Environmental quality standards (EQS) and priority substances
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 priority substances 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:
| Priority substances | Specific pollutants | |
|---|---|---|
| Who sets the list | the European Union - common to all member states | the member state, for its own waters |
| Which assessment they enter | chemical status | ecological status (a physico-chemical element) |
| The standard | EQS: annual average (AA) and maximum allowable concentration (MAC) | national threshold values for the classes |
| Matrix | water, and for some substances biota or sediment (mercury among them) | usually water |
| What exceedance means | chemical status below good - for the whole water body, on a single substance | the physico-chemical element falls below good, so ecological status cannot be higher than moderate |
⚠️ 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).
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.
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