SITE INFORMATION AND CORRESPONDENCE

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Mr. Vaughn • 3 • 12 May 1995 only if the technologies appear feasible and if additional testing is needed to select and design interim source control technologies. Wells proposed for installation in the STA include: a 6-inch diameter ground water recovery well; four 4-inch diameter combination SVE/ground water monitoring wells; and one 2-inch diameter air sparge well. STTC proposes to screen the 6 inch well from approximately 5 to 20 feet bgs, and to use it both for monitoring of the vadose and saturated zones and for ground water recovery during the ground water pumping tests. The four 4-inch SVE wells are to be screened from approximately 5 to 20 feet bgs and are intended for use as vapor extraction and vadose-zone monitoring points during SVE testing, and saturated-zone monitoring wells during ground water pumping tests. The air-sparge well is to be screened at a deeper interval (approximately 18 to 20 feet bgs) in order to allow injection of compressed air near the base of the A-zone. The configuration of the well array is intended to allow simultaneous monitoring of the vadose and saturated zones at distances of 5, 10, 15 and 20 feet from the SVE and air-sparge locations, based on the lower expected permeability of the clay and silt soils, and to provide saturated zone monitoring at points approximately 8, 23, and 28 feet from the ground water recovery well. The pilot-test program will include testing within the trench to evaluate: (1) free-product recovery from the trench using SVE; (2) air-sparging to promote in-situ air stripping of free and dissolved phases, and (3) air sparging to increase dissolved-oxygen levels within the trench for possible future enhanced-biodegradation studies. The method proposed for installation of the wells is hollow-stem augur. Soil samples will be collected at 5-foot intervals and will be logged for lithology. In the vadose zone, this may mean that the average number of soil samples collected per boring will likely be less than one, since the vadose zone is likely to be less than 5 feet thick in many areas and less than 10 feet in most areas. It is proposed to analyze the vadose zone soil samples for TPH-g and BTEX by USEPA methods (modified) 8015/8020. Well arrays proposed for the NTA and the proposed testing and sampling programs are similar. STTC proposes to use an existing monitoring well, TO/MW-1, as the fourth SVE/ground water monitoring well. Installation of the wells and implementation of the testing program will take place in the NTA only if results from the STA program warrant the additional testing. PROPOSED TEST PROCEDURES: Testing is proposed in the STA and the trench, and later, if applicable, in the NTA. Individual tests are proposed as described below for SVE alone, air sparging alone, and combined SVE and air sparging. In addition, ground water pumping testing is proposed. The SVE testing must be limited to 5 days or less to conform with the terms of the air discharge permit: SVE Tests : The STTC proposes short-term testing of the SVE on one well each in the STA and (as applicable) in the NTA. The SVE test will comprise a I-hour step test to evaluate the vacuum and air-flow characteristics of the SVE well, and then a 23-hour extended test to evaluate the stability of extracted HC concentrations and induced vacuum response in adjacent observation wells. Air will