Monitored natural attenuation to manage PFAS impacts to groundwater: Potential guidelines

Practical guidelines based on a three‐tiered lines of evidence (LOEs) approach have been developed for evaluating monitored natural attenuation (MNA) at per‐ and polyfluoroalkyl substances (PFAS)‐impacted groundwater sites using the scientific basis described in a companion paper (Newell et al., 2021). The three‐tiered approach applies direct measurements and indirect measurements, calculations, and more complex field and modeling methods to assess PFAS retention in the subsurface. Data requirements to assess the LOEs for quantifying retention in both the vadose and saturated zones are identified, as are 10 key PFAS MNA questions and 10 tools that can be applied to address them. Finally, a list of potential methods to enhance PFAS MNA is provided for sites where MNA alone may not effectively manage the PFAS plumes. Overall, a practical framework for evaluating PFAS MNA that can result in more efficient, reliable management of some PFAS sites is provided.