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' la wal Tte/rnfU 199 Pae 11 Gea g � Dual Phase Extraction Pilot Test Report Project No 723 2 November 12,2004 ' oxidants) while a negative value indicates a reducing environment APT staff monitored these parameters while the wells were sampled prior to injection during each event In the APT report's Figure 1 it can be seen that the ORP values rose right after the first injection event in wells MW-9 and GP-1 Conversely, well GP-2 exhibited a slow increase until a sharp decline in ORP on June 11 The differences in the wells' ORP response suggests that ' the fast response in GP-1 was due to the fracture/lithologic pathway discussed above while rr the response in GP-2 was more reflective of slow diffusion of th'einjectio en fects But the theory that a sand/silt layer exists that intersects both MW-9 and GP-1 wells screens but not ' the screen of GP-2 is not realistic given the short lateral distance separating the wells (24") Therefore, the fast response in GP-1 is probably a result of a drilling artifact fracture rather than a sand/silt preferential pathway ' The ORP values from GP-3 fluctuated without a trend for the first six events and then declined strongly on and after June 11 This suggests that the oxidant injection did not influence this well and the pilot test's definitive radius of influence was less than 48 inches from well MW-9 The TPH-G increases/ORP declines recorded at the end of the pilot test ' may have been the result of the injection pressures applied to the subsurface causing mobilization of contamination previously trapped in pore spaces 6.0 DISCUSSION ' The test demonstrated that oxidants can destroy the contamination in groundwater at the site but the radius of influence is limited due to the clayey soils at the site There were limitations in the test that should be taken into account regarding the radius of influence demonstration The length of the well screen in MW-9, 15 -- 35 feet bgs, was excessive as compared to air sparging wells which are typically constructed with screens less than 5 feet in ' lengtho p The depth of the well screen in MW-9, 15 — 35 feet bgs, exceeded the bottom of the GeoprobeTM well monitoring points screened at 20 - 30 feet bgs. It is possible that if ' a sand/silt preferential pathway existed below 30 feet bgs then the infected oxidants could have bypassed the monitoring points a Due to the tight formation it is possible that some of the infected oxidants may have �o A ' exited the top of the MW-9 well screen above the water table and the contaminant decay was weighted toward vadose zone sediments ' • The total volume of H202 solution injected was a fraction of the pore space within the monitored area In their report APT states that the amount of solution injected was less than 2% of the volume of water in a four foot radius centered on MW-9 If we ' assume the treated area is a cylinder centered on well MW-9 with a height of 20 feet (screened interval), a radius of four feet and a porosity of 50% (clay) this represents approximately 3,762 gallons* The total amount of solution infected was 15 gallons