A 60-Year Increase in the Ultrashort-Chain PFAS Trifluoroacetate\nand Its Suitability as a Tracer for Groundwater Age

Trifluoracetate (TFA)\nis an extremely mobile and persistent ultra-short-chain\nper- and polyfluoroalkyl substance (PFAS), found ubiquitously in the\nenvironment. Oxidation of certain fluorinated gases in the atmosphere\nis considered the primary source of TFA in the terrestrial environment,\nand the combined change in gas type and increased usage would therefore\nexpectedly lead to an increased TFA burden over the last few decades.\nHere, we show that the concentration of TFA in 113 Danish groundwater\nmonitoring wells is strongly correlated to the groundwater recharge\nyear, as derived from the tritium-helium dating method. TFA was not\ndetected in tritium-free groundwater recharged before 1960, while\nit was detected at low concentrations in most samples recharged between\n1960 and 1980. Groundwater recharged after 1980 had at least 0.1 μg/L\nTFA, and the concentration increased with time. Shallow groundwater\nfrom natural “background” areas revealed some variation\nof TFA even with diffuse atmospheric deposition as the only source,\nbut additional local sources must contribute TFA to some of the monitoring\nwells. As an important side aspect, TFA may prove very useful as a\nsimple indicator of groundwater age, such as the presence of young\nwater in a groundwater abstraction well.