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1 7.0 THORNTON NO. 2 MUNICIPAL SUPPLY WELL - CONSTRUCTION 1 Well construction information on the Thornton Municipal Supply Well No 2 was obtained from the California Department of Water Resources The well is located 115 feet north of the former tank pit and 85 feet from well MW4 The 245 foot well was drilled and constructed in August 1978 by Henning Bros Drilling Company of Modesto, California The well is completed with 14- inch diameter steel casing which is perforated from 135 to 150 feet, 160 to 185 feet, and 225 to ' 235 feet The well sand pack extends from 0 to 245 feet bgs A sanitary surface seal was installed which includes a 30-inch conductor casing cemented in place from 0 to 70 feet bgs ' The well log indicates the first significant impervious strata was encountered from approximately 60 to 94 feet bgs during drilling activities On May 17, 1982, a field inspection of the well by the California Department of Health Services indicated the static groundwater level at the time was 34 9 feet bgs and when pumping the water table dropped to 77 4 feet bgs, a draw down of 42 5 feet 1 8.0 DISTRIBUTION OF PETROLEUM HYDROCARBONS $_1 Soil The distribution of residual TPHg and benzene in soil beneath the site is generally between 11 and 30 feet bgs and extends approximately 70 feet from north of the former UST excavation near the Thornton Municipal Supply Well No 2 compound and associated underground water lines Benzene was generally detected between 20 and 26 feet bgs in the vicinity of borings SB2, SB4, SB6 and SB8 and at 11 feet bgs in MW4 The lateral extent of benzene 1n soil is shown on Figure 4 TPHg was generally detected between ll and 15 feet bgs in the vicinity of SB4, MW3 and MW4 The lateral extent of THPg in soil is shown on Figure 5 Low concentrations of toluene ' and xylenes were detected at 5 5 feet bgs in boring MW5 when monitoring well MW5 was installed in 1991 Soil analytical results are summarized in Table 1 ' 8$=2 Groundwater The estimated lateral extents of benzene and TPHg concentrations in groundwater during this subsurface investigation, utilizing recent quarterly groundwater data are presented on Figures 6 ' and 7, respectively As indicated on Figures 6 and 7, the dissolved petroleum hydrocarbon plume extends north of the former UST excavation, and appears to be centered in the vicinity of well MW4 The lateral extent of the plume has been essentially defined although a data gap is present to the northwest direction Here the lateral extent of the dissolved petroleum plume has been inferred based on site analytical data and field observations Based on groundwater analytical ' data from samples collected from boring SB7, the dissolved petroleum plume in the upper water bearing zone is at a least 30 feet from the Thornton Municipal Water Supply Well No 2 Groundwater analytical results are summarized in Table 2 w 194326\repmu\sumrpt dor. 5 1 • SN�TH r