SITE INFORMATION AND CORRESPONDENCE_PRE 2019

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3.2 Groundwater Pathway 3.2.1 Hydrogeological Setting. The site is located within the Great Valley geologic province in northern San Joaquin County. To the east are metamorphic and igneous rocks of the Sierra Nevada geologic province. The Stockton area is underlain by continental rocks and deposits from the Miocene to Holocene age. Heterogenous mixtures of poorly sorted clay, silt, sand, and gravel, as well as some beds of claystone, siltstone, sandstone, and conglomerate are present beneath the area to depths of about 2,000 feet. Informal units of significance to area fresh water supplies in the Great Valley, from ground surface downward, include: the Younger Alluvium, the Older Alluvium, the Turlock Lake Formation, the Laguna Formation, the Mehrten Formation, and the Valley Springs Formation. Continental and marine rocks and deposits occur below about 2,000 feet. These deposits are predominantly sandstone, with some conglomerate and shale, marine rocks, and deposits of clay, shale, siltstone, and sandstone. (11) Based on groundwater sampling events conducted in the onsite monitoring wells, the depth at which groundwater is first encountered beneath the site is approximately 80 feet bgs. The groundwater flow direction beneath the site is to the northeast (4). 3.2.2 Groundwater Targets. Domestic wells in the vicinity of the site draw water from a shallow drinking-water aquifer that occurs between approximately 40 feet bgs and 175 feet bgs (12). The population multiplier for the domestic drinking-water wells in the Stockton area is 3.3 (14). It is estimated that 29 domestic drinking-water wells serve approximately 95 people within 4 miles of the site. The nearest currently active domestic well is less than 0.25 mile from the site (9). Municipal wells in the vicinity of the site are screened in a deeper aquifer. The nearest municipal drinking-water well (Well 79-01), located less than 0.25 mile from the site, is screened between 298 feet and 498 feet bgs. The log of Well 79-01 indicates that a 20-foot-thick clay layer, which may be present beneath the site, occurs between the shallow aquifer and the deeper aquifer. (13) 3.2.3 Groundwater Pathway Conclusion. Domestic wells in the vicinity of the site are screened in a shallow drinking-water aquifer that occurs between approximately 40 feet bgs and 175 feet bgs (12). Depth to groundwater beneath the site is approximately 80 feet bgs. The groundwater flow direction beneath the site is to the northeast. (4) Analytical results of groundwater samples collected in 1993 from an onsite domestic well indicated the following concentrations of HVOCs: 1,1,-DCA at 63 ppb, 1,1,-DCE at 230 ppb, and TCE at 12 ppb (5). No background data are available. The onsite domestic well was taken out of service in 1993 (7). Municipal wells in the vicinity of the site are screened in a deeper aquifer. The nearest municipal drinking-water well (Well 79-01), less than 0.25 mile from the site, is screened between 298 feet and 498 feet bgs. The log of Well 79-01 indicates that there is a 20-foot-thick clay layer, which may be present beneath the site, that separates the shallow aquifer and the deeper aquifer. (13) None of the municipal drinking-water wells in the vicinity of the site is contaminated with hazardous substances attributable to the site (15). The population multiplier for the domestic drinking-water wells in the Stockton area is 3.3 (14). It is estimated that 29 domestic drinking- water wells serve approximately 95 people within 4 miles of the site. The nearest currently active domestic well is less than 0.25 mile from the site (9). 3.3 Surface Water Pathway Surface water runoff from the site flows northwest into the Stockton Diverting Canal, approximately 1 mile from the site. The Canal flows into the Calaveras River, approximately 2 PA Marlowe Property-SS • 6/95 6 Printed on 50% recyded paper.