Trifluoroacetate mobilization by subsurface stormflow: the role of soil layer connectivity and prior flushing history

Abstract. Trifluoroacetate (TFA) is an environmentally persistent degradation product of fluorinated compounds that is ubiq-uitously deposited through precipitation. While TFA retention in vegetation and organic soil has been documented, the hydro-logical processes governing its mobilization and export to surface waters remain poorly understood. Here we test whether TFA export depends on the hydrological activation of near-surface soil compartments and thus carries a legacy signal in comparison to classical chemical tracers. For this purpose, we used high-resolution sampling of subsurface stormflow (SSF) at a trenched forest hillslope in the Black Forest, Germany, combined with multi-tracer analysis of normalised tracer trajectories and qual-itative event-water inference during rainfall events with contrasting antecedent conditions. We also investigated TFA pools in organic-rich topsoil and litter.
Freshly fallen autumn litter showed the highest TFA concentrations of all sampled compartments (up to 1862 µg kg⁻¹), with  concentrations declining from old litter to mineral soil. TFA concentrations in SSF (median 1.96–2.06 µg L⁻¹) substantially exceeded open-field precipitation inputs (0.22 µg L⁻¹), with episodic peaks of up to 3.82 µg L⁻¹ during events following dry
antecedent conditions.
The prior flushing state of the soil profile emerged as the primary control on TFA export via SSF. TFA, together with potas-
sium and nitrate, departed from the shared dilution behavior of the other major ions only when the profile was unflushed, with recently flushed events showing little deviation. This flushing-state-dependent divergence reflects TFA’s near-surface source in litter and topsoil: mobilized TFA is carried down through the profile by rapid, macropore-dominated vertical percolation and diverted laterally into the SSF flow path once the wetting front establishes hydrological connectivity at that depth. Event magnitude and duration determined how deep, and for how long, this connectivity was sustained, producing either a single brief TFA spike or a sustained, multi-peak signal depending on event size. This event-scale variability was damped, though not absent, in streamflow, consistent with spatial integration across the catchment and dilution by TFA-depleted deep groundwater. These findings provide first insights into the processes that govern TFA mobilization in forested headwaters and call for further
research on TFA behavior in the unsaturated zone.