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L <br /> d Verify that air injection pressures are low enough to avoid fracturing the <br /> soil Pressures measured were low enough (17 5 psi to -26 psi) that they <br /> should not have caused fracturing in the wells Typically, 20 to 22 psi was <br /> necessary to overcome the formation in individual sparge wells <br /> e Verify that the contaminant concentrations are sampled and analyzed in <br /> accordance with a sampling and analysis plan designed to assess the <br /> system performance Determine if any additional monitoring is needed to <br /> evaluate the operating conditions The current sampling plan does provide <br /> enough information to accurately evaluate the effectiveness of the air sparging <br /> system System performance is interpolated based upon groundwater <br /> concentrations in monitoring wells located near the ISAS wells Air flow to ISAS <br /> wells AS-26 and MW-25D, located north of Robinhood Drive, have been reduced <br /> to avoid the possibility of moving the contaminants offsite <br /> f Are groundwater samples being collected at proper locations*> Yes There <br /> are forty monitoring wells installed at the site There are groundwater monitoring <br /> wells screened in each of the water bearing units <br /> 7 Typical Performance Problems <br /> a Is there evidence of short-circuiting along the well casing, through nearby <br /> utility corridors, or through soil fractures or other subsurface features? <br /> This may be due to poor well installation, preferential air flow paths and <br /> may require well replacement or relocation No No evidence of snort <br /> • circuiting has been observed <br /> b Have contaminant concentrations been declining in most of the target <br /> zone? if concentrations are not declining signiflcantly, consider the <br /> possible existence of additional source zones that may require remedial <br /> action, the likelihood of inadequate air distribution, or preferential air flow <br /> paths These conditions may require that additional wells be installed, air <br /> flow rates be increased, efforts be made to verify air flow paths, that ISAS <br /> be augmented with other treatment processes, or that ISAS be replaced by <br /> an alternate technology Yes, contaminant concentrations appear to be <br /> declining in most of the target zone wells ENSR has proposed that additional <br /> ISAS wells be installed in the vicinity of MW-3 and MW-11 <br /> c Did concentrations rebound following cessation of sparging? This may be <br /> due to short circuiting of air through the monitoring points (i e , monitoring <br /> well concentrations do not reflect aquifer concentrations), inadequate air <br /> distribution, or ongoing sources Consider replacing monitoring points, <br /> increasing cparning well dpnsify, incraasina air flow to each well, or other <br /> source removal alternatives No Concentrations rebounded while the SVE <br /> system was shut down between October 2003 and September 2004 During that <br /> period--September 2004 and November 2004--overall dissolved petroleum <br /> hydrocarbon concentrations appeared to decrease <br /> d Is there good evidence for adequate air distribution in three dimensions <br /> . based on relatively reliable indicators? Yes As discussed earlier in this <br /> document, groundwater data has shown a decreasing trend since the ISAS was <br />