SITE INFORMATION AND CORRESPONDENCE

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Mr. Vaughn • 5 12 May 1995 Ground Water Pumping Tests: Short-term step and constant-discharge tests are proposed. A sustainable pumping rate will be estimated from a 6-hour step test followed by a 24-hour constant-rate test and a 24 -hour recovery test. Test results will be used to evaluate aquifer parameters for future design or modeling. At least four observation wells will be monitored during the constant-rate test, using a datalogger and downhole pressure transducers. A set of ground water samples will be collected from the extraction well at the end of the 24-hour test. Analyses will be performed for TPH-g, TPH-d, BTEX, oil and grease, VOCs, semivolatiles, cyanide, priority pollutant metals, general minerals, and ethylene dibromide, to provide a data base for future uses. Pumping tests within the interceptor trench are not proposed, for the reasons that the trench is intended to evaluate product removal and enhanced-biodegradation techniques, and because the trench's relatively large storage of water in the backfill would necessitate extended pumping tests to minimize casing storage effects. ANALYSIS: 1. Appropriateness of Proposed Remedial Technologies: Based on the overall site characteristics of depth to ground water, lithologic types, and stratigraphic geometry, the STTC terminals area appears to be only marginally suitable for SVE and air sparging applications. In general, soil sections having significant proportions of low permeability soils, high stratification, and/or heterogeneous, discontinuous lithologies are not as amenable to SVE as sections with significant proportions of laterally continuous permeable units. Where sections are highly stratified with layers or lenses of differing permeability, vapors may flow preferentially through the more permeable (transmissive) units, and flow through the less permeable portions may be reduced. Remediation effectiveness is impaired and the time required for treatment is increased. In designing a SVE system in such a stratigraphic setting, special consideration must be given to extraction in the less permeable zones. Therefore, for soil vapor extraction, it is usually desirable to screen the extraction well over the less permeable zones. It is not always necessary to screen over the entire vertical extent of contamination; the principal determinant of the SVE well's radius of influence (ROI) is the intrinsic permeability of the soils . Regarding the application of air sparging, this is a technology that often has not worked well, especially in stratified sections where air/water flow may be restricted by strata of low permeability overlying higher-permeability strata, so that vapors are trapped or dispersed by the low-permeability units. Controlling the vapor flow can become a problem in this situation. Of greater concern here is the relatively small thickness of the vadose zone (varying by season and location from 3 to 9 feet below ground surface). This is at the limits of feasibility of SVE application. If the ground water surface rises under the influence of induced vacuum, there is a danger that the extraction screen will be inundated. Therefore, simultaneous ground water pumping may be needed to control (or increase) the depth to water table. Data from the constant- rate pumping test should be useful in designing a dual purpose pumping program for extraction and treatment of ground water and for shallow-zone dewatering,providing that the constant rate test is run long enough to allow adequate assessment of the aquifer properties.