ARCHIVED REPORTS_XR0009576 - Laserfiche WebLink

Home Browse Search

1 Additional Soil and Groundwater Investigation Report Canepa s Car Wash(Park Street) ' Juiy 12,2004 Page 9 . near the former USTs and down-gradient of the former USTs at all of the sample locations The results of ' the groundwater samples from the CPT3, CPT5, and CPT6 sample locations geperally indicate that the lateral extent of shallow groundwater contamination has been adequately investigated to the east, northeast, and west The deep intermediate water bearing zone at approximately 74 to 85 feet bgs appears ' to be the most highly impacted zone, and the lateral extent of contaminants in this zone, as well as the lateral extent of contaminants in the deep water bearing zones below approximately 98 feet, have not yet been defined It appears that the interpreted clayey horizon beginning at approximately 85 feet bgs is ' significantly impeding the vertical migration of contaminants in groundwater, as contaminant concentrations from samples collected below approximately 100 feet are generally one to two orders of magnitude less than the samples collected above the clayey horizon The highest concentrations of petroleum hydrocarbon constituents were detected in the shallow groundwater sample collected from the CPT7 boring in the presumed source area, at 140,000 µg/L (TPH- ' G) and 32,000 Nig/L (toluene) The highest concentrations of benzene were encountered in each of the three down gradient groundwater samples collected from intermediate groundwater depths at 6,000 µg/L in CPT3 (80-84 feet), at 8,500 pg/L in CPT4 (78-82 feet), and at 7,500 µg/L in CPT5 (82-86 feet) MTBE, DIPE, and 1,2-DCA were sporadically detected in groundwater samples at low concentrations up to 16 �tg/L, 4 1 lig/L, and 23 µg/L, respectively MTBE was generally detected at very low concentrations in the samples collected from shallow groundwater at the periphery of the site contaminant plume, reinforcing previous indications that the MTBE may be from an off-site source However, interference from high concentrations of non-target analytes may have masked lower concentrations of MTBE within the core of the site contaminant plume In summary, the results of the groundwater and soil investigation indicate that the lateral extent of contaminants in shallow groundwater and the vertical extent of soil contamination in the presumed ' source area at the site have been adequately investigated The lateral extent of contamination in intermediate, and to a lesser extent, deep water-bearing zones north, northeast and northwest of the site needs further investigation Four of the five CPT/hydropunch boring pairs encountered refusal prior to fully investigating the vertical extent of groundwater contamination Generally low concentrations of petroleum contaminants were encountered in the deepest samples collected, however 6 0 RECOMMENDATIONS Condor recommends additional groundwater investigation at the site to further evaluate the vertical and horizontal distribution of the contaminant plume Condor recommends the installation of five multi- screened, discrete depth monitoring wells to evaluate and monitor the extent and movenae to of f'ie contaminant plume int e�ii e dimension h laboratory samples and hydraulic head measurements A work plan addendum describing the proposed activities is included below 7 0 WORK PLAN ADDENDUM This work plan addendum was prepared by Condor to describe the installation of five proposed I monitoring wells at Canepa's Car Wash, located at 248 Paris Street in Stockton, California (Figure 1 and 2) The scope of work detailed in this work plan addendum is in addition to the installation of five CPT borings proposed in the Work Plan for Additional Soil And Groundwater ,Investigation by Cone Penetrometer Testing, dated November 1, 2002, prepared by Condor, and approved by SJCEHD on March 18, 2003 At the direction of Mr Wong of the SJCEHD, the above referenced work plan was to be implemented in phases, with the number and locations of the proposed deep monitoring wells to be dependent on the results of the CPT investigation The proposed work described in this work plan addendum is for the installation of five multi-screened, discrete depth groundwater monitoring wells t g CONDOR