Trifluoroacetate (TFA) – Potentials and limits of a new hydrological pollution tracer

Trifluoroacetate (TFA), the perfluorinated analogue of the acetate ion, is an emerging pollutant that is highly water-soluble, mobile and persistent in the environment. TFA has many precursors and a variety of atmospheric and terrestrial sources. This study investigates the suitability of this molecule to act as a hydrological pollution tracer to characterize water types and to identify young recharge components and river water infiltration. For this purpose, TFA was compared with classical environmental tracers (stable water isotopes and major ions) in depth-profiles of three groundwater wells and in surrounding rivers. The tapped porous aquifer supplies drinking water for the city of Freiburg, Germany, and is located in an area used by intense agriculture. Rising atmospheric inputs and, probably, the increased use of fluorinated pesticides led to a strong signal of TFA in recent soil water. This signal enabled efficient differentiation between different surface water types and groundwater of different ages. Dual-anion plots (TFA-Cl^- and TFA-NO₃^-) were more efficient than the traditional dual-isotope plot with ¹⁸O and deuterium. These plots allowed estimates about the approximate contribution of TFA from agricultural soil. Moreover, TFA traced the infiltration of river water into groundwater and contributions of relatively young soil water components down to deep groundwater. We see promising applications of TFA as a tracer in the future, provided that TFA input functions are well-defined, the major TFA sources are known, and other tracers are used in parallel to back up TFA-derived results.