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+.. Geological Technics inc. Page 2 City of Escalon Project No. 750.3 October 31,2000 GNP from borehole to borehole. A couple of non-continuous clay layers were encountered in a few of the boreholes. 1.2 Site Hydrology As of yet, monitoring wells have not been installed at the site therefore site-specific data is not availa e for groundwater gradient direction and slope. During the drilling of the previous boreholes, groundwater was encountered at 60-61 feet. The capillary fringe appears to extend less than 2 feet from the groundwater Ievel. The soil was generally moist from the surface to the groundwater interface. 2.0 VAPOR EXTRACTION—REMEDIAL ACTION Soil vapor extraction has proven to be an effective in situ remedial method for soils of a permeable nature and can also be used to mitigate volatile hydrocarbons in groundwater by drawing air through permeable geologic units above the capillary fringe. �. Vapor extraction is the process of removing volatile organic compounds in the subsurface by drawing air through the subsurface sediments, which allow a transfer of chemical mass from the liquid state to the vapor state. As vapor, the chemicals are removed from the subsurface in the extraction air stream and destroyed aboveground via thermal oxidation. It has been shown by Beckett and Huntley (Ground Water Vol. 32,No. 2, March-April 1994, Published by the National Ground Water Association), that not only horizontal flow of air in the subsurface, but also vertical flow is an important component of soil vapor extraction remedial process. This is especially true for mitigation of low permeable units such as silts and clays. This site is primarily sand, which is course-grained sediment. 2.1 Proposed Vapor Extraction Wells No additional vapor extraction wells (VEW) are proposed at this time. Data from the pilot test suggests that the vadose zone beneath the site is under the influence of the existing vapor extraction wells. 2.2 Proposed Air Recharge Wells The existing groundwater monitoring wells and unused vapor extraction will be used for air- recharge wells by fitting them with breather caps. The breather caps will be constructed so as to not allow rainwater to enter the wells. 2.3 Piping and Manifold System v It is proposed that the vapor extraction and treatment system unit (VETS - a blower and thermal oxidizer) be placed, west of the former UST location, on the:north side of the ,., building, for the purpose of maximum cost effectiveness of installation and operation. The PVC piping will be connected from the piping of the vapor extraction wells to the VETS enclosure via an underground trenching and piping system. A manifold valve unit will be used to control the direction of vapor extraction. The piping will be 3 inch inside diameter to ,. reduce the amount of airflow lost to friction.