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Vapor Intrusion Contingency Plan <br /> Mr. Roberto Cervantes <br /> January 17, 2014 <br /> CONTINGENCY PLAN <br /> Stantec will conduct quarterly soil vapor sampling of the APSV monitoring wells associated with each ASVE <br /> well while the onsite remediation systems are shut down for rebound testing. Rebound testing will include <br /> four quarters of sampling to account for all seasons of temporal variability. Soil vapor data will be <br /> immediately reviewed to verify that concentrations of total petroleum hydrocarbons as gasoline (TPHg), <br /> benzene, toluene, ethyl-benzene, xylenes, and naphthalene are not in excess of the California Regional <br /> Water Quality Control Board - Shallow Soil Gas Environmental Screening Levels for Evaluation of Potential <br /> Vapor Intrusion (May 2013) for residential receptors. Screening levels will be updated as guidance <br /> documents are updated. The environmental screening levels that are now used as triggers for the above <br /> referenced chemicals are presented in the table below (Note that units are converted from µg/L to ig/ma from <br /> the ESL document for direct laboratory comparison purposes). <br /> Residential Exposure <br /> Chemical Lowest Residential(µg/m3) <br /> TPH (gasoline- 150,000 <br /> volatile range) <br /> Benzene 42 <br /> Toluene 160,000 <br /> Ethylbenzene 490 <br /> Xylenes 52,000 <br /> Naphthalene 36 <br /> pg/m =micrograms per cubic meter <br /> TPH=total petroleum hydrocarbons <br /> Should a soil vapor sample contain a chemical analysis with a concentration detected at or above the Lowest <br /> Residential Exposure concentration, the soil vapor extraction well(s) associated with the high chemical <br /> concentration will be opened to the onsite remediation system which will be restarted. Stantec uses a mobile <br /> lab to collect real time data; thus, preliminary results will be available the day of sample collection and will <br /> enable Stantec to expedite restart of the remediation system. The system will remain on for the associated <br /> wells with concentrations above the Lowest Residential Exposure concentration for a minimum of two <br /> quarters to allow for two respective soil gas sampling events and review of laboratory data to confirm whether <br /> system shutdown is again warranted or if the system should remain on. Should laboratory data indicate shut <br /> down should occur again, Stantec will collect at least four quarters of data from the associated soil vapor <br /> wells following shut down to complete the rebound test. <br /> Once there are four quarters of analytical results illustrating soil vapor is below the Lowest Residential <br /> Exposure concentration, the system will then be abandoned. <br /> Should you require further information regarding this plan please do not hesitate to contact the undersigned <br /> at any time. Stantec looks forward to your response. <br /> Sincerely, <br /> Stantec Consulting Services Inc. <br /> � a - W <br /> Tony Giglini. <br /> Associate Scientist <br /> Tel: (916) 384-0764 <br /> tony.giglini@stantec.com <br /> 2 <br />