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Effectiveness of Air Sparge System Performance groundwater monitoring has been conducted on a monthly basis since March 2012, prior to the start of the air sparge remediation system. As shown in Table 1, monitoring wells OW-1 and OW-2 have reported a significant increase in DO levels. OW-1 Monthly performance monitoring demonstrates that the air sparging in OS-1 is actively increasing DO levels in monitoring well OW-1, which is approximately 12 feet down-gradient. DO levels have increased from 0.29 mg/L in September 2012 to 1.19 mg/L in April 2013 and remained elevated during the May 2013 monitoring event at 0.97 mg/L following eight (8) days of system non-operation. OW-2 During the January 2013 performance monitoring event, monitoring well OW-2 was monitored prior to, during and immediately following a 15 minute air injection cycle. DO levels were 5.92 mg/L prior to injection, 8.03 mg/L at the end of injection and stayed steady above 6.0 mg/L for the 15 minutes following the end of injection. This demonstrates that the air sparging in OS-2 is actively increasing DO levels in monitoring well OW-2, which is approximately 5 feet down- gradient. Despite the effectiveness of the air sparging system in wells located near the edge of the groundwater plume (OW-1 and OW-2), monitoring well MW-2 has shown a minimal increase in DO levels with no decrease in contaminant concentrations. Currently there is only one (1) air sparge well (AS-1)operating within the core of the plume (>5,000 pg/L TPH-G). Additional Air Sparge Wells Due to the fact that monthly performance monitoring has not shown a significant increase in DO levels within the core of the groundwater plume, GZA recommends that three (3) additional air sparge wells (AS-4, AS-5 & AS-6 ) be installed and operated within the core of the plume, as shown in Figure 2. GZA believes that these additional wells will yield increase DO levels in the core of the plume and reduce contaminant concentrations as demonstrated in the vicinity of air sparge wells OS-1 and OS-2. Based on the elevated DO levels in monitoring wells OW-1 and OW-2, the air sparging wells appear to have a radius-of-influence of at least 12 feet, with significant influence occurring within 5 feet of the injection wells. This was taken into account while planning the placement of the proposed air sparge wells. The existing air compressor and manifold can be easily modified to include the additional wells. Well Installation The air sparge (AS) well boreholes will be drilled by a licensed driller, using an 8-inch outside diameter, continuous flight, hollow stem auger to a depth of 25 feet below grade. The AS wells will be constructed similar to air sparge wells AS-1, AS-2 and AS-3, using 1.0 inch diameter PVC casing with flush threads. The sparge point will consist of a 5-foot long fine screened interval designed to produce fine bubbles with maximum surface area. A #12 sand filter pack will surround and extend I foot above and below the sparge point. A three-foot transition seal will be installed above the sand pack by adding weighted bentonite pellets. A surface seal will be achieved by pumping a neat cement grout (augmented with<4%bentonite)via a tremie tube. The tops of the wells will be provided with an injection wellhead.