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MOW e <br /> N: IN <br /> i <br /> 3. CONTAMINANT ASSESSMENT <br /> f . <br /> 3 . 1 PRIOR INVESTIGATIONS <br /> ' a The site was closed after a product line failed in November 1985. <br /> The amount of fuel lost was unknown. Subsequent tank and product <br /> line removal was supervised by GTI. GTI installed two monitoring <br /> wells in January 1986 (MW1 , MW2, Figure 5 ) and an additional well <br /> in December 1986 (MW3 ) . On the basis of the boring logs, the <br /> materials immediately underlying the site consist predominantly <br /> of clays and sands, with lesser amounts of silt, silty clay, and <br /> silty sand with increasing depth. Two more monitoring wells <br /> (MW4, MW5 ) were installed by GTI in May 1987. <br /> F•-d <br /> 3.2 SOIL VAPOR CONTAMINANT ASSESSMENT <br /> On 23 and 24 November 1987, EA conducted an SVGA survey at the <br /> Stockton site. Deep (approximately 20 feet) soil gas samples <br /> •"') were collected from six locations on-site (V1-V6, Figuxe S ) , and <br /> monitoring well headspace samples were taken from the five <br /> monitoring wells. A follow-up investigati-7n fQr the pretence of <br /> shallow hydrocarbon levels was conducted c- i 22 December 1987. <br /> Shallow samples were collected from five :,oc''.taons (V7-V11 , <br /> Figure 5 ) . <br /> w„, •' <br /> 3. 3 GROUND-WATER. CHEMISTRY <br /> a The results of ground water monit:, - ;,g i",-Ummarfzed Lor samples <br /> collected in November 1986, June -vd 7�-_l3ruary ''."T-able <br /> 2) Through all three periods toad. the hi()hest” 35ydro <br /> PA carbon. levels of the the five .fells.. ': WIe ; well is ;located in the <br /> £ former tank field area. Sinco monitc. in began in November 1986, <br /> " s there has been a general declipO in t,y&c-o, arbi)ns in ground water <br /> beneath this site (Figure 6 ). nexhgp,:, most V.z levant is the <br /> �r ! recent sampling, in which berz<sne, jluz�ie; xylene, and total <br /> { petroleum hydrocarbons be2,11)+i t_tc l detection •limits in <br /> t all wells except MW3. Hydroca�Euvn �:. r.� e st.All present at MW3, <br /> but at significantly lower. level% Cnd-ji thew recorded in previous <br /> samplings. Benzene concentration in particular had dropped byan <br /> order, of magnitude, to 0.3 ppm. Laboratory reports are provided ; <br /> i in Appendix A. Weekly well. gauging has shown no evidence of free <br /> g { product in any of the wells. <br /> --- 3.,4-__SOIL-CHEMISTRY <br /> - - <br /> j : Soil samples were :collected during well drilling and analyzed for <br /> fuel constituents (GTI 1986, 1987) . Soil .samples from MW3 <br /> E kA contained .very low levels of benzene, toluene, xylenes, and total` <br /> petroleum hydrocarbons (Table 3 ) Soil samples from MW4 and MW5 <br /> contained nondetectable levels of benzene, toluene, xylenes, or <br /> total petroleum hydrocarbons (Table 3 ) . The soil vapor assess- <br /> ments conducted by EA in November and December 1987 examined . <br /> soils throughout the site at depths of up to 23 feet without <br /> 4 <br />