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AGS 4.0 SITE CLOSURE REQUIREMENTS The approach for evaluating the potential for site closure will follow guidelines outlined in the Tri-Regional Board Staff Recommendations for Preliminary Investigation and Evaluation of Underground Tank Sites (RWQCB, 1990); including Appendices A (RWQCB. 1991) and B (RWQCB. 1996). Specifically, each of the OWS sites will be evaluated with respect to low risk soil and/or groundwater designation, as outlined in Appendix B the or the Tri-Regional Staff Recommendations (RWQCB, 1996). Low risk classification results will be used to evaluate the need for active and/or passive remediation. and to obtain a "no further action required" (NFAR) letter. 4.1 LOW RISK VADOSE ZONE DESIGNATION For OWS sites where contamination is limited to unsaturated soils specified criteria will be evaluated for obtaining a low risk vadose zone designation. Per RWQCB guidelines (1990. 1991, and 1996), the following must be demonstrated in order to achieve a low risk vadose zone designation: 1 . Demonstrate that only the vadose zone has been affected; 2. The release or leak has been stopped and source(s) of hydrocarbons in the vadose zone have been removed or permanently contained; 3. Petroleum hydrocarbons remaining in the vadose zone (or surface soils) do not impact or threaten waters of the State; and 4. No significant risk to human health and safety exists or is anticipated following remediation. For such sites, petroleum hydrocarbon detections in soils will be compared to the 100 .parts per million (ppm) screening threshold outlined by Tri-Regional staff as indicative of a significant contaminant release (RWQCB, 1990). The potential presence of remaining contaminant sources, including non-aqueous phase liquids (NAPLs) and mobile 17 �.\War jn601WPD0CS05-C_TS\1995\950304p\Work Plan\Work Pian ooc 08/10/99