| 22 Jul 2025
Diffuse sources of TFA: atmospheric and terrestrial inputs, retention and pathways at the catchment scale
Abstract. Trifluoroacetate (TFA) is a contaminant from various human sources. The degradation of fluorinated gases in the atmosphere leads to a ubiquitous input through precipitation. Degradation of certain agricultural pesticides and wastewater-borne pharmaceuticals adds to the amount of TFA pollution. Once released into the aquatic environment, TFA is nearly conservative due to its negative charge, high water solubility, and absence of degradation pathways. Consequently, TFA concentrations in the environment are constantly increasing, following the production of precursor substances. Previous studies suggested the accumulation of TFA in plants and its retention in organic soil. This knowledge, however, is based on a small number of environmental samples or laboratory experiments. Catchment-scale studies are so far missing. In particular, hydrological processes controlling the retention and mobilization of TFA are poorly understood. Therefore, we analyzed a two-year dataset of weekly water samples for major ions and isotope tracers with TFA in the mountainous Dreisam catchment (Black Forest, Germany). We sampled precipitation, the discharge of three nested catchments, and a hillslope spring. A balancing approach suggested that TFA was not permanently retained in forested headwaters. Therefore, we were able to estimate evapotranspiration in the sub-catchments from two years of TFA concentrations in streamflow. In agricultural areas, we found a surplus of TFA, which totaled an annual input of 11.4 ± 3.9 kg km-² for arable land. A correlation analysis using environmental tracers, combined with knowledge of runoff generation in the study catchment, suggested that previously retained TFA was flushed from soils under wet conditions, with subsurface stormflow serving as a primary transport path. These findings indicate that TFA concentrations in soils may be higher than average concentrations found in rain or streamflow. Therefore, future research should focus on TFA retention in the unsaturated zone.
General comments:
The authors made a comprehensive and interesting study on the size and applicability of TFA concentration on catchment scale. The study includes many data that may be used by others interested in similar catchment scale studies or in other study types. In general discussions and conclusions are both interesting and justified. However, I am skeptical about the justification of the calculations on agricultural input. While the higher TFA concentrations at the Dreisam Gauge is likely to be caused by an input from agricultural activity, as the authors suggest, I think the numbers used in the calculations in combination with the general understanding of agricultural sources are so uncertain that the calculated mass of TFA from agriculture makes no sense and should be removed from the abstract and probably from the article and more emphasis should be put on the need for more research into this important aspect of the study. Please see specific comments for more details on this.
Specific comments:
Line 51 "the lack of transformation rates". Not only transformation rates are lacking but for most potential TFA-pesticides also whether the pesticides are transformed into TFA at all.
Line 93: How were HFCs (and which) used as tracers? Normally you would expect CFCs to be used as tracers. Are HFC/CFC data in Uhlenbrook et al, I cannot find them?
Line 95-96: “HOF" and "SOF" are these abbreviations necessary - they are not used any further?
Line 143: "a polyethylene (PE) tank located below the funnel". This is not much information on the precipitation collection. What material was the funnel made of? Did you test that it did not leach or adsorb TFA? Was it assured that there was not evaporation from the tank? Was it a constantly open funnel, so that also some of the dry deposition would be sampled? In general, dry deposition is not considered in the study, but some studies claim that it is a substantial fraction of atmospheric TFA input (most extreme case by Zhuang et al (https://www.sciencedirect.com/science/article/abs/pii/S0304389424009622), and most studies estimate at least some dry deposition. I guess there will also be quite some fog/mist in the area, that can be captured by especially conifers, but will probably not be captured by the precipitation sampler, or?
Line 147: "Storage time was up to four months". Stored how?
Line 275: "the highest TFA levels in the Dreisam River were observed during the 2023/2024 winter". To me this is not clear from the figure - it looks relatively constant, when considering the fluctuation from sample to sample...?
Line 277: "TFA concentrations increased with discharge". Is this statistically significant - it is not so easy to see from the figure, I think. Sometimes there seem to be a clear positive relationship between the two, sometimes not...
Line 334: "The Dreisam River exported 48 ± 21 % more TFA." This seems like a very uncertain calculation, since according to the figure in 2023 there was no surplus and in 2024 the surplus was around 100%? Taking average of two such different numbers makes no sense to me. Furthermore, how can the uncertainty be so small when taking average of two so different years? There should be input from agriculture in both years unless there is a very good (hydrological) reason for this to be so different. Although quantifying the contribution from agriculture is very important, I am not convinced it is scientifically reasonable to do from these data as apparently uncertainty is very high (which is actually expected for a whole catchment exercise)
Line 341: "main Dreisam catchment" is this the same as what is denoted DRC throughout the text?
Figure 4: Why use standard error, not standard deviation, so that it is easier to compare statistical significance?
Table 5: “Eq. 9” Do you mean Eq.8? I see no eq. 9.
Line 382: In general, water isotopes seem to show minor seasonal fluctuation, compared to what you would expect in the precipitation (though precipitation isotopes for the present study seems not to be shown). It guess this would suggest the rivers are mainly fed by groundwater? Does this fit with your understanding of the system? Does it fit with the fluctuations in TFA? Do you have stable isotope data for precipitation that could be included?
Section 4.2: Although highly interesting, this section seems very speculative and I think it should be minimized and uncertainties put more forward.
Line 421-427: Taking an average of two very different years (in terms of hydrology and TFA balance) doesn´t make sense to me even if the average precipitation volume of the area lies between the two.
Line 444: 100% molar yield is highly unlikely (pesticides are rarely degraded 100% to one compound, and a significant fraction of TFA and possibly the pesticides themselves will be removed with the crops). It is important to mention that this is highly unlikely, and also important to mention that so far, TFA from PPP is mainly hypothetical/potential (Joerss et al use the term "estimations of TFA formation potentials "). For the few compounds, where TFA was demonstrated in the EFSA conclusions and elsewhere, (much) smaller fractions than 100% was found.
Line 445: I doubt that manure will add much TFA compared to pesticides, but I guess you could calculate from published values for TFA in manure, mentioned in the Introduction?
Line 454: “Eq. 9” Do you mean Eq.8? I see is no eq. 9.
Line 458-465: What about the year-to-year variation that may be quite large, as you show in table 4 and which has also been shown by others (much higher than the 5% you mention as an average yearly increase). Maybe this should be discussed here as well?
Line 475: "reported for the degradation of precursor PPP". I would suggest to change into "reported for the potential degradation of precursor PPP"
Line 478: "mean residence times of 2-5 years". Why not shorter than 2 years? Is it not applicable for one year (11-16 months, line 460) as in the present manuscript?