Exposure-Integrated Risk Prioritization of Legacy and Emerging PFAS: A Case Study of a Major River Basin in South Korea via High-Resolution Mass Spectrometry

Water is the silent carrier of our modern world, moving not just through rivers and pipes, but through the very chemistry of our daily lives. For decades, a group of synthetic chemicals known as per- and polyfluoroalkyl substances, or PFAS, has been used to make everything from non-stick cookware to water-repellent clothing. These molecules are famous for their durability; they resist heat, oil, and water so effectively that they are often called "forever chemicals" because they do not break down easily in the environment. As global regulations have begun to restrict the older, most toxic versions of these chemicals, manufacturers have shifted toward newer, alternative compounds. The hope was that these replacements would be safer and easier for nature to handle. However, a critical question remains: are these new alternatives truly less harmful, or have they simply traded one set of risks for another? To answer this, scientists must look beyond just identifying what chemicals are present; they must understand how much of them is there, where they come from, and how their presence combines with their inherent toxicity to create real-world danger.

In a recent study focused on the Geum River basin in South Korea, researchers set out to map this complex chemical landscape. They chose this specific river system because it flows through a mix of farmland, dense cities, and heavy industry, making it a perfect mirror of the diverse ways humans interact with the environment. The team did not rely on a single method to find these invisible contaminants. Instead, they combined a targeted search for thirty-nine known PFAS compounds with a broader, more open-ended screening technique capable of spotting unfamiliar chemical structures. This dual approach allowed them to see both the well-known players and the emerging newcomers. They collected water samples from twelve different locations, ranging from the main river channel to its smaller tributaries, ensuring they captured a complete picture of how these chemicals move through the watershed.

The results revealed a river system teeming with these synthetic substances. The total concentration of PFAS in the water varied widely, ranging from 7.20 to 86.2 nanograms per liter, with a median level of 31.5 nanograms per liter. While these numbers might seem small, they are significant in the context of drinking water, as the river supplies a major portion of the nation's water resources. The study found that the smaller tributaries feeding into the main river were significantly more polluted than the main channel itself, carrying concentrations roughly 1.6 times higher. This pattern pointed directly to local sources, such as municipal wastewater treatment plants and industrial complexes, acting as the primary entry points for these chemicals. The most abundant compound detected was not one of the old, heavily regulated "legacy" chemicals, but rather a shorter-chain alternative called perfluorobutane sulfonate, or PFBS. This single substance accounted for nearly one-fifth of the total PFAS burden in the river, signaling a clear shift in the chemical makeup of the environment as industries adopt newer substitutes.

Perhaps the most intriguing discovery came from the broad screening of unknown substances. The researchers tentatively identified a modified version of PFBS, where a fluorine atom in the molecule had been replaced by a hydrogen atom. This specific variant, known as H-PFBS, was found at high levels in a tributary flowing through a major city, but was completely absent in a tributary near heavy industry. This difference provided a crucial clue about the source of the pollution. The presence of H-PFBS in the urban area suggests that the chemical is being transformed by bacteria inside wastewater treatment plants, a process that breaks down complex precursors into this new form. In contrast, the industrial site appeared to be releasing fresh, unaltered chemicals directly into the water, bypassing the biological processes that create these transformation products. This finding highlights that the river is not just a passive recipient of pollution, but an active chemical reactor where human waste and industrial effluents are constantly being altered.

When the team evaluated the risks, they uncovered a surprising disconnect between what is dangerous in theory and what is dangerous in practice. Using computer models to predict the inherent toxicity of each chemical, they found that the long-chain, legacy PFAS compounds—such as perfluorodecanoic acid and perfluorooctanoic acid—were the most hazardous. These molecules are predicted to persist in the environment for a long time, accumulate in living organisms, and cause significant harm. If the assessment had stopped there, these older chemicals would have been the top priority for cleanup. However, the researchers then integrated real-world data on how much of each chemical was actually present in the river. This exposure-based calculation completely reshuffled the priorities. Because the short-chain alternative PFBS was found everywhere and in such high quantities, it emerged as the highest management priority, surpassing the more toxic legacy chemicals. The study concluded that while the old chemicals are inherently more dangerous, the sheer volume of the new alternatives currently poses the greater immediate threat to the ecosystem.

This research underscores a vital lesson for environmental management: the danger of a chemical cannot be judged by its structure alone. A substance that is theoretically less toxic can become the most pressing problem if it is released in massive amounts and spreads widely. The Geum River study demonstrates that as the world transitions away from legacy PFAS, the new alternatives are not merely stepping stones but are becoming dominant forces in aquatic ecosystems. The presence of transformation products like H-PFBS further complicates the picture, suggesting that our current wastewater systems are creating new chemical entities that we are only just beginning to understand. The authors emphasize that while the levels of the most toxic legacy chemicals have dropped significantly compared to previous decades, the rise of these ubiquitous short-chain alternatives demands continuous monitoring and a cautious approach to risk management. The river is telling a story of chemical evolution, and the next chapter will depend on how well we can track and manage these invisible, ever-changing contaminants.