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If significant rebound occurs according to the above criteria, pulsing operations will be reinstated. 4.6 Soil Gas Cleanup Goals In addition to the SVE system performance goal of VOC asymptotic stabilization, which is based on operating the system to its technically and economically practicable limits, it may also be appropriate at some sites to evaluate SVE system performance by measuring its effect on reducing VOC concentrations in soil. Where appropriate, it is proposed that SVE systems operating in Lodi also have as a performance goal achieving cleanup levels in vadose zone soils set in accordance with the 'Interim Guidance for Remediation of VOC Impacted Sites" published by the California Regional Water Quality Control Board for the Los Angeles Region in January 1995 ("RWQCB Guidance"). The RWQCB Guidance presents VOC cleanup standards for vadose zone, unsaturated soils based on attenuation factors derived from equations using site-specific soil vapor, chemical, and other physical parameters. This guidance document specifically indicates that the procedure assumes that VOCs are in liquid phase and that only the liquid phase is moving downward to impact groundwater quality. The document further assumes that VOCs in residual and gaseous phases are considered as lost mass in the subsurface. Therefore, the attenuation factor method is likely not applicable for sites where the majority of the contaminant mass in the vadose zone is in a gaseous phase. It is not currently known whether soil cleanup standards established by applying the RWQCB Guidance are achievable even if SVE systems are operated to the fullest technically and economically practicable extent. On the other hand, it may be possible to achieve these soil cleanup levels before the SVE systems reach asymptotic mass removal. As a result, the overall performance goal proposed for SVE systems in Lodi would allow their operation to be terminated upon achieving either asymptotic stabilization, as described in Section 4 above, or RWQCB Guidance soil gas cleanup standards. DTSC has calculated preliminary soil cleanup levels and soil gas cleanup levels based on the RWQCB Guidance. The preliminary DTSC-established soil gas cleanup goals will be refined when site specific data are collected. Site specific data, including soil moisture content, porosity, bulk density, and organic fraction, will be used to determine site-specific soil gas cleanup goals rather than the mean values used by DTSC for the preliminary estimates. Soil gas cleanup goals will be to assess SVE performance at all sites. 8