Implications of Using Literature Values in Modeling PFAS Fate and Transport

Common question when modeling contaminant fate and transport (F&T) is acceptable use of literature vs. site-specific values for key input parameters. This question is accentuated in the case of PFAS, arguably the hottest emerging group of contaminants about which little is still known in terms of their long-term F&T in groundwater including related quantitative parameters. A typical example is the use of sorption (adsoprtion-desorption) derived from the organic carbon partitioning coefficient (K_oc). The reported literature values of K_oc vary greatly for individual PFAS and there is also a significant overlap between reported ranges for PFAS generally considered to be of different mobility (e.g., PFOA, and PFOS). Most of the common PFAS of concern are anionic and tend to sorb better at lower pH. This relationship has yet to be quantified, but the spatial distribution of groundwater pH at a site, and other factors may play an important role in creating defensible sorption in a model and more accurately matching PFAS concentrations observed in the field. It is therefore preferred to develop a site-specific partitioning coefficient by collecting co-located aqueous phase and saturated soil samples from the saturated zone in the source area.

While most perfluoroalkyl acids (such as PFOA and PFOS) are highly persistent due to the strength of the C-F bond, many polyfluoroalkyl substances (currently not routinely analyzed for at contaminated sites) can be degraded via different biological and abiotic mechanisms and act as precursors to PFAAs (perfluoroalkyl acids). These precursors, which are currently not routinely analyzed for at contaminated sites, occur in different environmental media and can result in complicated spatial and temporal distribution of PFAAs.

A case study illustrates these concepts by showing modeled development of a PFAS plume using various values of model input parameters, including site-specific, and their combinations.