Classes and status of surface water quality - how to read them

Knowledge base · July 2, 2026

When we say that a river is “in good status” and a lake “in moderate status”, we are referring to a coherent assessment system stemming from the Water Framework Directive (WFD) - Directive 2000/60/EC. Its aim is for waters to reach at least good status, and for results from different places and years to be comparable. For anyone who runs monitoring or reads reports on water, command of this “grid of concepts” is fundamental - without it, the figures from studies are hard to interpret.

The assessment rests on two pillars: ecological status and chemical status. Together they make up the overall status of a water body (JCWP) - the basic unit in which water management is assessed and planned.

Ecological status - five classes

Ecological status describes how far the ecosystem departs from reference conditions, that is from the state expected in a water of this type without human pressure. Hence five classes:

It is decided above all by the biological elements (organisms living in the water), supported by physico-chemical and hydromorphological indicators (WFD, Annex V, 1.2). This hierarchy has a concrete consequence that is easy to misread: physico-chemical indicators play a supporting role and can cap the assessment from above - at moderate status at most. They cannot push a water body down to poor or bad: those are decided by biology alone. Poor physico-chemistry on its own will therefore never produce class V.

For heavily modified and artificial water bodies (e.g. canals, impounded reservoirs), instead of “status” one assesses ecological potential (maximum / good / moderate / poor / bad; WFD Art. 4(3)) - because the reference point here is not a natural river but the best achievable state given the particular modification.

Chemical status - two classes

Chemical status concerns the presence of priority substances and other hazardous pollutants (among them heavy metals, polycyclic aromatic hydrocarbons, certain pesticides), assessed against environmental quality standards (EQS). The list of substances and the standards themselves are set by Directive 2013/39/EU. There are two classes:

• good,
• failing to achieve good (bad).

Why “failing to achieve good” is seen almost everywhere

Reading assessments, it is easy to draw a wrong conclusion: if chemical status is bad, the water must be locally polluted. Usually it is not. After the list of priority substances was extended with ubiquitous, persistent and bioaccumulative compounds (above all mercury and brominated diphenyl ethers), the EQS for those substances are exceeded in practice almost universally - including in waters without any local pressure, because the source is atmospheric deposition and historical contamination. The “worst decides” principle means that one such substance settles the outcome of the entire chemical pillar.

So a result of “chemical status failing to achieve good” says nothing in itself about local water quality. To draw a conclusion about pressure in a given place, you have to look at which substance decided it.

Why five classes, and only two for chemical status

This question comes up almost always - and the answer is logical, not arbitrary. Ecological status measures a gradient: departure from reference conditions can be graded, because communities of organisms change smoothly. Chemical status is a compliance test: for each substance there is an EQS which a result either meets or does not. There is no third possibility, so there is nothing to grade. More on the difference between these pillars: ecological versus chemical status.

Overall status - the worse decides

The overall status of a water body is the worse of the two results: it is good only when both the ecological and the chemical status are (at least) good. This reflects the principle of “the worst decides” (one-out-all-out), which guards against “averaging out” problems.

The same principle works within ecological status: the class is decided by the worst of the assessed biological elements, not by their average. That is why a single degraded indicator can lower the assessment of a whole water body - and why assessments are sometimes “worse than most of the measurements would suggest”.

How old is the assessment you are reading

The WFD works in six-year planning cycles (Art. 13(7)): river basin management plans are reviewed and updated every 6 years, and monitoring is aligned with them. This has a practical consequence when reading reports: the status assessment of a water body is not a current reading - it rests on multi-year data and may date from several years back. Comparing it with your own fresh measurement compares two different things.

Where to look for assessments and regulations

• The EU-level acts above (WFD and the EQS directive) apply in every member state, but the detailed thresholds and the classification method are set by national implementing regulations, which differ between countries and are amended more often than the directive itself. Always check the version in force and its date of application.
• In Poland, GIOŚ runs the State Environmental Monitoring and publishes water status assessments - see GIOŚ data - and Wody Polskie manages waters and is responsible for the river basin management plans.

In practice

Behind each of these classes lie measurement series of many indicators, which first have to be reliably collected and organised. If you run your own monitoring, in LimnoLog you can record measurements with guidelines (e.g. thresholds relevant to your assessment) for each indicator and see the exceedances at once. On the measurements themselves we write in the Method Guide - including oxygen, macroinvertebrates and transparency.