A 60-Year increase in the ultrashort-chain PFAS trifluoroacetate and its suitability as a tracer for groundwater age

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