PFAS and the water sector: what do we know at the moment?

PFAS can enter the soil in various ways, examples being atmospheric deposition and in pesticides. There are many questions about what happens then. It is, for example, unclear how quickly PFAS enter the groundwater. This is an important question for drinking water utliities: PFAS in groundwater can spread faster, with the danger of them entering drinking water sources. In addition, PFAS that stay in the soil pose a long-term risk.

PFAS transport

Crucial questions therefore include: where do the PFAS stay after they enter the soil? And what exactly happens to them? The project PFAS leaching in Dutch soils, which was conducted under the umbrella of Waterwijs – the collective research programme of the drinking water utilities, established a clearer picture of PFAS transport. The project was a follow-up to earlier Waterwijs research, which included looking at which models are suitable to get to grips better with this subject. “All PFAS research is actually quite recent,” says Sija Stofberg, a hydrologist at KWR. “What we know is that PFAS have special properties. The substances are transported by water but they also stick to soil particles, and they stay in the zone where water and air meet. All these factors are present in the unsaturated zone, in other words the soil below the groundwater level. Even though groundwater in the Netherlands is not deep – around one or two metres in many places – an awful lot can happen in the soil above.”

Dutch soils

Stofberg explains that, when research began, a limited number of studies from the United States were available looking at PFAS transport in the unsaturated zone, with measurements and calculations for just a few (synthetic) soil types. “You can’t simply apply those results to the Dutch situation. We have different soil types here, and relatively shallow groundwater levels; that is very different from the American soils that were studied. And so we wanted to conduct a sensitivity analysis, a study where, in this case, we looked at three different factors that play a role in PFAS transport in the unsaturated zone: soil type, groundwater level and type of PFAS. Our aim was to establish how these factors may affect leaching. We used the HYDRUS model code, which emerged from our earlier project as very suitable. This software is used for flow and transport in unsaturated soils and, for some years now, it has had a module for calculating PFAS transport. It includes the air-water interface which is characteristic of the unsaturated zone. You can use a model of this kind to describe the soil as simply as possible. You put in all kinds of information such as precipitation, evaporation and vegetation, as well as data about the soil that are available in the bodemfysische eenhedenkaart, a map of the Netherlands designed for calculations of this kind. It shows all kinds of soil types for which the characteristics are known. Our focus was on groundwater protection areas where drinking water utilities have their sources. We then entered the groundwater depths that apply to those soils and the properties of twelve PFAS components. We selected substances that are relatively common, with different chain lengths, and about which information is available.”

PFAS type is the most decisive factor

Because of the numerous factors, a huge number of simulations were required. “That was quite a job,” says Stofberg. “Because you are working with extremes. There are soils that can become very dry, and other soils are actually very wet. It was a considerable challenge to make calculations for all those differences.” The results indicate that the type of PFAS is the main determinant of how long it takes for it to leach through to the groundwater. And there was a sizeable spread in this respect for the twelve substances. Stofberg: “PFAS with very short chains like TFA leach almost as fast as chloride, for example, which flows at the same speed as the water itself. Depending on the soil type and groundwater level, this can take less than one year. By contrast, longer chains such as PFOS and PFDA can stay in the unsaturated zone for decades, or even centuries. As we expected, leaching is faster than in sandy soils than in clay soils. Another factor is the composition of the soil: which minerals and how much organic matter does it contain? And how big is the air-water interface? This means that the transport speed of PFAS can vary by up to a factor of ten between different soil types.”

Shortage of field studies

If Stofberg were to sit around the table with people working at a drinking water utility, what would she advise on the basis of these results? “I would say they need to take into account not only PFAS substances that they could already find in their sources in the short term, but also substances associated with a long delay. These substances can show up in groundwater even a long time after PFAS have been released into the environment, or after their use has been banned. At the same time, I would like to nuance this result. We are talking about model results here: field studies are needed to compare what the model tells us with the actual situation. Does the model provide us with a realistic picture? Or does it need to be modified? For example, in practice, we see specific PFAS in very deep locations, even though we thought they would accumulate in the root zone for centuries. That could mean, for example, that there are still places in the soil where leaching is faster. There may be variations in the soil, or perhaps a mole has disturbed the soil, with this acceleration as a result. So we urgently need data that have been systematically collected in the field to complete the picture. RIVM’s PFAS research programme represents a major collaborative effort to remedy this gap with a nationwide monitoring campaign.”

Health issues

For drinking water utilities, research like Stofberg’s is actually always too slow. They want an overall picture of the PFAS problem as soon as possible so that they know what they are dealing with. With all the disciplines that KWR has in-house, researchers are working day in and day out to put together all those pieces of the puzzle. To provide a description of the variety, we also talked to Milou Dingemans, the Chief Science Officer and a toxicologist who specialises in research on health issues relating to substances in water. “KWR has been a Collaborating Centre of the World Health Organization WHO since back in 2013,” she says. “On the basis of that role, we are involved in substantive issues relating to water quality and public health. PFAS are super-relevant here. For example, the RIVM asked us to talk with them about their assignment from the WHO to develop an approach to mixture toxicology in relation to PFAS.”

Expert workshop

For this assignment, RIVM wants to work out how to determine when PFAS in food or drinking water are hazardous to health – not just on a substance-by-substance basis but also as mixtures that people ingest every day. The development of new assessment methods, data protocols and limit values will provide a scientific basis for progress towards legislation and regulation for PFAS and public health. “Accordingly, RIVM organised an expert workshop in October 2024 to which KWR was invited,” says Dingemans. She was one of fifty participants from research institutes, universities and government agencies worldwide. The goal of the workshop was to launch an open scientific discussion about different approaches to mixture toxicity relating to PFAS, including their strengths and weaknesses, and their suitability for the assessment of PFAS in food and drinking water. “One of the things I contributed on behalf of KWR at that time was the importance of focusing on the most relevant PFAS.”

Health effects of PFAS

Another workshop on PFAS at which Dingemans participated was organised by the European Food Safety Authority (EFSA) in November 2025. The idea behind the workshop was to provide authorities and relevant stakeholders with an update on the health assessment for PFAS. Dingemans: “Because PFAS spread so widely, international measures are needed to mitigate the effects. PFAS are forever chemicals. They are still being produced and used. So different disciplines and European member states and authorities need to persist with their joint efforts.” Dingemans was pleased to hear a contribution from WHO saying that RIVM had come up with a new approach to establish an overview of the most relevant PFAS and their most relevant health effects, including identifying and prioritising the most important intake of these substances. “A preliminary approach has been developed that will be able to derive guidance values for PFAS that people ingest through food and drinking water. These steps mean that the stage has now set for the next phase in which more specific priorities can be established for specific groups of PFAS and the health effects associated with them, and in which health guidance values will be proposed. That phase began late last year. Some eighteen PFAS and six health effects are being scrutinised. The results will probably be discussed in an expert workshop in early 2027.”

Safe intake level for TFA

Dingemans also heard at the workshop that the European Commission has asked EFSA to derive a safe intake level for TFA. “TFA is a short-chain PFAS and a product of the breakdown of longer-chain PFAS found in, for example, specific pesticides,” explains Dingemans. “The outcome of this research may carry over into the authorisation of these pesticides, in other words into legislation and regulation. There will also be more research into the formation of TFA in soil and water, including the processes that are important in that respect. This safe intake level for TFA will be published in 2026. I am following this closely because this is also important knowledge for the drinking water utilities.”

The polluter pays

A third theme that Dingemans identified at EFSA’s highly informative workshop was that the European Environmental Agency will be publishing a report on the ‘polluter pays’ principle. KWR is also working on this topic, she says. “For drinking water utilities, ‘the polluter pays’ is an important factor. At the end of the chain, they are saddled up with the problems caused elsewhere by the production and use of substances such as PFAS. Drinking water utilities put a lot of effort into monitoring and removing those substances. Extending producer responsibility could change this.”

Future mixture toxicology and PFAS

Looking at the field of mixture toxicology and PFAS, what are Dingemans’ ambitions for the direction that KWR’s research should take? “I think we are on the right road. And I expect that we will have standards for PFAS mixtures within a few years. KWR has been involved in the discussions about the indicative drinking water guidance value proposed by RIVM for a mixture of relevant PFAS. That value is 4.4 nanograms of PFOA equivalents per litre. The term ‘equivalents’ indicates that the focus is not on a single specific PFAS but on the sum of several substances for which the amounts are converted to a baseline substance, in this case the widespread substance PFOA. This guidance value is a recommendation for the Ministry. However, the move to a policy-based standard is on hold pending the WHO report. In the meantime, we are working on expanding our knowledge in the practice-based PFAS research being conducted by KWR and the drinking water utilities. For example, we are looking at which groups of PFAS could pose a threat to public health, and therefore which should be included in the PFAS health guidance value. So there is plenty to do!”

Would you like to know more about PFAS and the latest knowledge that you, as a water expert, mustn’t miss? Read the stories we published in February and in July last year. You can also follow our news page to stay informed!