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L EM 1110-1-4001 3 Jun 02 Close isolation valves • Disconnect electrical power to equipment + Log the event Depending on the reason(s)for shutting down the system,decontamination procedures could be more stringent c If the operator believes that the site is approaching"clean" or if there is interest in trying to understand and quantify the extent of rate-limited mass transfer(e g ,diffusion from low permeability strata),then a rebound test may be performed Shutdown for rebound tests should proceed according to the shutdown procedures described in the site specific O&M manual However,prior to shutdown for rebound testing,it is important to collect a complete set of system performance data This data should include • Flow rate and concentration(s)from each extraction well and vacuum applied to each well (for SVE systems) • Infection rates to each infection well (if applicable) • Soil gas concentrations of site specific VOCs,gaseous Oz,CO2,and methane from both monitoring points and extraction wells + Vacuum/pressure distribution within the treatment area,e g, vacuum measurements at vadose zone monitoring points and nearby water table wells • Groundwater elevations (if the contaminated zone is near or within the capillary fringe)in the treatment zone,including degree of upwellmg at extraction points The extracted gas concentrations and flow rates may have to collected multiple times if a model is used to quantify diffusion limited mass transfer(see paragraph 9-9) Groundwater and soil VOC concentrations are not particularly useful to evaluate rebound d Remediation shutdown In the later phases of remediation,extraction wells may be shut down one at a time A wellhead valve can be included to isolate each well when cleanup criteria are met The system should be designed to operate at reduced airflow rates without jeopardizing the performance of the system Final shutdown of the system should follow the same activities as those for maintenance shutdown Decontamination procedures should be followed to minimize loss of contaminated materials to surrounding areas 9-7 Closure Report Once remediation has been completed, a closure report/construction documentation report should be prepared to verify and document the activities and results of the remediation project It should be noted that prior to the preparation of the closure report,the design team must determine if the acting regulatory agency has a specific format to follow and/or additional forms to be filled out 9-8. Long-Term Monitoring Requirements Long-term monitoring requirements following system shutdown, if any,will be defined in the SAP and perhaps modified pursuant to data collected during the operation of the system Typically,analyses will be 9-9