SITE INFORMATION AND CORRESPONDENCE FILE 1

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I J r �f/ Mr.Denis Brown Re: 620 W. Charter Way, Stockton, CA Page 2 of 3 i • Elevated concentrations of TPH-g and BTEX in groundwater samples collected from the 45- to 50-foot bsg interval from CPT-1 through CPT-4 and CPT-10 lead Cambria to recommend installation of additional groundwater monitoring wells screened at that depth interval near the current USTs; in the former UST excavation area; on the site's northeast corner; on the Best Western property to the east(2 wells); and on the Delta Market property northeast of the subject site, on the north side of Charter Way. • Installation of deeper monitoring wc:iis, screened at approximately 65 to 77 feet bsg and located in the same areas noted immediately above, were also recommended based on elevated concentrations of TPH-g and BTEX in groundwater samples collected from this interval at six CPT borings. • A well screened at 90 feet bsg in the current UST area was recommended to further assess the vertical extent of elevated concentrations for TPH-g and BTEX encountered at this depth in CPT-5. • The widespread distribution of 1,2-DCA lead Cambria to recommend advancing a boring up gradient on the site to assess the extent of 1,2-DCA contamination in soil and groundwater. The EHD concurs with the recommendation for advancement of one up-gradient boring to investigate the presence of 1,2-DCA in soil and groundwater, and with the exception of this one boring, the EHD concurs with Cambria's recommendation for no additional soil assessment at this time. To evaluate the other recommendations, the EHD prepared cross sections and maps showing the distribution of coarse-grained and fine-grained sediments, and the distribution of contaminants. Intervals of high and low permeability, inferred from the pore-pressure response of the CPT tool, were included on the cross sections. Five sand units were identified, of which two were mappable, and three were apparently of limited distribution. Based on the EHD maps and cross sections, EHD approves the recommended wells screened in the 45- to 50-foot depth interval and the 65- to 77- foot interval, but notes the.following observations and recommendations regarding the proposed wells: The 45-to 50-foot depth interval is characterized by moderate to high permeable soil that is laterally equivalent to a sand unit in the southwest portion of the site and in the northeast area north of Charter Way. This interval rests on a variable low permeable zone of limited lateral extent generally between 50 and 60 feet bsg, and occurs below a mappable sand unit locally between 25 and 35 feet bsg. Hydropunch groundwater samples from the sand units have not contained detectable concentrations of the contaminants of concern, whereas samples from the finer-grained portions have been impacted, suggesting that contaminants, if bled into the sand units, are rapidly dispersed or diluted, while those remaining in the finer-grained portion of the interval are subject to both reduced dilution and retarded transportation.