Several hundred thousand chemicals are considered as potentially environmentally relevant. Scientists from the RPTU Kaiserslautern-Landau in Germany show that monitoring data for surface waters are only available for a very small fraction of these chemicals.

In their article, published in Science, the authors also demonstrate that the environmental risks of highly toxic chemicals might be overlooked, because these chemicals affect ecosystems at concentrations that cannot be detected on a regular basis.

“We analyzed a very extensive US database for the presence of chemicals in the US surface waters and compared it with the toxicity data of these chemicals for aquatic organisms, such as plants, insects or fish,” says Ralf Schulz, environmental scientist from Landau and senior author of the article. “This way, we could assess how the dataset reflects chemical risks of the last six decades.”

According to the article, the major problem for comprehensive risk evaluations today is the insufficient chemical coverage of monitoring. Only less than 1% of the 300,000 chemicals, which the US Environmental Protection Agency lists as potentially environmentally relevant, has in fact been monitored.

The article reveals interesting patterns by linking 64 million monitoring records for 1,900 chemicals, 300,000 sites and the time span from 1958 to 2019 with toxicity thresholds indicating risks for aquatic organisms. For example, it shows increased numbers of threshold exceedances throughout the US in the 1970s for a relatively small number of chemicals, including heavy metals, such as copper, lead and zinc. But it also shows that the subsequent measures for emission control led to a decline in threshold exceedances of these inorganic chemicals.

In the 2000s, another peak of threshold exceedances can be observed, but this time spread out over a much larger number of mainly organic chemicals, such as pharmaceuticals and pesticides. The number of the exceedances has dropped since then, too. However, because the monitoring of these chemicals was stopped, it is not possible to state whether the environmental risks of these chemicals decreased as well.

“If you stop monitoring a problematic chemical, you lose the capability to track its actual presence in the environment. Without the monitoring information, it becomes very hard to understand how potential risks are developing,” comments Sascha Bub, environmental scientist and lead author of the article.

The article also presents the evaluation of 37 million records of analytical limits from the US database. Analytical limits describe the lowest concentration at which a chemical can be found in the environment.

For inorganic and most organic chemicals, the analytical limits are low enough to detect them at all concentrations that affect aquatic organisms. However, for some pesticides, and especially some insecticides, typical analytical limits are not sufficient to cover all concentrations that are associated with risks, because their analytical limits are close to their toxicity thresholds.

As a result, some concentration ranges that are associated with risks for aquatic organisms cannot be captured, and potential effects on the ecosystem remain undetected.

One group of insecticides, pyrethroids, which play an important role in today’s agricultural practice and belong to the most toxic chemicals for aquatic organisms, has conspicuous analytical limits that are almost entirely above the toxicity thresholds. The actual environmental risk of pyrethroids can, therefore, only be assessed to a very limited extent.

According to the authors of the article, their results can presumably be transferred to many other regions of the world. However, the data for the conduction of similar analyses is most often missing.

Bub emphasizes, “Our results illustrate the importance of analyzing the environmental data on large temporal and spatial scales. We need such analyses to be able to guide the monitoring and assessment of the rapidly increasing number of chemicals in use.”