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ENSR ; 35 Groundwater concentration trends Quarterly groundwater sampling at the site started in June 1998 with wells MW-1 through MW-3 Wells MW-4 and MW-5 were added in August 1989, MW-6 and MW-7 were added in March 1992, MW-8 and MW-9 were added in November 1994, MW-10 and NP-1 were added in February 1999, MW-11 through ' MW-19 were added in June 2001, and MW-20 and MW-21 were added in April 2002 A historical summary of the groundwater)oricentrattan data are summarized in Table 3-2 A Mann-Kendall trend analysis was performed on three most impacted wells (i e , MW-2, MW-10, and MW- 11) in close proximity to the source area and the ozone sparge system to evaluate groundwater trends for TPHg and benzene (refer to Appendix C) The trend analysis for TPHg indicated a downward trend in MW- 2, and no trend in MW-10 or MW-11 As for benzene, a downward trend was reported for MW-10, and no trend in MW-2 or MW-11 These findings may suggest that there is still enough source material remaining whereby these monitoring wells are not on the decline yet ' 3.6 Biodegradation parameters Biodegradation parameter data collected from the site included dissolved oxygen (DO), nitrates, sulfates, iron, and others Refer to Table 3-5, Groundwater Analytical Results of Natural Attenuation Parameters and Table 3-6, Dissolved Oxygen Concentrations for additional detail The DO readings indicate a background levet (upgradient location) of 10 mg/l as measured in well MW-8 DO readings m the source area were 17 5 mg/l in MW-2, 3 6 mg/l in MW-10 and 16 7 mg/l in MW-11 The data suggests that the sparge system is providing DO to wells MW-2 and MW-11, but not to MW-10 as this well is probably beyond the zone of influence DO readings further downgradient of the sparge wells and outside of the impacted area increased to aerobic levels of 8 mg/l (MW-5), 4 8 mg/I (MW-17), and 8 3 mg/l (MW-4) ' Natural attenuation parameters have also been collected at the site for nitrates, sulfates, ferrous iron, and others Background concentrations of these parameters as measured at well MW-8 indicate nitrate levels of about 21 mg/I, sulfate levels of about 160 mg/l, and ferrous iron levels of about 0 035 mg1l The data for the wells in the source area as measured at MW-2, MW-10, and MW-11 suggests aerobic conditions have been depleted in this area and that based on the changes on electron donors, anaerobic metabolism of hydrocarbons is occurring This metabolism is less efficient than aerobic metabolism, and thus ' biodegradation might be enhanced by the addition of oxygen to the aquifer 37 Ozone sparging system operational status Th- remedial system at the site consists of nine multi-level ozone sparge tnjells installed at three locations (SP 1, SP-2 and SP-3) and three vapor extraction wells (VW-1, VW-2, and VW-3) as depicted in Figure 2-2, Site Plan The C-Sparge system includes an ozone generator, an air compressor, and a programmable timer/controller The ozone is entrained into the air stream at low flow rates of 2 to 6 cubic feet per minute The C-Sparge system utilizes ozone microsparging a process where ozone is entrained into an airstream ' and introduced into groundwater through specially designed sparge points to create "microbubbles" As these microbubbles rise through the water column, dissolved-phase VOCs, including petroleum hydrocarbons, are oxidized The system operated at a delivery rate of 18 cycles per day, with each cycle approximately 9 minutes long The well construction details are summarized to Table 3-2 The sparge system was started in August 2002 and shut off in December 2005 Three of the eight sparge points (SP-1A, SP-2A, and SP-3A) were not turned on until July 2004 as the water table had dropped 3-4 hpr6l 25 1(06