2021

Tineke Slootweg about the project ‘Evaluation of drinking water-relevant substances’

Tineke Slootweg is a chemical water quality consultant at Het Waterlaboratorium. "Recently, it's all been about the assessment of new substances that are emerging. With questions such as: what new unknown substances are we finding in the sources of drinking water; are these substances removed during purification; what is the risk of the substances to the drinking water companies for example?"

New substances, new lists

Tineke herself has been involved with this topic since 2011 and drafted an evaluation for the first time in 2015. "In the meantime, I almost know the list by heart." In 2021, the list of drinking water-relevant substances was re-evaluated. "That was a lot of work. Particularly due to the trend of the new substances turning up. We've made a wide collaborative project out of it, together with Aqualab Zuid, the Belgian drinking water company water-link and RIWA-Meuse.”

Assessment system

The question is: how do you arrive at a common list of substances that the drinking water companies will monitor? According to Tineke, there is a whole system behind this, which has been gradually further refined in the course of time. She describes how it works. "We first check whether the substance arises at multiple places in the Meuse, and whether it also appears regularly. Then we check whether the substance exceeds specific target values, and whether it has already been detected recently."

However, this is by no means the end of it. "We also define a number of properties of the substance that give an indication of whether it is possibly relevant: for example, how well it dissolves in water, and how easily it binds to active carbon. Based on this, you can make a good estimate of how a substance will behave in a drinking water purification plant. We use this to estimate how well the substance will be removed during natural purification."

List 1

All this yields a score that determines whether the substance appears on List 1 or not. "Once they appear on List 1, the substances are monitored by all the drinking water companies along the Meuse using target analyses. This means that the concentrations are measured, and that it becomes possible to determine the risks. There are also substances that are disregarded from List 1 with the passage of time. This applies for example to prohibited pesticides that are no longer detected. For example, as pyrazole, with which the industrial sector has done a great deal to reduce its emissions. As a result, this substance has ended up below the relevant concentration. However, new substances end up on List 1 as well."

The primary criterion for being added to List 1 is whether the substance poses risks to human health. "To this end, we look at the concentration at which we expect no effect at all. RIVM (the Dutch National Institute for Public Health and the Environment) does this as well. For substances that are detected in the Meuse at high concentrations, we ask RIVM for advice about their risks. They then use the available data to calculate a safe concentration or standard.”

Information sources

The next question that arises is: how difficult is it in fact to assess a substance? "That all depends. For biocides, it’s easy to derive the risks, because legislation has been created for this that stipulates that the assessment must already have been done before the product comes on to the market. So, before these substances are even allowed to be produced, a calculation has already been done of the concentrations safe to humans. We only need to consult the dossier. We also use information from studies by the RIVM or the US EPA. For substances present in consumer products, such as in shampoo or foodstuffs, it's also easy to obtain information.

The assessment becomes harder when we're looking at industrial substances used as intermediate products or by-products, because there's often no information about them. In this case we assume a maximum permitted concentration of 0.1 micrograms per litre. This is a generally accepted toxicological threshold value: hardly a single substance still has an effect on humans below this threshold. Therefore, this also becomes the target value for surface water.”

Screening and drinking water-relevant substances

In a nutshell: the assessment system works as a kind of flowchart or decision tree. Important factors are checked step by step: for example, whether a substance goes straight through the water purification plant, or whether a substance has an effect on humans at low concentrations, and naturally whether the substance is actually detected in the water.

New in this system of drinking water-relevant substances is the extra focus on the use of screening techniques. Thanks to this, many new substances can be identified quickly. How does it work? Tineke: "The starting point is a list of 2,000 known substances; the substances library. Next, water samples are analysed using liquid chromatography, in combination with high-resolution mass spectrometry.

This yields a pattern of peaks that can be compared to the peaks of the known substances in the library. This gives us an indication of the substances present without us knowing their concentrations. Using this screening method, we can look for more substances simultaneously than with target analyses."

Tineke predicts that the arrival of this screening technique will mean a lot for the monitoring of new drinking water-relevant substances. "By using screening, we can in fact also look for substances that are suspected to be relevant to the drinking water sector, but about which we still know too little, because they're not yet monitored. We add such substances to the library, and then include them in the screening.

This yields a general picture of where such substances arise, and how often. We call these substances 'candidate drinking water-relevant substances.' They end up on List 2B."

Candidate 2A status

"If we actually find the substances on List 2B in all the sources (at water-link, WML, Evides and Dunea), the substance is shifted to List 2A in the next evaluation round. This means that we will develop a target method for it, so that we can also determine its concentrations and health risks in the future.

List 2A therefore includes substances that have emerged from the screening and are seen as important. Substances that we not only measure at many places in the Meuse, but that we also sometimes detect in the drinking water.

 Moreover, List 2A also includes substances that have come up as relevant in specific monitoring programmes. For example, the KWR water research institute has recently created a method for very polar substances. If these are actually seen in the Meuse at concentrations above the 0.1 micrograms per litre, then the drinking water company also has to start to monitor it."

Monitoring

Now that the idea behind the three lists is clear, the question arises of what happens after going through the step-by-step plan? "The drinking water sector itself determines which 2A substances will be monitored using target analyses. After all, it means an expansion of the measurement package. This expansion is phased, so that the extra monitoring efforts are spread out.

In the first year for example, we’re focusing on 10 new substances. We follow these for a year. The year after that, we select another 10. It's not realistic to suddenly start monitoring 40 extra substances in one year, on top of the existing monitoring programme."

Importance of the list

It is clear from Tineke's account that there is much knowledge and expertise behind the system. Why is the list so important?

“The strong point of the list of drinking water-relevant substances is that it forms a common monitoring list for all the drinking water companies. We therefore have a specific list of substances that we can use jointly, and with which RIWA-Meuse can really get started to address emissions. The list of drinking water-relevant substances has for example already been shared with Rijkswaterstaat, along with the request to start monitoring these substances in the Meuse. This is so that targeted actions can then be put in motion to reduce the substances in the Meuse.”

Tineke also indicates the importance of the substance lists in relation to recently amended legislation. "According to the Drinking Water Directive, drinking water companies must start to use risk-based monitoring. It's then useful to be able to use this approach (determination of drinking water-relevant substances)."