Many of the PFAS found in AFFF formulations are highly soluble and migrate rapidly, although others are far less mobile. The more soluble PFAS are likely to become depleted through flushing from source zones over time. Other PFAS may be retained in the source zone, with varying degrees of potential for mass transfer into the aqueous phase with subsequent infiltration into groundwater. Of particular concern are PFOS and PFOA, which are relatively mobile with environmental fates complicated by the presence of potential precursors for these compounds in PFAS mixtures such as AFFF. SERDP and ESTCP efforts are focused on developing in situ treatment methods to reduce the need for ex situ treatments such as granular activated carbon and other treatment methods that generate ancillary waste streams.