What is in our water? Known unknowns, and unknown unknowns

ducks rest as sunset turns the lake and sky rose red
CC BY 2.0 USDA

What is in your drinking water? It turns out there may be more than we know.

In general, we are enjoying a golden era of clean water, with healthy refreshment available at the twist of a tap to over 70% of the population of the world (source: WHO). But we are threatening this most valuable of resources. In many high-profile cases, such as microplastics and rocket fuels, people are beating the drums of change and regulators are acting. But we can't control what we don't know.

Target chemicals don't tell the story

When it comes to monitoring water supplies for safe drinking water, typically less than 100 chemicals or chemical groups are regulated. Another 100 may be on a list of contaminants of emerging concern (CECs), which water authorities are monitoring but are not yet regulating, as the concentrations and occurrences are below a health concern level. Or in the worst case, the CEC may be unsafe but the process of proving that to meet the standard required to regulate can take years.

These contaminants are known in the industry jargon as "knowns" or "target pollutants." The word "target" explains it: these are the chemicals we know might be there and we are aiming to find them and quantify their levels to ensure they are below levels of concern for public health.

But while we are looking for a couple hundred bad actors, there are approximately 30,000 chemicals being sold and used in the modern world. And that is not counting the degradation products which are formed when these chemicals break down to other chemicals (transformation products or metabolites) in the environment. Usually, that breakdown is a good thing - most of these chemicals break into harmless bits and no longer pose problems. But the breakdown can end at smaller molecules that are as dangerous as the starting molecules, and may be more mobile which means they could get into water sources more quickly.

Which chemicals are worth looking for?

At a recent conference hosted in Berlin, Germany, representatives of the water and chemical industries, the regulatory authorities, and scientists got together to talk about how we can get better at predicting which of these 30,000 chemicals we need to prioritize for better control. The answer builds on success stories in identifying chemicals like Persistent Organic Pollutants (POPs) or Persistent, Bioaccumulating, and Toxic (PBT). You know these as the chemicals that build up in the food chain, contaminating even remote polar bears.

A similar new class of chemicals is proposed: Persistent, mobile, and toxic (or even very persistent and very mobile, because these will build up to concentrations that may be an issue even if the toxicity is lower). The acronyms PMT and vPvM were proposed but may not survive because M already gets used for mutagens (chemicals which can cause heritable genetic damage).

One attendee commented that while POPs and PBTs accumulate in the food chain, building up to potentially dangerous levels in the fat of animals at the top of that chain like polar bears or humans, the persistent, mobile substances build up in the water cycle. They are persistent (not degrading to harmless bits), so they continue to concentrate in the environment. They are mobile, which means they don't stick to the earth but move easily with run-off and percolate into our sources of fresh water. Instead of building up inside us, they build up outside us, and we are faced with a repeated exposure to these low, but increasing, concentrations of contaminants.

A good start has been made in laying down the rules for how to identify the chemicals that may have these persistent and mobile properties, based on data which is often available. That brings us to the next obstacle: even when resources are available, testing water and finding these contaminants is not so easy.

Looking for the known unknowns and the unknown unknowns

With constant, daily exposure, even low concentrations of contaminants may be of concern. To find these, scientists have to separate these low levels of chemicals out of the water in order to identify them, which can require variations in methods depending on the source: e.g. surface water as opposed to treated drinking water.

There is a chemist's alphabet soup of methods which can find thousands of chemicals in water. But the chemicals with the sneakiest abilities to hide in fat (= bioaccumulative) or stay well dissolved among the water molecules (= mobile) can be the hardest ones to find. Fortunately, that leaves some good opportunities for Ph.D. candidates to push the limits of analytical chemistry.

Once a method has been developed for analyzing the chemicals, scientists get something that is analogous to a fingerprint at a crime scene. It is now possible to detect thousands of chemicals in a sample, each leaving its unique fingerprint. But like in the crime lab, they may not know the identity of the chemicals leaving many of the fingerprints, and especially not which ones belong to the criminals.

They turn to catalogs with the "fingerprints" of thousands of known chemicals. In this way, any "known unknowns" - that is, chemicals which are documented in literature but that weren't specifically expected in the water - can be given a name. It may even be possible once the chemical is known to quantify it (this is tech-speak for determining the concentration, and important piece of data because the dose makes the poison). And there may be information on whether there are known hazards associated with the chemical, or chemists can use computers to see if the chemical looks like others known to be hazardous. So that's a bit like finding the fingerprint of a criminal with a record, and checking out its rap sheet.

That still leaves the unknown unknowns. These are the chemicals that are not adequately documented in literature, not found in the databases that could help identify what is leaving that fingerprint. The crime analogy holds: just because we don't have a rap sheet doesn't mean we aren't looking at a previously unknown criminal. Scientists may need representatives from industry to provide samples from their facilities, or share proprietary analytical methods developed for manufacturing or quality control. Identifying degradation products unknown even to the industry is even more difficult.

But the tools are getting better all the time, so you can expect to hear more on this topic. One thing was clear at the conference in Berlin: the European regulatory authorities are poised to act if they do not see the chemical industry take up the proposed methods for identifying chemicals of concern based on their knowledge of their products - before they can become contaminants of concern. What a great opportunity for meeting commitments to sustainability and avoiding the burden of regulation.

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