SITE INFORMATION AND CORRESPONDENCE

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GEOMATRix advanced and four groundwater monitoring wells(MW-1, MW-2,MW-3, and MW-4) have been installed at the site to depths of approximately 37 to 39 feet below ground surface (bgs). Results of these investigations indicated the presence of petroleum hydrocarbons in site soil and in groundwater. These investigative activities and pertinent results are summarized below; a more comprehensive description is provided in Geomatrix's February 1999 Site Investigation and Screening-Level Health Risk Assessment. Soil and groundwater sampling locations are shown on Figure 2. Historical soil analytical results are presented in Tables 1 and 2. Histori- cal, and current, groundwater analytical results are presented in Tables 3 and 4. 2.2.1 Soil Conditions Petroleum hydrocarbons were reportedly observed in soil borings SB-2 and SB-8 and the bor- ing of monitoring well MW-1 at depths between 10 and 50 feet bgs. Petroleum-hydrocarbon affected soil was not observed in borings SB-1, SB-3 through SB-7, or in MW-2 through MW-4. The lateral extent of residual petroleum at the site has been assessed in previous inves- tigations and appears to be constrained within the area of SB-2, SB-8, and MW-1 and bounded in four directions by borings SB-3 through SB-6 (Figure 2)_ The vertical extent of residual petroleum has been identified above and below the water table between depths of approxi- mately 15 and 50 feet bgs using field observations; based on experience at other OVP sites, visual observation has been effective to delineate the extent of residual petroleum,particularly within the saturated zone. 2.2.2 Groundwater Conditions Based on historical and current water level measurements, groundwater is encountered between approximately 31 to 37 feet bgs(69 to 74 feet mean sea level) and has a general flow direction to the east-northeast. Historical groundwater monitoring and sampling was performed by Geomatrix in July and November of 1998. As shown in Tables 3 and 4,total petroleum hydrocarbons as motor oil (TPHmo),total petroleum hydrocarbons as gasoline(TPHg),benzene,toluene, ethylbenzene, and xylenes(BTEX), and polynuclear aromatic hydrocarbons (PAHs) were not detected in groundwater samples from either the July or November 1998 sampling events. In the July 1998 samples,TPHd was detected at concentrations slightly above water quality objectives' (WQO) at concentrations of 230, 53, and 150 micrograms per liter{µg11} in monitoring wells MW-2 1 The water quality objective utilized by the Central Valley Regional Water Quality Control Board for diesel fuel in 100µg11 and based on secondary criteria(i.e.,taste and odor threshold). lloak1ldOtdatalDoc__Safe1500ft501 LOMRepomSoil 2nd GW Repomdoc 3