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Geological Tecuks I= Page 6 ' Groundwater Monitonng Report Project No 662 2 July 7, 2005 � I ' 3.0 CONCLUSIONS AND RECOMMENDATIONS ' Conclusions 1 The historical shallow groundwater trend is easterly 2 The deeper aquifer direction was to the east-northeast but additional monitoring is needed ' to determine trends It is noted that the three wells used for monitoring this level (MW-5, MW-6 & MW-104) are in a fairly linear geometry that should be viewed with caution 3 Shallow extraction well EW-2 is located in the former UST excavation and contained ' moderate MTBE (3770 ug/l) for the second consecutive event This is the second discrete data from this well so a trend analysis cannot be made However, data from previous borings CPT-3 and CPT-4 indicate that higher concentrations of MTBE (10,000+ µg11) ' were present in groundwater beneath the former UST excavation area in December 2003 The decrease note for this event may be a result of the dual phase extraction pilot test performed in December 2004 The pilot test resulted in over 32,000 gallons of extracted ' groundwater being removed from the subject area 4 Based on previous discrete groundwater sampling, the shallow MTBE groundwater plume does not extend to the down gradient location of MW-5 The CPT-7 boring was advanced adjacent to the MW-5 location and its groundwater sample at 29 feet bgs was free of contamination 5 The deeper aquifer MTBE plume is present in well MW-104 and attenuates to the ' location of MW-5 and MW-6 A sand layer is present at a depth of 38 — 48 feet across the site and it appears to act as a preferential pathway for contaminant migration ' 6 There are low levels of TPH-G, 270 - 890 µg11, present in shallow groundwater beneath the former UST excavation area. It is noted that the laboratory reported that the sample chromatograms do not match a gasoline standard and result from one or two peaks within the gasoline range The CPT investigation completed in December 2003 revealed high tlevels of TPH-G beneath the former UST excavation (CPT-4 22,000 µgll TPH-G) As with the MTBE, the decrease in TPH-G concentrations may be a result of the dual phase ' extraction pilot test that resulted in over 32,000 gallons of extracted groundwater being removed from the subject area 7 The presence of TPH-G in down gradient well MW-6 but not in MW-5 is anomalous but ' these wells have only been sampled twice The lack of BTEX constituents suggests that this may be a weathered gasoline possibly from an old release ' Recommendations 1 Continue quarterly groundwater monitoring 2 Perform interim remedial action by installing a high vacuum dual phase extraction unit to ' address the on site contamination Dual phased extraction should be performed on well EW-2 with a separate dedicated groundwater pump on EW-1 The treatment system should include carbon filtration of extracted groundwater to be discharged to City of Manteca public treatment works This scope of work has been approved by SJC EHD