Rainwater and runoff from ponds and lakes gradually infiltrate the soil, filtering through tiny fissures to replenish underground aquifers. Per- and polyfluoroalkyl substances (PFAS), commonly known as forever chemicals, have the potential to enter groundwater, which can eventually be used for drinking. In a study published in ACS’ Environmental Science & Technology Letters, researchers examined water from more than 100 wells in Denmark to investigate one notably resilient PFAS: trifluoroacetate. Their findings indicate a continuous rise in the concentration of this persistent chemical over the last few decades.
Trifluoroacetate is produced when fluorinated gases, like refrigerants, and certain fluorinated pesticides partially decompose in the environment. As water travels through the air and soil, it can absorb trifluoroacetate, carrying the persistent substance into aquifers. Unfortunately, groundwater sources for drinking have not been extensively tested for trifluoroacetate due to the absence of a specific regulatory threshold for it, aside from the European Environment Agency’s (EEA) guideline restricting total PFAS in drinking water to 0.5 parts per billion (ppb). Therefore, Christian Albers and Jürgen Sültenfuss aimed to comprehensively evaluate the groundwater in Denmark for this contaminant, examining potential shifts over the past 60 years.
The researchers gathered samples from 113 groundwater monitoring wells across Denmark. They tested these samples for trifluoroacetate and employed a well-established tritium-helium isotope technique to estimate how long ago the water entered the aquifers. Overall, their findings revealed a consistent upward trend in trifluoroacetate levels since the 1960s. Specifically, groundwater from:
- Before 1960 had no detectable levels.
- From 1960 to 1980 averaged 0.06 ppb.
- From 1980 to 2000 averaged 0.24 ppb.
- From 2000 to the 2020s averaged 0.6 ppb, surpassing the EEA’s total PFAS limit for drinking water.
The researchers link the variations in concentration across different time frames to changes in atmospheric deposition, plant absorption, and local pesticide usage. For instance, pesticides that could lead to the formation of trifluoroacetate have been used in agricultural regions of Denmark since the late 1960s. Based on these observations, the researchers suggest that trifluoroacetate levels may serve as a reliable indicator for the age of groundwater in aquifers, distinguishing between water that entered after 1985 and that which arrived before 2000, rather than relying on more complex and labor-intensive dating techniques involving isotopes. Furthermore, Albers notes that some unusually high trifluoroacetate levels in groundwater that is less than a decade old may indicate that local sources, such as the recent application of fluorinated pesticides, have become increasingly significant.
