October 21

IDEM Approves the Use of the U.S. EPA VI Attenuation Factors

Today, the Indiana Department of Environmental Management (IDEM) has announced the approval of the use of the updated EPA VI attenuation factors presented in the recently released U.S. EPA Final VI Guidance. In general, the U.S. EPA attenuation factors are based on more recent scientific studies and are less conservative.  This allows a greater concentration of contaminant vapor to exist in the subsurface without fear of a completed vapor intrusion exposure pathway.  This change is effective immediately, and will have a significant impact on the VI investigative and mitigation process in Indiana.

As you are aware, the United States (U.S.) Environmental Protection Agency (EPA) released their Final Vapor Intrusion (VI) Guidance Documents, “OSWER Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air” and “Technical Guide for Addressing Petroleum Vapor Intrusion at Leaking Underground Storage Tank Sites” in June of this year. This was thirteen years in the making and includes extensive updates from the 2002 Draft Guidance document. One of the most notable guidance updates includes the recommended vapor attenuation factors for risk-based screening of the vapor intrusion pathway. Given the size of the EPA Guidance Documents, it will most likely take some state regulatory agencies a good deal of time to digest and implement changes to state-specific VI guidance documents. A few select states, like Indiana and Wisconsin, are ahead of the curve in adopting portions of the guidance by issuing short memos and/or addendums to their VI Guidance Documents.

About the Author:    

meganhamiltonmain-137x137Megan Hamilton
Director of Vapor Intrusion and Risk Assessment



Ms. Hamilton has over fifteen years of experience in environmental regulatory oversight and consulting, with a focus on risk assessment and vapor intrusion expertise. Her diversified professional experience includes research, policy development, technical writing, public outreach, vapor intrusion investigation and remediation, data analysis and interpretation, human health risk assessment, and conceptual site model analysis. Ms. Hamilton served as a technical, scientific, and risk assessment policy resource for the Indiana Department of Environmental Management’s (IDEM) Office of Land Quality for nine years. She also served as the coordinator and team leader for the IDEM Vapor Intrusion Workgroup for six years and is the main author of the current Indiana Vapor Intrusion Guidance. She has helped design and implement numerous vapor intrusion investigations for chlorinated and petroleum contaminated sites throughout Indiana.  Ms. Hamilton is experienced in evaluating human health risk assessments, as well as vapor intrusion risk assessments for sites regulated by all of IDEM’s remediation programs, including:

  • Brownfields Program
  • Leaking Underground Storage Tank Program
  • RCRA Program
  • State Cleanup Program
  • Voluntary Remediation Program

Ms. Hamilton develops, helps implement, and oversees the VI investigations and mitigation for all of EnviroForensic’s projects. She is also the main contact for risk communication issues and community outreach development. Ms. Hamilton has presented at several National Conferences, is a member of the National EPA VI Science Advisory Committee, and currently serves on the Board of Directors for the Midwestern States Environmental Consultants Association (MSECA).