We often hear that we need clean water to live, that access to good quality water is a basic human right and need, that clean water is a prerequisite for the proper functioning of ecosystems, and much more in this vein. This is the absolute truth, an indisputable fact. But have you ever wondered what “clean water” means? Clean, meaning what kind of clean?

Different applications, different criteria

Typically, water purity is assessed in terms of its suitability for consumption. It seems that if it is suitable for drinking, it is basically suitable for all other uses. Well, not necessarily. The requirements are different and this should not surprise anyone. However, a deeper analysis of them can lead to interesting conclusions. According to current regulations, the overall salinity, expressed in terms of specific electrolytic conductivity, for inland waters (at least those not subject to seawater) should not exceed 850 µs/cm, those captured for public water supply – 1000 µs/cm, while those already intended for human consumption – 2500 µs/cm. So what? The water flowing from the tap could theoretically be in poor ecological condition. The same is true of mercury concentrations. The standard for this element for good chemical status is 0.07 µg/l, while in drinking water it can be up to 1.0 µg/l (14 times more!). That is, again – the water in the tap may be drinkable, but be in poor chemical condition.

Such examples can be multiplied, as both the levels of acceptable indicators themselves and their selection vary widely in different evaluation systems. The condition of the water is assessed on the basis of more than 100 indicators, the quality of the water captured for public supply – almost 40 indicators, and water intended for human consumption – more than 50. The waters used for bathing, on the other hand, are evaluated primarily on the basis of bacteriological indicators, which are not considered at all in the assessment of water status. That is, it is theoretically safe to bathe in water that is not suitable for aquatic organisms. I am not positing that the criteria are wrong or that they should be standardized. I’m just citing examples to emphasize that talking about “clean” and “dirty” water requires a certain precision and awareness of what we are talking about.

Clean water, dirty water?

The closest way for us to determine the “purity” of water is through organoleptic evaluation, that is, finding out how it looks, smells and even tastes. But here again we have a game of appearances. Of course, water with a greenish-blue cyanobacterial crust or a stain of an oil-based substance certainly does not meet quality standards, no one will drink it or bathe in it. But already brown water with a high content of organic matter, the so-called “brown water”. Humic, even if it is not suitable for drinking, can meet the conditions of ecological quality, i.e. appropriate to nature. More, in natural oxbow lakes and small bodies of water, even greenish water with increased conductivity can be a completely natural condition and not indicative of any pollution. It may not impress with its crystallinity, but fixing it is not necessary. Conversely, seemingly crystal clear water can contain harmful, though invisible to the naked eye, contaminants, such as pesticides or heavy metals, which disqualifies its use for food purposes. So, let’s not be fooled by appearances – what is dirty and what is clean depends on the needs and the definition adopted.

Bad water condition in Poland, but maybe not that bad?

And one more important point. Recently, the water assessment maps of Poland, spectacularly red and indicating a globally bad condition, have been frequently cited, especially in the context of the events in the Oder River in the summer of 2022. Sure, the quality of Poland’s water is not good. Nevertheless, when looking at such maps, it is worth being aware of the method of their preparation. Considering that the overall condition of the waters presented on these maps consists of an assessment of more than 100 indicators, and exceeding the standard of even one of them results in red marking, it is really hard to be surprised by the scale of the phenomenon.

On the other hand, in terms of ecological status, unacceptable condition is defined by as many as three classes – moderate, poor and bad (marked in yellow, orange and red in the figure below). On the final map, all three will be marked in red. None of them are satisfactory, but a moderate state is not yet an ecological disaster. And this is the condition that affects 53% of the more than 3,500. rivers and 49% of the 749 lakes in Poland assessed in the previous planning cycle (2016 – 2021)[1]. During the same period, poor ecological status/potential affected 13% of rivers and 15% of lakes. There is no reason to be proud, but not an absolute tragedy either.


Ocena stanu ekologicznego rzek w latach 2016 2021 Wodne Sprawy
Clean water, or what kind of water? 1



Ocena stanu chemicznego rzek w latach 2016 2021 Wodne Sprawy
Clean water, or what kind of water? 2



Ocena stanu rzek w latach 2016 2021 dla stanu ogolnego Wodne Sprawy
Assessment of the condition of rivers monitored in 2016-2021 (data source: GIOŚ, maps: editor’s own elaboration)

Poor water status is due to worse than good ecological status and/or poor chemical status. The latter is rated zero-one. If even one of more than 50 indicators exceeds the standard, the condition will be bad, regardless of the values of all others, both chemical and ecological. This should largely explain the redness of the general condition map.

So I return to the question from the title: is the water, in which mercury concentrations of 0.08 µg/l (i.e. just above the standard for chemical status) were found, and all other indicators, including biological ones, indicate at least good condition, clean or dirty? It will be red on the map. And I anticipate the question – yes, such situations occur in Poland, and not at all infrequently. Of the more than 200 river water bodies in good or very good ecological status, half have poor chemical status, mainly due to exceedances of mercury and benzo(a)pyrene (the latter is a component of smog and enters waters mainly from the air). That is, again – the state of the water bad, but whether so unequivocally bad, I am not convinced. Such ambiguities, moreover, are plentiful. More about the nuances and sometimes absurdities (unintentional) of water assessment is beautifully written by our editorial colleague in an article about the devil being in the details. I am not criticizing the current system of water assessment, nor am I requesting that it be changed. I am merely pointing out that oversimplifications in interpretation lead to highly inaccurate conclusions that are not befitting those considered experts in their field.

[1] Planning cycle – the six-year period during which a water management plan is in effect. Subsequent planning cycles include 2003-2009; 2010-2015; 2016-2021; 2022-2027.

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