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”, we are referring to a consistent system of assessment stemming from the Water Framework Directive (WFD). Its aim is that waters should reach at least good status, and that results from different places and years should be comparable. For anyone who runs monitoring or reads reports on water, familiarity with 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 close the ecosystem is to its natural state, and is divided into five classes:

  1. high (I),
  2. good (II),
  3. moderate (III),
  4. poor (IV),
  5. bad (V).

It is decided above all by biological elements (organisms living in the water), supported by physico-chemical and hydromorphological indicators (more in a separate article).

For heavily modified and artificial waters (e.g. canals, dam reservoirs), instead of “status” the ecological potential is assessed (maximum / good / moderate / poor / bad) - because the reference point here is not a natural river but the best achievable state given the particular alteration.

Chemical status - two classes

Chemical status concerns the presence of priority substances and other hazardous pollutants (including heavy metals, polycyclic aromatic hydrocarbons, some pesticides), assessed against the environmental quality standards (EQS). There are two classes:

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

Overall status - the worse decides

The overall status of a JCWP 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.

Where to look for assessments and regulations

  • GIOŚ runs the State Environmental Monitoring and publishes water status assessments (including the water-quality portal).
  • Wody Polskie manages waters and is responsible for the river basin management plans.
  • The detailed thresholds and the method of classification are set out in the regulation on the classification of the status of surface water bodies - it is worth consulting its current version.

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.

← Knowledge base