ARCHIVED REPORTS XR0009904 - Laserfiche WebLink

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Corrective Action Plan Page 12 Controlled Excavation to Remove Severely Affected Soil Based on the guidance provided by the SJCPHS, the distribution of contaminated soil in the subsurface was re-examined with specific reference to the reduction in concentrations of components of hydrocarbons with depth below the water table and the relatively low concentrations that appear to be present in, and adjacent to, the strata of very loose, medium to fine sand that are present in the subsurface The feasibility of safely and cost- effectively remediating at least the preponderance of the contaminated material located beneath the 152 East 11th Street property was explored Following that study it was concluded that, if the existing building could be demolished, it night be possible to rremove a significant volume of contaminated soil from the subsurface, even if considerations related to the safety of neighboring streets and structures, methods for restoring the site in compliance wrth the requisite geotechnical specifications and other practical issues preclude removal of every trace of soil affected by components of gasoline With respect to limitations on the depth to which a remedial excavation can be made, there are three principal concerns, a) the excavation must not penetrate into the loose sands found in an extensive stratum beneath the site at a approximately 12 ft to 15 ft beneath the surface, or come sufficiently close to that depth that the bottom of the remedial pit heaves and sand flows into it, b) if any excavation is performed beneath the water table it must be adequately controlled and c) a means of back-filling the pit with material meeting the applicable engineering specifications must be available Secondary, but important, considerations include the need for sufficient control of excavation work so that raveling or piping from local lenses and pockets of loose sand or silt that have been logged at elevations higher than the extensive stratum of loose sand at a depth of 12 ft to 15 ft (e g the silty sand encountered between b ft to 8 ft beneath the ground surface in the boring ifor monitoring well NAV 3 - see Figure 7) does not compromise the stability of the walls of the excavation or cause subsidence of adjacent property Data from the available boring logs (see Appendix A) and from analyses of soil samples recovered from the borings (see Table 2) and their interpretation in the form of the stratigraphy and estimated distribution of components of hydrocarbons in the subsurface that are shown on Figures 3, 4, 7 and 8 reveal the following a ) The highest concentration of components of hydrocarbons beneath the 152 East 11th Street property have been found in soil located within one or two feet above and below the water table, i e in a zone between approximately 7 ft and 10 ft below the ground surface (Refer to Figures 3 and 4 ) b ) There is a very rapid decrease in the concentration of components of gasoline with increasing depth below the water table Results of analyses of soil samples recovered from depths greater than 10 ft show that contaminant concentration at those depth are more