter is presented in Appendix B. In a letter to the SJC/EHD, dated August 19, 1991, Ms. Thayer raised the following concern about the soil excavation: "Conts.•ni„wnts in five soil samples ranged from 60 ppm to 1100 ppm oil and grease hydrocarbons. Apparently, additional lateral excavation (over-excavation) occurred towards the northeast. However, it is unclear from either the report of the addendum whether the excavation was extended vertically below 10.5 feet." She raised the following concern about soil samples from the installation of well MW- 3B: "The soil samples from MW-3B indicated soil contamination of 770 ppb total recoverable hydrocarbons at d feet and 20 ppb at 9 feet. The fractions of hydrocarbons were not identified." Based on these concerns, the RWQCB did not concur with site closure. Ms. Thayer - stated that, "Orchard Supply .iardware must present a precise description of the over- excavation activities before closure may be considered, including the fallowing information: 1. Total depth of excavation below samples MW-1, NW-1, MN-1, NE-1, and ME-1; l � _ l 2. The depth to water below grade and below the excavation; than 10.5 feet �Y3 3: If the excavation was deeper , the reasons for not collecting confirmatory.soil samples from the base:of the excavation." We prepa.►.ed this summary report to consolidate and clarify reports, addendums, and letters,-a;id personal communication that transpired between Clayton and the 9;. SJC/EHD. ?t 1.3 MROGEOLOGY Soils underlying the site generally consist of clayey sands, sandy clays, and gravelly sands with gravel being more abundant near the surface. Depth to gt'oundwater'rangm approxi $to 12 feet bgs,ln the three onsite r wells (Figure 2). In the vicinity of the excavation (southwest corner of property) the depth to groundwater is appropmately 10.5 to 12 feet bgs. Well elevations were surveyed to datum sea level by Mr. Bruce Tronoff, a licensed surveyor, and were plotted on a sealed map (Appendix C). From the well elevation data,groundwater f now direction was calculated to be to the north. The grouniiwater gradient is 0.3 foot elevation drop per 100 feet of horizontal distance. 2 . 4