SITE INFORMATION AND CORRESPONDENCE

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page 2 1604 B Street Review of the historical soil analytical data for this site indicates that, with the exception of the tank pull results at 12 to 14 feet bsg, soil contamination is present at intervals from approximately 35 to 66 feet bsg. There is no indication from the available data that there is soil contamination above 25 feet bsg in the area of the proposed SVE well. Section 5.0, Proposed Pilot Test and Rationale, states that "Airflow pathways are predicted to be radial outward from the air sparge well through the permeable sandy formation overlying the clay boundary at approximately 27 feet bsg." This statement is unclear, as the stated depth of the AS well screen interval is from 57 to 60 feet bsg. Boring logs for the site indicate a sand lens extending from approximately 23 to 27 feet bsg, with clayey silts, clayey sands and silty clay from 27 feet to approximately 53 feet bsg. Below 53 feet bsg the lithology is mostly sand to 70 feet bsg. PHS/EHD is concerned about the efficacy of a SVE well which is screened to 25 feet bsg in being able to capture vapors associated with an AS well screened at 60 feet bsg, when there is a 25 to 30 foot interval of silts and clays between them. The EPA document How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank Sites, Chapter 7-Air Sparging, page VII-6, states that air sparging cannot be used to treat groundwater in a confined aquifer because the injected air would be trapped by the saturated confining layer and could not escape to the unsaturated zone. The document further states, on page VII-14, that stratified or highly variable heterogeneous soils typically create the greatest barriers to air sparging. Both the injected air and the stripped vapors will travel along the paths of least resistance (coarse-grained zones) and could travel a great lateral distance from the injection point. Section 5.0 also discusses the use of a tracer gas (helium) as an indicator compound of the radius of influence (ROI) of the injected air. Section 6.0 Pilot Test Objectives states that the first objective is to "Estimate the air sparge ROI by using tracer gas in the injected air and by monitoring vadose zone soil gas composition to detect its presence at nearby observation points." However, details of how the vadose zone soil gas will be monitored, or specifically what 'observation points' are being referred to, are not included in the work plan. These points must be clarified in writing to PHS/EHD. Further in Section 6.0, under the title "Air Sparge and Limited Vapor Extraction Test," the work plan states that"the vapor extraction unit will produce a vacuum cone of depression within the area of air injection and toward the three grouped vapor extraction wells." The meaning of this statement is also unclear. Please note that 'cone of depression' is a hydrogeologic term referring to the cone shaped lowering of the potentiometric surface of groundwater as a result of pumping influences. A statement is made that equations that will be used to calculate the AS ROI are included in the attachments [of the work plan]. However, no attachments were included in the submitted work plan. PHS/EHD approves the proposed geoprobe boring and depth discrete water sampling. This work is currently scheduled to be performed on May 22, 2001. PHS/EHD will proceed with issuance of the boring permit for this phase of the work plan. In discussions with PSI, it was agreed that the SVE well would be built to 40 feet bsg, instead of 25 feet bsg. As discussed above, PHS/EHD has serious reservations about the objectives and potential effectiveness of the proposed AS/SVE pilot test. However, if