Researchers have identified increasing concentrations of trifluoroacetate, a degradation product of HFOs and other f-gases, in Denmark’s groundwater over the last 60 years, according to a new study.

The study – “A 60-Year Increase in the Ultrashort-Chain PFAS Trifluoroacetate and Its Suitability as a Tracer for Groundwater Age” – was published on September 4 in Environmental Science & Technology Letters. It received funding from the Danish Environmental Protection Agency and the Danish Groundwater Monitoring Program.

Trifluoroacetate (abbreviated as TFA in the study and in this article) is formed when the highly acidic trifluoroacetic acid (also abbreviated as TFA elsewhere) mixes with water. Because of their carbon-fluorine bond, both are considered ultrashort-chain examples of PFAS (per- and polyfluoroalkyl substances), also known as “forever chemicals” for their persistence in nature.

 One of the primary sources of TFA in recent years is HFO-1234yf, which oxidizes 100% in the atmosphere in about two weeks to form trifluoroacetic acid, which is absorbed in rainwater and infiltrates the environment. TFA has other sources, notably pesticide breakdown, the study noted.

In this study, the researchers – Christian Albers, Senior Research Scientist for the Geological Survey of Denmark and Greenland; and Jürgen Sültenfuss, Research Scientist at the University of Bremen, Germany – tied the concentrations of TFA to different time periods across 113 Danish groundwater-monitoring wells, a source of drinking water. The time periods were derived from the tritium-helium dating method, which used the half-life of tritium (about 12.3 years) to estimate the age of samples. (Tritium is a radioactive isotope of hydrogen.)

“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,” said the study abstract.

The recharge year is the estimated year when groundwater was last replenished in an aquifer, calculated by measuring the amount of tritium and helium-3 present in a water sample. The researchers suggest that the TFA level by itself could be a simpler way of estimating the age of groundwater.

According to the abstract, 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 mcg/L of TFA, and the concentration increased with time.

More details on the study were provided in an article in SciTechDaily. According to the article, the following TFA levels were found in the wells:

• From 1960 to 1980: 0.06 ppb (0.06mcg/L) on average.
• From 1980 to 2000: 0.24 ppb (0.24mcg/L) on average.
• From 2000 to the 2020s: 0.6 ppb (0.6mcg/L) on average.

The study suggests that the growth in TFA in groundwater could be linked to the increasing use of HFO-1234yf in recent years as a substitute for HFCs. “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,” said the study abstract.

Drinking water standards

The European Environment Agency (EEA)’s limit on total PFAS in drinking water, which takes effect in 2026, is 0.5 ppb (0.5mcg/L); this was exceeded by the most recent TFA average in the groundwater study. However, the EEA’s limit pertains to the total PFAS contained in a sample and may not include TFA as part of that mix.

Some European countries have established limits for TFA in drinking water. Denmark’s is 9mcg/L; in the Netherlands it is 2.2mcg/L; and Germany has a health guideline value for TFA of 60 mcg/L and an advisory level of 10 mcg/L.

The Pesticide Action Network Europe (PAN Europe), which has recently conducted studies of TFA in surface and drinking water in Europe, is calling on European regulators to set an EU-wide safe drinking water limit for TFA, according to an article in Food Safety Magazine. Revisions to the EU Water Framework Directive, which sets drinking water standards, are underway and expected to be finalized in a trilogue by the end of 2024, the article said.

Another study this year of drinking water and mineral water in German cities and Brussels, Belgium, conducted by BUND (German Federation for the Environment and Nature Conservation), found several chemical pollutants, with trifluoracetic acid the most frequently discovered chemical.

The human health impacts of TFA are still being assessed. In July, the German Federal Office for Chemicals (Bundesstelle für Chemikalien or BfC) submitted to the European Chemical Agency (ECHA) a proposal linking reproductive toxicity to trifluoroacetic acid and its inorganic salts. In addition, the proposal categorizes TFA and its inorganic salts under hazard classes PMT (persistent, mobile and toxic) and vPvM (very persistent, very mobile).

The European Partnership for Energy and the Environment (EPEE), which includes chemical producers among its members, did not respond to a request for comment on the groundwater study.

The chemical industry addressed the environmental deposition of TFA in an October 2021 study funded by the Global Forum for Advanced Climate Technologies (globalFACT), which represents f-gas producers Chemours, Honeywell, Arkema and Koura (and equipment manufacturer Daikin). The study concluded that “with the current knowledge of the effects of TFA on humans and ecosystems, the projected emissions through 2040 would not be detrimental.” But the study also acknowledged that “the major uncertainty in the knowledge of the TFA concentrations and their spatial distributions is due to uncertainties in the future projected emissions.”

“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.”

From the study, “A 60-Year Increase in the Ultrashort-Chain PFAS Trifluoroacetate and Its Suitability as a Tracer for Groundwater Age”