WWEM - Abstract

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Presenter Name: Dr Ian Ross
Company/Organisation: Tetra Tech
Country: United Kingdom

Abstract Information :

INTRODUCTIONrnPoly- and Perfluoroalkyl substances PFAS are used in a wide range of industrial applications and commercial products due to their unique surface tension and levelling properties. PFAS are also major components of firefighting foams such as Aqueous Film Forming Foam AFFF and fluoroprotein foams including FP and FFFP. The PFAS group of compounds consists of both perfluorinated compounds or perfluoroalkyl acids PFAAs, where all carbons are saturated with F atoms, and polyfluorinated compounds, where both fluorine saturated carbons and carbons with hydrogen bonds are present. Polyfluorinated precursor compounds biotransform to produce PFAAs as dead end extremely persistent daughter products. The understanding of the fate and transport of these compounds in the environment is complex and challenging and will be discussed. The concepts of in situ generation of perfluroalkyl acids PFAAs via precursor biotransformation will be used to explain how significant PFAS mass remains hidden in source areas in an analogous manner to NAPL residuals for hydrocarbon or chlorinated solvents.rnrnMETHODSrnPFAA precursors are so named because they transform slowly over time through abiotic and biological processes to the PFAAs. There is a natural biological funneling in which a whole host of PFAA precursor compounds containing a range of perfluorinated alkyl chain lengths and functional groups, aerobically biotransform to persistent PFAA products.rnFire-fighting foam formulations and many fluorochemicals used across multiple industries are composed of many PFAS that are PFAA precursors. Unlike the PFAAs, these species are not strictly anionic, as some contain multiple charges zwitterionic and some are positively charged cationic. These zwitterionic and cationic PFAA precursors are currently undetected by conventional analytical tools but can be quantified using more advanced approaches such as the Total Oxidizeable Precursor TOP assay. A significant mass of PFAA precursors in addition to the PFAAs have been detected in both fire-fighting foam-impacted soil and groundwater. A conceptual site model describing PFAS fate and transport at a firefighter training area is hypothesized and will be presented, as described below.rnCationic and zwitterionic precursors will be retained in the soils at the source zone via strongly binding ion exchange processes. The source zones will be anaerobic as a result of the presence of residual hydrocarbons used in firefighter training, so these strongly sorbing cationic precursors will biotransform very slowly to simpler, anionic PFAA precursors and PFAAs under these redox conditions. Anionic PFAAs and PFAA precursors will migrate away from the source as they enter the redox recharge zone where conditions become increasingly aerobic thus promoting in situ generation of detectable PFAAs from the hidden anionic PFAA precursors. PFAAs will not break down further, and will continue to migrate as a plume with shorter chain PFAAs generally migrating further.rnrnRESULTS AND DISCUSSIONrnThe concepts of biological funnelling show that PFAS behave significantly differently to other contaminants and management plans need to be adapted to adequately understand the fate and transport of these contaminants. Examples of plans from AFFF impacted sites will be presented. The common breakdown products from PFAS in other sources, such as landfills and waste water treatment plants will be described.rnrn

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