PFAS research for the water sector: what are the issues?

First, we look at our coastline. Dune water utilities there use infiltrated surface water as a source of drinking water. We know that river water contains PFAS. What is the situation with respect to PFAS concentrations in the water from dune water sources?

Shift in thinking

“In about 2008, the first reports appeared with monitoring series in the dunes showing that there are PFAS here,” says KWR researcher Bas van der Grift. “At that time, PFAS were not yet seen as a problem. There was still no legal standard for them. But there was increasing interest in them in the global scientific community.” The realisation dawned that these man-made chemicals are very persistent and that they are not broken down with time. And that they may also be toxic. In about 2012, there was a shift in how we thought about them: it was not until 2017 that RIVM (the Dutch National Institute of Public Health and the Environment) adopted the first preliminary health advisory levels. In 2020, RIVM set a more stringent drinking water guideline value for PFAS of 4.4 nanograms per litre, stated as PFOA equivalents. This guideline value could possibly be included as a statutory quality requirement in the Drinking Water Decree in the future.

What happens to PFAS in the dunes

The collective research programme of the dune water utilities turned to the question: what happens to PFAS in the dunes? Van der Grift: “The dune water utilities wanted to know where PFAS came from. And what the implications are for the future sustainability of extracting dune water. To answer that question, we measured PFAS concentrations in infiltration ponds and the associated extraction wells and groundwater monitoring wells. We used existing technologies that can detect low concentrations of PFAS. The results show that PFAS concentrations increase between the infiltration and the extraction of water. We still haven’t established precisely why that happens. The dune water utilities can use the results to understand developments in concentrations of PFAS in groundwater and adjust their operations accordingly.”

Sea spray

Van der Grift refers to ‘sea spray aerosols’ as a possible source of PFAS in the dunes. These are tiny particles formed directly on the sea surface. The wind takes them into higher air strata. These particles have recently been linked to PFAS contamination in the atmosphere. “Sea spray aerosols are deposited on the dunes,” explains Van der Grift. “Through the soil, they can reach the groundwater and enter the extraction wells. In a new project with the dune water utilities, we will be investigating the influence of sea spray aerosols further. We are taking bore samples from the unsaturated zone of the dunes, in other words above the groundwater. We want to measure how PFAS levels develop there at different depths. In this way, we hope to discover whether the leaching of PFAS into groundwater is a result of the deposition of sea spray aerosols.”

Legacy from the past

For now, the most logical explanation for the higher PFAS concentrations in extraction wells is that this is a legacy from the past, continues Van der Grift. “The river water that was once used for dune infiltration contained higher PFAS concentrations than at present. That water went into the subsurface. Some PFAS bond to soil particles, and especially organic matter. On the other hand, the infiltration water contains less of these substances, and the process is in the other direction. The PFAS are released, which means the sediment will pass them on. In other projects, we are looking at another issue: how does that adsorption between PFAS, water and the subsurface work? PFAS molecules have a head and a tail with different properties. They have their heads in the water and their tails in the air. As a result, they like to be located in the interface between those two media. There are a lot of these interfaces in unsaturated soil zones and that affects the transportation of these substances.”

The fate of PFAS during the reactivation of activated carbon

The following example of PFAS research comes from KWR researcher Martijn van Veggel and it was conducted as part of the water sector’s collective research programme, Waterwijs. “During drinking water preparation, activated carbon is used to remove PFAS from the water so that the drinking water meets the statutory standard. The PFAS ‘sticks’ to the carbon, which therefore has to be reactivated after a while before being reused. The presence of PFAS means that reactivation has to be more frequent. The treatment involves using high temperatures in an atmosphere without oxygen. Drinking water utilities wanted to know what happens during this reactivation process because PFAS are so persistent. Do the PFAS simply evaporate or are they also broken down? Does this process break the stubborn bond between carbon and fluoride?”

No ongoing pumping

To answer these questions , Van Veggel simulated the reactivation process on a small scale. He used PFOS and PFOA – two known persistent types of PFAS – as model substances and saturated activated carbon with them. At the University of Bath (U.K.) the researcher used a special furnace with features that made it possible to conduct the required experiments at different temperatures. “From 500 degrees Celsius upwards, we didn’t find any PFAS on the carbon,” says Van Veggel. “The processing companies that conduct the reactivation work at temperatures of 800 degrees Celsius. So that is safe. During the analysis of the gas flow released when the carbon was heated, we did not find the original PFAS. And our measurements showed minimal quantities of small breakdown products. This means that drinking water utilities are not pumping PFAS further into the environment. They capture it with the activated carbon which they send for reactivation. Although we were not able to provide a numerical picture of this process, we could see that the amount of PFAS by-products is lower than the amount originally bonded to the carbon as intact PFAS. That makes it reasonable to assume that the PFAS are converted, broken down or mineralised during reactivation. So drinking water utilities are now using the best method that is currently available. And they are contributing to the reduction of the overall amount of PFAS in our environment.”

Short and long chains

The efficiency of PFAS removal from water with activated carbon depends on the types of substances involved. PFAS molecules with long chains – such as the PFOS and PFOA mentioned earlier – do ‘stick’ to the carbon. Molecules with short chains slip through. KWR researcher Elvio Amato knows the questions this raises for the water sector. “Short-chain PFAS, such as trifluoroacetic acid, or TFA, are increasingly a feature of the scientific debate. This is because very high concentrations of them are found in many places, including water. TFA is a non-degradable end product of the breakdown of other compounds that are used, for example, in refrigeration technologies or in pesticides. At present, RIVM believes that TFA is less toxic than other PFAS. Unlike long-chain PFAS, short-chain PFAS have less tendency to accumulate in the tissues of living organisms. But they are extremely mobile in the environment, making them difficult to remove from, for example, water. So they represent a major headache for drinking water utilities. In the Waterwijs programme, we have been conducting research to understand the sources of TFA, the treatment technologies that are useful, and whether they are found in groundwater. The results were published recently.”

Falling from the sky

The literature review conducted to identify the sources of TFA shows that fluorinated gases in the atmosphere and pesticides are the most important. Mainly as a result of atmospheric deposition and pesticide breakdown, TFA reaches the ground and enters the soil in rain. “For drinking water utilities, this means that, as long as there are molecules that result in TFA, they have to take them into consideration,” says Amato. “It just falls out of the sky; they can’t do anything about that. We conclude that reverse osmosis is the only effective way to remove TFA from water. This filtration method with high-grade membranes is already being used in a number of locations by drinking water utilities. However, it results in concentrate flows. In addition, reverse osmosis requires higher water intake, even though that water is not always available.”

Old and young groundwater

Another result of the study shows that no TFA was found in old, non-vulnerable groundwater, as had already been assumed. The situation is different with young, vulnerable groundwater: low TFA concentrations were found there. Amato: “Non-vulnerable groundwater is very deep in the ground, or it is protected by clay layers that ensure that it will not readily be contaminated with TFA. In the case of vulnerable groundwater, TFA can penetrate more deeply. In addition to recent monitoring, we also included old datasets from drinking water utilities in the study. We see the same patterns there. Although there are not yet any statutory standards for TFA in drinking water, I expect to see them in the future. If the concentrations continue to rise or statutory standards become stricter, that could put more pressure on drinking water utilities. They are obliged to meet increasingly stricter water quality requirements. TFA levels in the environment are already high and regulations are needed for emissions of this substance.”

Practical key

When asked about the future knowledge needs of the water sector relating to PFAS, all three researchers confirm that there is still a lot of work to be done for the time being. For example, there is a need for both an improved understanding of the behaviour of PFAS in different soil types and for the optimisation of wastewater treatment. Water quality and health is one of the social tasks that KWR focuses on. “At KWR, we approach PFAS in many different ways,” says Van Veggel. “We appreciate the value of existing technologies for the water sector and we apply them quickly. And if there is no tailor-made solution as yet, we design one ourselves. Our work is all about finding the practical key for the right answer.”

Would you like to know more about PFAS and the latest knowledge that you, as a water expert, mustn’t miss? Read the story we published on this topic earlier this year here . You can also follow our news page to stay informed!