The Crack Factor in Vapor Intrusion Calculations. S. L. Sager, L. D. Braddy, and C. H. Day, Geraghty & Miller, Inc., 2840 Plaza Place, Suite 350, Raleigh, NC 27612
Risk-based corrective action (RBCA) for petroleum release sites is a methodology that is gaining acceptance within the state regulatory communities. The vapor intrusion exposure pathway frequently dominates the target levels or cleanup goals selected for the site, in states that use this pathway as part of their suite of exposure pathways. There are a number of conservatisms built into the pathway evaluation. Once that can have a significant influence on the cleanup levels is the crack factor. The crack factor represents the fraction of the foundation that is cracked and can allow vapors to migrate from the subsurface soil into a building. The significance of this factor is important since vapors may not enter the building if there are no cracks in the foundation. The literature values range over four orders of magnitude. This paper examines the influence of the crack factor on the calculation of cleanup goals and proposes a realistic one for use in vapor intrusion calculations. This paper does not focus on the models that are used to estimate the flux of volatiles from the source (either subsurface soil or groundwater).