A "forever" chemical is becoming more abundant in groundwater

Rain and water from ponds and lakes slowly seep into the ground, passing through tiny cracks to replenish underground aquifers. Per- and polyfluoroalkyl substances (PFAS), often described as forever chemicals because they are so difficult to break down, can seep into groundwater that is then extracted for drinking.

Christian N. Albers of the Geological Survey of Denmark and Greenland (GEUS) and Jürgen Sültenfuss of the University of Bremen in Germany tested water from more than 100 wells in Denmark for a particularly persistent PFAS: trifluoroacetate. They found that levels of the everlasting chemical have been steadily increasing over the past few decades.

Trifluoroacetate is formed when fluorinated gases such as refrigerants and fluorinated pesticides partially degrade in the environment. Water passing through the air and soil picks up trifluoroacetate, transporting the mobile and persistent compound to groundwater aquifers. However, testing for trifluoroacetate in drinking groundwater sources is not very common because there is no regulatory limit other than the European Environment Agency (EEA) limit for total PFAS in drinking water of 0.5 parts per billion (ppb). Against this background, Christian Albers and Jürgen Sültenfuss wanted to comprehensively assess groundwater in Denmark for the presence of this contaminant and look for possible changes over the past 60 years.

The researchers collected samples from 113 groundwater monitoring wells across Denmark. They analyzed the samples for trifluoroacetate concentration and applied an established tritium-helium isotope method to calculate how long ago the water had been penetrating underground aquifers.

Water in a cave. (Photo: Scott House/Ozark National Scenic Riverways/NPS)

Overall, their data showed a trend of increasing trifluoroacetate concentrations since the 1960s. Specifically, groundwater had the following concentrations:

-Before 1960, they had non-measurable levels.

-Between 1960 and 1980, they contained an average of 0.06 ppb.

-Between 1980 and 2000, they contained an average of 0.24 ppb.

-Between 2000 and 2020, they contained an average of 0.6 ppb, which exceeds the total limit for PFAS in drinking water set by the EEA.

The researchers attribute the concentration differences between these time periods to changing atmospheric deposition, plant uptake, and local pesticide application. For example, in agricultural areas of Denmark, pesticides that could be precursors to trifluoroacetate began to be applied in the late 1960s. Based on these observations, the researchers say that trifluoroacetate concentrations could be used to define when groundwater entered aquifers—for example, after 1985 or before 2000—rather than using more sophisticated and tedious dating methods that require isotopes. In addition, Albers says that some particularly high concentrations of trifluoroacetate in groundwater less than 10 years old could suggest that local sources have recently become important, such as fluorinated pesticide applications.

The study is titled “A 60-Year Increase in the Ultrashort-Chain PFAS Trifluoroacetate and Its Suitability as a Tracer for Groundwater Age.” And it has been published in the academic journal Environmental Science & Technology Letters. (Source: American Chemical Society)