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property north of the site It is believed that a gasoline service station was once located on this vacant property This former service station . may be a source of the 1,2-DCA 1,2-DCA was also detected in several borings north of CPT-26, generally to groundwater samples collected from deeper zones The only sample that contained 1,2-DCA in a relatively shallow zone was a groundwater sample collected from a depth of 38-feet bgs in boring CPT-18 See Figure 15 for an illustration of which borings and wells in the site vicinity have shown detectable 1,2-DCA concentrations 13 . 0 RECOMMENDATIONS The on-site borings indicated that groundwater contamination was present beyond the calculated capture zone of the existing groundwater remediation system ASE recommends that additional recovery wells be drilled at the site west of the existing recovery wells The on-site remediation system should also be modified to accommodate these new pumping wells ASE recommends that the previous Remedial Action Plan be modified and further remediation should be completed at the site to further reduce the on-site concentrations Significant groundwater contamination is present to deeper water-bearing • zones across West Yosemite Avenue ASE suspects that an improperly abandoned well on the vacant lot across West Yosemite Avenue may be a conduit for this downward movement of MTBE through the aquitard, which appears to be generally competent at the site ASE recommends that a detailed survey be completed on this property in an effort to locate any possible abandoned wells or other conduit This task should also require the review of historical aerial photographs as well as subsurface electromagnetic surveys and ground penetrating radar Any conduit located should be sealed immediately An evaluation of the feasibility of groundwater remediation in the deeper zones in the vicinity of CPT-26 should be evaluated Because there is a n aquitard above the zone located at 48-feet bgs, air sparging would not b e feasible in this location Pump and treat in this deeper zone may be a feasible alternative However, further evaluation should be conducted since further pumping stress in this deeper zone could actually draw additional contamination downward if a conduit or other incompetence in the aquitard above is present In-situ enhanced bioremediation using a product such as Oxygen Releasing Compound (ORC) may be a feasible remediation technology in this area Even though MTBE is generally slow to breaking down biologically, breakdown compounds such as TBA are Franks One Stop Report — March 2001 -22-