API Publ 4674:1998 pdf download

API Publ 4674:1998 pdf download.SITE-SPECIFIC ALTERNATIVES TO GENERIC ESTIMATES.
When soils are impacted by leaks or spills, or wastes are placed in impoundments, the potential exists for contaminant vapor migration to enclosed spaces (buildings, conduits, etc.) and leachate migration to groundwater. Regulations have historically considered the leachate impacts on groundwater; however, the issue of vapor migration has only recently been formally and quantitatively considered. This focus has been brought about in large part by the move toward more structured risk-based corrective action (RBCA) approaches (e.g., ASTM 1995), coupled with an increased awareness of this pathway.
The significance of the vapor intrusion pathway and natural attenuation of vapors in the vadose zone is currently the subject of intense debate. When common screening-level algorithms (e.g., Johnson and Ettinger 1991, Little eta!. 1992) are combined with conservative soil properties, geometries, and exposure assumptions, the resulting risk- based screening levels (RBSLs) are very low. In fact they are often one-tenth to one- thousandth the existing cleanup guidelines in many states. For example, the sample calculation in the ASTM RBCA Standard (ASTM, 1995) suggests that benzene concentrations in excess of 5 rig/kg-soil could be of concern if one wishes to be protective to a lO excess cancer risk level.
Many intuitively feel that the current generation of screening-level predictive models is too conservative and leads to unnecessarily low cleanup levels. Some point to the fact that the algorithms generally do not account for biodegradation and other possible vadose zone attenuation mechanisms. It is reasonable to expect that some chemicals of interest degrade as they migrate, especially those originating from petroleum spills (e.g., benzene). If this is true, then these chemicals should be found at concentrations much less than those predicted by the current generation of screening level algorithms. This hypothesis is supported to some degree by the Fitzpatrick and Fitzgerald (1997) Massachusetts indoor air survey, the data of Fischer era!. (1996), and others who have observed and reported on petroleum hydrocarbon biodegradation in the vadose zone under natural conditions (e.g., Ostendorf and Kampbell 1991).
Unfortunately, little data exist to refute or support existing algorithms, or to quantify the degree of over-conservatism. This lack of data is a result of many factors, including the fact that interest in this pathway is relatively new. From a comparison of model predictions with published radon intrusion data, Johnson et al. (1991) and Little et al.