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i � 'yl • PH October 3,,2002 2002 Page 4 of 5 of the former UST (exclusive of the on-site water supply well) is located more than 1,000 feet from the former UST while the contaminant plume apparently extends less than 100 feet from the former UST The site facility well is located approximately 0 feet northwest (downgradient) of the ormer UST The closest upgradient well to the former UST is the domestic well located at the residence on the southwest corner of Almondwood and Oleander This well is located approximately 80 ft south-southeast of the former UST Well construction details for this well and for the facility well were not available A copy of the table and figure from the well receptor survey are included in Attachment E + SJC/EHD expressed concerned that Ground Zero performed no fate and transport modeling to support the theory that this is a low risk site Response: In our opinion, the data collected from this site during the course of this investigation are more than adequate to illustrate that this is a low risk site The tank removal �,'v, ° aitd minor overexcavation�have removed the contamination source from the soil Dissolved Vey'�lhydrocarbons in groundwater have declined significantly in all wells MTBE is present in 'groundwater beneath the site, but has generally declined and is currently only marginally above the secondary MCL in well MW-1 (6 0 ppb in March 2002), is below the MCL for wells MW3 and MW4, and is below the laboratory detection limit in well MW2 Contaminant • concentrations in groundwater and soil are expected to continue to decline through natural attenuation However, to address your concerns we modeled the transport of dissolved contaminants from the source area to the downgradient facility well and also to the nearest upgradient domestic well For the groundwater transport modeling it was assumed that the hydraulic conductivity of the aquifer is 10.2 cm/sec, the hydraulic gradient is 0 001, the plume width at the source area is 60 feet, the vertical groundwater mixing zone is 8 feet, the distance from source to receptor is 190 feet to the facility well and 80 feet to the domestic well, effective porosity of the aquifer material is 35%, the organic carbon content of the aquifer material is 0 1%, contaminants disperse with transport and the contaminants decay at the first order rates estimated by Howard The source zone concentrations (mean concentrations in MW I) and predicted concentrations at the receptor wells are summarized below Compound Source Conc Facility Well Conc Domestic Well Conc Benzene 1 7 ppb 0 024 ppb 0 32 ppb Toluene 0 8 ppb 4 0 E-13 ppb 6 5 E-8 ppb Ethybenzene 13 4 ppb 9 7 E-5 ppb 0 034 ppb Xylenes 117 ppb 0 0039 ppb 0 29 ppb MTBE 8 6 ppb 0 037 ppb 1 0 ppb G 1Data1GROUNDZEChifiBoy\Corresp10902gpsrevd doc