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�ZNMA Quarterly Report and Closure Request,Site Code 1203 Stockton Regional Wastewater Treatment Facility . 2500 W Navy Dnve,Stockton,CA Project No W0366 01 Page 4 LABORATORY ANALYSES Collected groundwater samples were placed on ice, secured, and transported under chain-of- custody protocol to State-certified RESNA Environmental Laboratories in Fremont, California Samples were analyzed for BTEX and TPHD by EPA methods 8020 and modified 8015, respectively No petroleum hydrocarbon constituents were detected from any on site monitoring well (Table 2) Laboratory data sheets are included as Attachment 2 RECOMMENDATIONS Analytical results from groundwater samples collected on December 28, 1992, indicate no petroleum hydrocarbon constituents are present in groundwater underlying the site These results are consistent with previous quarterly sampling rounds which began in October, 1989 A review of the groundwater analytical data collected from the subject site indicates benzene has never been detected above the method detection limit of 0 5 parts per billion (ppb) Historic water level measurements at the site depict a consistent northeast gradient As discussed in RESNA's quarterly report dated August 14, 1992, and more recently, RESNA's October 6, 1992, site closure request, soil samples collected from borings SB4, SB5, and SB6 exhibited very low levels of toluene, ethylbenzene, and xylenes No benzene was detected in any soil samples collected at the site TPHD was detected in boring SB4 (near the former tank) at only 1 9 ppm (Table 1) All residual diesel hydrocarbons detected in soils at the site are well below the State of California's Primary Maximum Contaminant Levels (MCLs) established for drinking water TPHD, a heavy hydrocarbon, detected near the method detection lima (1 0) in SB4 has relatively non-mobile characteristics in fine grained soils such as those encountered at the subject site (i a silts and clays) The relatively low permeability of these soil types, which occur to a depth of 25 feet (the maximum depth explored) will limit the rate of vertical migration In addition, the surface above and surrounding this area is covered by impermeable asphalt and is graded to prevent ponded water, which greatly limits infiltration of surface water RESNA, on behalf of the City of Stockton, respectfully requests case closure It is the intention of the City to discontinue monitoring and sampling at the site, and to apply for monitoring well abandonment The City anticipates PHS/EHD written concurrence for the closure of this site 4QR-1992 FNL