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CLEAkWA'TE-R I c a oy P Environmental Services I the source of the majority of the dissolved-phase plume appears to be this area Based on assessment results, an estimated 4,030 cubic yards (in-place) of soil with TPHg concentrations greater than 1 mg/kg reside in this area However, due to practical constraints of equipment ' limitations, excavation sloping, and soil decompaction, it is estimated that excavation in this area would generate approximately 15,200 cubic yards of soil ' Based on assessment data, shallow soils (i e less than 19 feet bgs) in the area of the sorbed-phase footprint are free of gasoline contaminants Assuming this clean soil were segregated during excavation, approximately 5,250 cubic yards of contaminated soil would be generated for treatment The space available at the site is large enough to stockpile and aerate soil, so off-site treatment of contaminated soil is not the only option If off-site treatment were necessary, NDS in Richmond, CA is the closest facility This option would also require the destruction of wells MW-1, MW-3 and MW-5B (see Site Plan Figure 2) New wells should be reinstalled following excavation to continue monitoring of maximum dissolved-phase contaminant concentrations Another consideration is that not all contaminated soil could recovered through excavation as a portion of the sorbed-phase plume hes beneath a building, and the furthest eastern edge is under South Fresno Street, where the city of Stockton has already denied encroachment permits for monitoring well installations I6 2 2 Soil Vapor Extraction and Treatment Soil vapor extraction (SVE) involves the removal of hydrocarbon vapor from soil pore space in the vadose zone by applying a vacuum to wells to induce airflow through the contaminated zone Volatile hydrocarbon constituents are released from the soil into the air stream and treated at the surface as required SVE most effectively removes hydrocarbons from the vadose zone in situations where soil permeability is moderate to high, and where the contaminants consist primarily of the more volatile hydrocarbon compounds (i a gasoline constituents) The relatively fine-grained sediments observed beneath the site are not very conducive to any technology relying on the transmission of air or water through the subsurface Therefore, the soils beneath the site are not well suited for SVE alone Also, shallow water tables can make SVE problematic (i a upwelling and occlusion of well screen), the relatively low permeability of . soils at the site would require relatively high vacuums to produce sufficient air flow rates compounding the problem Additionally, SVE alone in areas of high water table would be IZB 171C CAP 13 November 20,2002