SITE INFORMATION AND CORRESPONDENCE

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Review for No Further Action - 3 - 15 June 1995 Major Mart, 1235 East Alpine Ave. �l Stockton, Yolo County error and that the gradient may actually have been to the south. IEMunicipal Well #77 is located approximately 450 feet southeast of the site. However, #77, has not been used in the recent past due to solvent contamination from the nearby Kearny-KPF site. Kearny-KPF has been conducting ground water remediation of several water bearing zones and hasp been pumping large volumes of { ground water. Apparently, some recent analyses have shown that #77 has not contained contamination, therefore, the well may have been used at some time during the Major Mart investigation. Both the use of #77 and the Kearny-KPF dewatering may have had an impact on the direction of ground water flow. Monitoring wells MW-1, MW-2, and MW-3 formerly contained significant levels of TPHg, benzene, and xylene contamination, with highs of 4100 ppb TPHg , 19000 pb benzene, and 180 ppb xylene in MW-2 in late 1991. MW-4, MW-5 and MW-6 had low tonon-detectable levels of petroleum hydrocarbon contamination. Since the overexcavation in early 1993, levels of BTEX and TPHg in ground water dropped significantly to nondetectable levels or very low concentrations. No petroleum hydrocarbons have been detected in any of the wells'since August 1993. (See attached map and table of ground water analyses.) Solvent analyses were conducted only in June and October •1994.' DCA was detected in MW-2 and MW-5 at up to 1.6 ppb and PCE was detected in MW-4 and:MW-5 at up to 1.2 ppb. PCE was not used at this site but was detected in the monitoring wells closest to the streets. Secor investigated other possible sources of PCE and found several to the north along West Lane. Sewer lines run down the centers of both Alpine Avenue and West Lane. PCE has been known to migrate from clay sewer pipes into surrounding soil and ground water. 9 � Apparently, in April 1994, Secor submitted soil samples from SB-1 through SB-4 to a lab for bacterial analyses to determine if bacteria may have been the cause of the drop in the contamination of the ground water. Hydrocarbon consumi' g bacteria were present in the saturated zone. If MODELING ;r Secor modeled the movement of BTEX, DCA, and methyl naphthalene using SESOIL. . Residual BTEX and DCA in the former tank pit were•modeled based on a conceptual soil column consisting { of 62 feet of silty clay underlain by 8 feet of silty sand, representing the ground water zone. The chemical distribution of BTEX and DCA within the model extends: in a column from the base of the former tank pit excavation (approximately 32 feet bgs)to the itop of the sandy zone at 62 feet. Based on the parameters used and assumptions made, the modeF'showed only benzene and DCA impacting ground water. Average concentrations of 9.9 ppb beniene and 33 ppb DCA in the soil column between 32 and 66 feet bgs were assumed. Based on these parameters, soil moisture k concentrations were 2.4ppb benzene and 33 ppb DCA at the ground water interface. Based on gradient calculations, the transport rate of DCA ranged from 2.7 to 16.1 feet per year and benzene ranged from 2.4 to 15.4 feet per year. Based on these rates, DCA contamination may reach #77 in 28 years without factoring in degradation rates and variations in gradient. Benzene would not impact the well. �$ iE ;p