Self-purification of waters - how a river copes on its own
A river or a lake is not passive in the face of pollution - it has a natural capacity for self-purification, that is, the gradual removal or transformation of substances that reach it. Understanding these processes helps you interpret results and appreciate why protecting water is not only “do not pollute” but also “do not overload”.
Three groups of processes
- Physical - dilution, sedimentation (the settling of suspension), sorption, aeration on riffles.
- Chemical - oxidation and other reactions transforming pollutants.
- Biological - the decomposition of organic matter by micro-organisms and the uptake of nutrients by algae and plants. This is usually the most important mechanism.
What it depends on
The efficiency of self-purification rises when the water is well oxygenated (the decomposition of matter uses up oxygen!), warmer (up to certain limits, it speeds up biological processes) and when there is enough time (flow, the length of the reach). That is why regulated, oxygen-poor watercourses cope worse.
The limits of this capacity
This is crucial: self-purification has a limited capacity. Under too large a load - intensive decomposition exhausts the oxygen, deficits and an oxygen crash appear, and an excess of nutrients drives eutrophication. In other words: water will “clean up” after moderate pollution, but overloaded - it will collapse.
Why know this
- Interpretation: an improvement in indicators downstream can be the effect of self-purification, not the absence of a pollution source.
- Protection: the point is not to exceed the natural capacity of the waters.
In practice
Self-purification is best seen in a series of measurements along a watercourse (above and below a source) and over time. In LimnoLog you can run several stations in one project and compare how, for example, the oxygen or the load changes between points.
← Knowledge base