ARCHIVED REPORTS_XR0004448 - Laserfiche WebLink

Home Browse Search

' Groundwater Investigation,Monitor Well Destruction and Installation and Interim Remediation Pump cTest Report Boyett Petroleum,419 S.Main St,Manteca,CA ' January 30,2004 Page 9 • 3.6 DISCUSSION Four deep CPT boring pairs and five shallow direct push borings were advanced between October 20 and 23, 2003, to evaluate the lateral and vertical extent of groundwater contamination at the site. CPT-3, CPT- 4, and CPT-6 were advanced to depths of approximately 125 feet bgs; CPT-5 encountered refusal at approximately 66 feet bgs. Five direct push borings, HP-1 through HP-5, were advanced to depths of approximately 28 feet bgs. ' Cross-section locations are shown in Figure 3, Appendix A; completed cross-sections are shown in Figure 6 and Figure 7, Appendix A. From the surface to approximately 22 feet bgs (upper horizon), soils at the site appear to be predominantly interbedded silts and sands with interspersed beds of clay. Silt and sand ' interbeds form continuous interfingering horizons of variable thickness. Within this upper horizon, a continuous clayey silt layer was noted in the northeastern portion of the site from approximately five to ten feet bgs (Figure 7, Appendix A). An undulatory silty clay to clay horizon of variable thickness was encountered in the CPT borings at approximately 22 to 28 feet bgs; this clay horizon corresponds to a clay unit noted in existing monitoring well borehole logs and a previous CPT site investigation (Figures 7 and 8, Appendix A). The apparent ' lateral continuity of this clay horizon suggests it may have acted as an aquitard to the vertical migration of contaminants. However, the construction of monitoring well MW-1 bridged this clayey unit and may have created a conduit for vertical contaminant transport (Figure 6, Appendix A). ' Frorn 28 feet bgs to the maximum depth of the investigation (125 feet bgs) the soil profile appears to transition from that dominated by fine-grained soil types to one characterized by medium- to coarse- grained soil types interbedded with clay horizons. A continuous sand horizon with interbedded gravels was encountered in the CPT borings directly below the continuous clay horizon to approximately 53 feet bgs; a thin, discontinuous, clayey silt to silty sand layer was noted in some borings transitioning the contact between the clay unit and the underlying sand horizon. At depths greater than approximately 53 feet bgs, clay horizons form relatively continuous undulatory interbeds of variable thickness. Silt horizons form relatively thin, discontinuous lenticular features intercalated within the dominant sand and clay horizons. Although the lower horizon clay units appear to form relatively continuous, laterally and ' vertically uniform Iayers across the site, they do not appear to effectively impede vertical contaminant migration (Figure 6, Appendix A). This apparent transmissivity could be attributed to the clay units containing a significant silt to sand-sized component. Groundwater was first encountered in the CPT and ' direct push borings at depths ranging from 22 to 28 feet bgs. Laboratory analytical results indicate that groundwater has been impacted by petroleum hydrocarbons ' constituents to the maximum depth of the investigation (at least 122 feet bgs; see Figure 6, Appendix A). Petroleum hydrocarbon constituents, with the exception of MTBE, TBA, TAME, and 1,2-DCA, were not detected at or above laboratory reported detection limits in the groundwater samples collected during the investigation. MTBE was detected in the groundwater samples collected from CPT-3 from depths of 24- 29 feet, 39-44 feet, and 66-71 feet bgs. MTBE, TBA, TAME, and 1,2-DCA were detected in the groundwater samples collected from CPT-4. MTBE was detected in the groundwater samples collected I from CPT-5; a low concentration of 1,2-DCA was detected in groundwater sample collected from 56-28 feet bgs. Low concentrations of MTBE and 1,2-DCA were detected in the groundwater sample collected from CPT-6 at 88-93 feet bgs. MTBE was detected in the groundwater samples collected from HP-1 and HP-3; 1,2-DCA was detected in the groundwater sample collected from HP-3 from 23-28 feet bgs. Isoconcentration contours representing the estimated lateral and vertical extent of the MTBE plume in groundwater are provided in Figures 5 through Figure 7, Appendix A. Laboratory analytical results for ' the November 26, 2003 groundwater monitoring event conducted at the site are provided in Table 3, CONDOR