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EM 1110-1-4001 3 Jun 02 Table 9.1 Possible Criteria to be Considered in Evaluating Shutdown of SVEIBV System Sod Sample Analysis Recommended,provided proper sampling strategy and methods used • Sod constituents collected from the area being remediated indicated levels below regulatory requirements or levels of nondetection (confirmatory analyses) • If soil constituents collected from the area being remediated indicate levels above regulatory requirements and operation times and cost have been exceeded the operator may request a variance from the regulatory agency to accept remediated levels(refer to paragraph 9-5,Evaluation of Results) b Shutdown strategies can be based on • Soil sampling results compared to specific regulatory or nsk-based soil concentration standards • Soil gas sampling,compared to specific soil gas concentration standards • Soil gas concentration rebound following shutdown for reasonable period(see Section 9-9 and Appendix F) • For BV systems,attainment of low respiration rates In all cases,the design of both the remediation and the monitoring system must be evaluated to ensure it is in accordance with good practice,as defined by this manual C Shutdown strategies based on the need to protect ground water are becoming more common In most cases,the removal of contaminant mass in the vadose zone must continue until the residual mass will not leach to the ground water in quantities that would cause exceedance of ground water quality standards This typically is evaluated through the use of leaching models and the assumption that some mixing of the leachate and ground water occurs below the water table The details of this mixing(i e ,thickness of mixing zone,etc )are often the subject of debate It is unrealistic to neglect this mixing,but it is not conservative to overestimate the thickness of the mixing zone in the absence of hydrogeologic data The observed thickness of the ground water plume or the typical length of either monitoring well screens or production well screens may be a basis for the thickness of the mixing zone The acceptable soil concentration may be determined through the modeling(DiGiulio et at , 1999),or,by assuming equilibnum between phases,an acceptable soil gas concentration can also be computed At an Air Force Base in California, further analysis has been done to determine the acceptable soil remediation by SVE considering the cost of simultaneous ground water extraction The evaluation considers the cost of removing additional mass through the SVE system,versus the additional cost for future ground water extraction accounting for the added leachate from the vadose zone if SVE is stopped The operation of the SVE system would continue only if the reduction in the cost of future ground water extraction was more than the cost of the additional SVE This requires the use of both vadose zone leaching and ground water contaminant transport models and is probably justified only on larger SVE sites where ground water is already impacted d The USACE Remediation System Evaluatton process can also be used to help identify when the system may be ready for shutdown or when enhancements may be necessary to reach remediation goals Refer to the Soil Vapor Extraction Subsurface Performance checklist available at iittp,/w�tiw Eng nonmcrit it �rs�ce ac my n�rl'ltbiaru�cl� rs�c hl.rrsc�fil`html The checklist guides the user in analysts of data to determine if the system has reached a limitation,if operation needs to be changed to 9-2