SITE INFORMATION AND CORRESPONDENCE_1

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I i Page 2 April 30, 1996 .i i cause an increase in the release of volatile compounds. Thus, one would expect to see lower concentrations with higher purge rates. The point made in the report is that we saw the opposite effect. In the protocol developed by the US Air Force at Hill Air Force Base, for intrinsic bioremediation (iTB) sampling, it is recommended that well draw down should be limited to less than 10 percent of the wetted screen interval to achieve minimum aeration well purging. The well purge rates applied for ITB demonstration sampling at the Disco site were determined by continuous water level sounding during the purge process. Well draw down was limited to less than 0.2 feet or'1 percent of the wetted screen interval of 20 feet. Paragraph 4 - Page 1 We agree that caution should be used in the evaluation of BTEX concentration reduction for MW- DIS-1. If air sparging is discontinued,we expect concentrations to climb back up to some equilibrium concentration. f Paragraph 5 - Page 1 Groundwater level has been higher than the top of well screens since March 1996, and it is a consideration as samples collected may have artificially low concentrations. We expect to see the highest BTEX concentrations at the groundwater surface. We will add top of well screen elevation to report Table l to call attention to the relationship between water level and top of well screen. Sample dilution is not a rel8vant consideration as it is a function of the analytical method. Results reported for samples that require dilution are corrected for dilution and the amount of dilution is reflected in the method detI etion limit reported by the laboratory. Paragraph 1 Page 2 We do not expect to find MTBE in the gasoline contamination at the Disco site because the Disco Gas Station went out of business in 1976,three years before EPA approved MTBE as an additive. Unleaded gasoline was produced as early as 1974, but lead substitutes other than MTBE were used until 1979 including t-butyl alcohol. We do not believe there is adequate justification for the extra expense involved in sampling for MTBE. Paragraph 2 - Pagl 2 We agree with PHS/EHD's recommendations to reduce monitoring frequencies. However,we need to complete the ITB demonstration which requires quarterly sampling until August 1996. t Paragraph 3 - Page 2 Soil and Water Generation/Disposal A description of the soil generated from construction of the monitoring wells and purge.water is j attached to this letter in Attachment 1. � I i I 3 PHSLETLDOC 'F