In 2023, USGS researchers led by hydrologist Kelly L. Smalling conducted a study on PFAS in US tap water. They published their findings in Environment International. The study focused on the quality of drinking water in the United States. It specifically examined the presence of per- and poly-fluoroalkyl substances (PFAS) in tap water.

Understanding PFAS

PFAS are man-made chemicals. They are often called “forever chemicals.” These chemicals have raised concerns among scientists and environmentalists. This is due to their widespread presence in the environment and their persistence over time.  Their persistent nature is troublesome when considering their toxicity, and the potential risk they pose to human health through exposure.

The Purpose of the Study

The researchers embarked on a national reconnaissance. The objective was to compare human exposure to PFAS from two different sources of water. These were unregulated private wells and regulated public water supplies.

Methodology of the Study

The team collected tap water samples from 716 locations across the United States. This collection process took place over a period of five years, from 2016 to 2021. This extensive collection period allowed for a comprehensive analysis of PFAS presence in tap water.

PFAS Findings in Tap Water

The study found a varying range of individual PFAS in the samples. The number of PFAS observed in the samples ranged from 1 to 9. The corresponding cumulative concentrations of these substances ranged from 0.348 to 346 ng/L. Out of all the PFAS, seventeen were observed at least once. Among these, PFBS, PFHxS, and PFOA were the most frequently observed. They appeared in approximately 15% of the samples.

The researchers modeled these results and found on average at least one PFAS in about 45% of US drinking-water samples. The results show rural areas (8%) are less likely to have PFAS in water than urban areas (up to 70%).

Comparison Between Private Wells and Public Supplies

The study made a key comparison. It compared the PFAS profiles and median cumulative concentrations in private wells and public water supplies. The study found that these were similar in both sources. The researchers estimated that at least one PFAS could be detected in about 45% of US drinking water samples. However, these detection probabilities were not uniform across all locations. They showed spatial variation. There was also limited temporal variation in the concentrations or numbers of PFAS detected.

Risks Associated with Human Exposure to PFAS

The study concluded with an important finding. It found that the potential risk of human exposure to PFAS was dominated by two specific substances. These were PFOA and PFOS. This was the case when they were detected. In addition to this, the researchers discovered a correlation. This was between potential source and land-use information and the cumulative PFAS concentrations. It was also related to the number of PFAS detected.

Call for Further Research

The study underscores the urgent need for further research. This research should look into the cumulative health risks posed by PFAS. This is particularly important when considering PFAS as a class of chemicals. It is also important when considering them in combination with other co-occurring contaminants. The need for further research is especially critical in the case of unmonitored private wells. In these wells, information about PFAS presence and concentration is often limited or not available at all.


The study by Kelly L. Smalling and her team is a significant contribution to the field of environmental science. It provides valuable insights into the presence of PFAS in drinking water. It also highlights the potential risks associated with these chemicals. The findings of this study underscore the need for further research and regulation in this area. This is to ensure the safety and health of all individuals who rely on both private wells and public water supplies for their drinking water.

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