ARCHIVED REPORTS_XR0004864 - Laserfiche WebLink

Home Browse Search

5.0 CONCLUSIONS ' American Geological Services has calculated that 1,522 cubic yards of gasoline contaminated • Y soil was excavated during the performance of the approved remedial excavation. Soil sample analytical data collected from known or suspected contaminated areas indicate that gasoline and gasoline constituent levels were higher than determined from previous soil boring data. A ' calculation of mass of gasoline removed from the two excavations using average contaminant concentrations indicates that approximately 4,000 gallons of gasoline were present in the removed soil. Residual soil contaminants remain along the excavation sidewalls to the north, northeast, and east of the former UST. These areas are not accessible to excavation due to the presence of structural features. Monitoring well MW-6 is located downgradient from these areas and has only shown very low levels of gasoline and BTEX in past quarterly monitoring events. The fine- grained texture of the soil is believed to strongly adsorb the contaminants thereby immobilizing ' them in the soil and minimizing the potential risk to ground water quality. The presence of an areal extensive clay layer at an approximate depth of 18 feet bgs is believed ' to have limited the vertical migration of contamination. The boundary between gasoline impacted soils and the clay layer was marked by a distinct color change and a sharp decline in PID values. Based on these observations and data it is believed that the excavation to ' approximately 18 feet was warranted. lie 6.0 RECOMMENDATIONS Based on the results of the remedial soil excavation AGS makes the following recommendations: i ' • Install one ground water monitoring well near the former location of the UST to replace MW-1. This well should be installed to a depth of 18 feet bgs, and include 10 feet of screened well casing. The well would be completed above grade in a monument that extends ' approximately 30 inches above the existing ground surface. Figure 5 shows a typical well construction diagram for the proposed well. • Evaluate effectiveness of the remedial program over several quarterly ground water sampling events. Reductions of gasoline and BTEX levels at the proposed monitoring well and ' stability of levels at MW-6 would indicate a successful remediation. A request for site closure will be prepared when ground water analytical data indicate that the risk to ground water quality has been minimized. ' American Geological Services,Inc. CA95 DE-029 17