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�rrins OFsr+Rsca,amlwrS low groundwater levels have resulted in the water table level dropping into Zone 4 Typically, silt/clay type materials are difficult to remediate by AS because they have low permeabilities, are elastic and tend to channel the injected air. At present, the groundwater in the upper sand/gravel zones could be effectively stripped, however, removing contaminants from the lower silt zone may be difficult This technology is most effective when soil and groundwater concentrations are high and is much less effective when contaminants reach an asymptotic state. Based on Figure 10, benzene ' concentrations in groundwater are near the asymptotic state; this strongly suggests that AS/SVE would not be as effective in removing additional contamination. While AS/SVE can be implemented at sites where contaminant levels have decreased to an asymptotic state, the return, or amount of contaminants recovered, per dollar spent is not as favorable as it would be at more contaminated sites Implementability Implementing this alternative would be very difficult because it requires installing wells in the streets, it would also be necessary to install wells inside buildings.— Trenches uildings. -Trenches for underground piping would need to be constructed and backfilled. Well and trench installation would require closing local streets for long periods of time In addition, there are several utilities in this location which would require working around or relocating This alternative has the potential to cause significant disruption of the local area, because all utilities, including electrical, water, sewer, gas and telephone lines, are installed underground Cost The cost of this alternative includes the necessary piping, connections, compressors, vacuum pumps, air monitoring equipment, and treatment system. It is assumed that the system would operate for 18 months The overall cost of this alternative is estimated to be approximately $598,000 including groundwater monitoring for 2 years Costs are shown in Appendix B, Table 1 5.2 ALTERNATIVE 2 INTRINSIC REMEDIATION WITH MONITORING ' Alternative Description ' This alternative consists of monitoring the groundwater plume to verify that natural attenuation continues to degrade the contaminant concentrations to cleanup criteria. Bi-annual (i.e twice per year) groundwater monitoring would occur. Groundwater wells to be monitored would ' consist of source area wells (MW-2 and MW-2A) to confirm that concentrations were decreasing as anticipated, downgradient wells (MW-7) to confirm that the leading edge of the plume is not significantly expanding and unimpacted wells (MW-3, MW-6 and MW-8 and MW-9) to confirm ' that the plume does not migrate. Groundwater samples would be analyzed for BTEX, pH, eH, and dissolved oxygen One deep (screened at 65 to 75 feet bgs) and one shallow (screened across the groundwater surface)monitor well would be installed in the vicinity of MW-2A M After the first 2 years of monitoring, if trends show that the plume is still shrinking in size, monitoring for the remaining years may be reduced to once per year. Based on the observed LTROJECPSZANJOQIHSAWGTIONPL 37