This Explanation of Significant Differences (ESD) for the Final Record of Decision/Remedial Action Plan (ROD/RAP), Operable Unit 1B (OU-1B), Former Marine Corps Air Station (MCAS) Tustin, Tustin, California (United States Department of the Navy [DON], 2004) addresses potential vapor intrusion (VI) risk as a component of the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) remedy for Installation Restoration Program (IRP) Sites 12 (OU-1B North) and 3 (OU-1B South), identifies appropriate VI institutional controls (ICs), and modifies one existing groundwater IC. It has been prepared in accordance with A Guide to Preparing Superfund Proposed Plans, Records of Decision, and Other Remedy Selection Decision Documents (United States Environmental Protection Agency [U.S. EPA], 1999) pursuant to CERCLA Section 117(c) and the National Oil and Hazardous Substances Pollution Contingency Plan (NCP; 40 Code of Federal Regulations [CFR], Section 300.435(c)(2)(i)). Figures 1 and 2 present the locations of Former MCAS Tustin and OU-1B, respectively.

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FINAL EXPLANATION OF SIGNIFICANT DIFFERENCES FOR FINAL RECORD OF DECISION/REMEDIAL ACTION PLAN, OPERABLE UNIT 1B, INSTALLATION RESTORATION PROGRAM-3 - PAINT STRIPPER DISPOSAL AREA, INSTALLATION RESTORATION PROGRAM-12 - DRUM STORAGE AREA NO. 2