Environmental quality standards (EQS) and priority substances

Knowledge base · July 8, 2026

Behind the shorthand assessment “good / failing to achieve good chemical status” lies a specific mechanism: comparing the concentrations of selected, hazardous substances with established environmental quality standards. It is the complement to the picture of water assessment - and a topic worth knowing when reading the results of chemical tests.

Priority substances

Priority substances are a list of particularly dangerous pollutants, established at the level of the European Union. It includes, among others, heavy metals (e.g. cadmium, mercury, lead, nickel), polycyclic aromatic hydrocarbons (PAHs), some pesticides and industrial substances. It is these that decide the chemical status of a water body.

Environmental quality standards (EQS)

An environmental quality standard (EQS) is the permissible concentration of a given substance in the aquatic environment (sometimes also in organisms or sediments). Two values are usually defined:

• the annual average (AA-EQS) - protects against long-term exposure,
• the maximum allowable (MAC-EQS) - protects against short, high peaks of concentration.

Chemical status is good when none of the assessed substances (the priority substances and a few other pollutants from the same assessment) exceeds its EQS; the exceedance of even one means status failing to achieve good.

Not to be confused with “specific pollutants”

This is a common confusion. Alongside the priority substances (chemical status) there are specific synthetic and non-synthetic pollutants - assessed within the ecological status, as one of the physico-chemical elements (see elements of assessment). In other words: the same seemingly “chemical” indicators can belong to two different assessments - depending on which list they are on.

The difference is easiest to see side by side - these are two separate assessments, not two levels of the same one:

This division will change. Directive (EU) 2026/805 (in force since 10 May 2026) moves specific pollutants - in its wording “river basin specific pollutants” - from ecological status to chemical status, and part of their list will be set by the Union. In each EU Member State (Poland included) this will start to apply once the directive is transposed into national law (deadline: 21 December 2027), and the standards for these substances enter into force at the start of the next river basin management plan after they are set. Until then the table above applies.

⚠️ The same substance can fall into a different box in a different country - the priority list is EU-wide, but the lists of specific pollutants are national. When comparing results with foreign ones, check which list your parameter sits on before you compare thresholds.

How it combines into a whole

Chemical status (based on EQS) and ecological status combine into the overall status of a water body - the worse of the two results decides (see Ecological vs chemical status and Classes and status of water quality).

Sources

• What will change: Directive (EU) 2026/805 (in force since 10 May 2026, deadline for transposition into national law 21 December 2027) - amended standards for 14 substances and 25 new substances with effect from 22 December 2027; river basin specific pollutants move to chemical status. We do not copy new values until they take effect. Checked 20 October 2026.

In practice

Determinations of priority substances are carried out by a laboratory - in the field what counts above all is the correct sampling and preservation of the sample (see water sampling). It is worth keeping the lab results together with the rest of the monitoring and relating them to the standards; in LimnoLog you record them next to the right sample, with guidelines (e.g. the EQS values) for the given indicator. The in-app alert compares every single result with the guideline, so it works best with the maximum allowable standard (MAC-EQS); compliance with the annual average (AA-EQS) is judged on the mean for the year.