This section aims to explain how water quality is assessed in rivers in the European Union. The ecological status (in natural and semi-natural rivers) or ecological potential (in heavily modified water bodies) is evaluated based on biological, hydro-morphological, and chemical quality.
INTRODUCTION TO WATER QUALITY IN RIVERS
The self-purification ability of standing and running waters was generally sufficient for coping with pollution in rivers until the 18th century. Before then, there was less wastewater produced, and the pollution was mostly of organic origin. It was possible for different organisms (microorganisms, algae) to decompose and degrade it into mineral forms, taken by algae from the aquatic system (biogeochemical cycles). After a certain period, the water was cleaned due to the natural recycling capacity.
However, in the last century, water in rivers became much more polluted. The problem occurs when large amounts of organic sewage are discharged, surpassing the self-purification capacity of water systems, or if the sewage contains substances of artificial origin that are biologically non-degradable or even hazardous. These substances influence the metabolism and life processes in watercourses, either because the microorganisms taking part in the biodegradation processes cannot decompose them, or because the decomposition processes consume too much oxygen, dramatically degrading the environmental conditions. As a consequence of lower oxygen content, most aquatic organisms die and accumulate at the bottom of the water, consuming even more oxygen for decomposition. This is how aerobic conditions turn into anaerobic conditions, where only a few species can survive.
The organisms decomposing the organic waste require oxygen. Therefore, we express the amount of organic substances that decompose in the environment with a biochemical oxygen demand at 20 °C in five days as “BOD5”. If BOD5 is high, this indicates a large amount of organic substances in the water and intense activity of the microbial community. For the majority of unpolluted watercourses, BOD5 ranges between 1and 10 mg/l, while communal wastewaters may range between 100 and 300 mg/l. Today, more precise methods are used by autoanalyzers, quantifying directly the concentration of TOC (total organic carbon), POC (particulate organic carbon), and DOC (dissolved organic carbon).
ECOLOGICAL WATER STATUS
Ecological quality (chemical, hydro-morphological, biological)
Starting with the year 2000 with the adoption of the Water Framework Directive (WFD), water policy in Europe changed significantly: rivers are regarded as part of a river basin, and their quality is assessed not only by water chemistry but by integrating the habitat integrity and the status of the aquatic communities. For the first time, the connection of the aquatic species with their habitats was taken into account in the assessment of the ecological status, with aquatic communities playing a key role in ensuring the functionality of the ecosystems.
- For this assessment, three groups of parameters should be monitored: chemical quality: general parameters (temperature, pH, oxygen, salinity), nutrients (nitrates and/or phosphates), specific pollutants (EU Member States are required to prepare a list of specific river basin pollutants that are recognized as problematic and their environmental quality standards in the aquatic environment for the purpose of evaluating the ecological status of water), priority substances (a priority list of 33 substances that pose a threat to or via the aquatic environment was established as a part of the Water Framework Directive) or other substances discharged in significant quantities into water bodies;
- hydro-morphological quality: hydrological regime, water flow (quantity and dynamics), connection to groundwater bodies, river continuity, morphological conditions, river depth and width variation, structure and substrate of the river bed, the structure of the riparian zone;
- quality of the biological communities: composition and abundance of aquatic flora and invertebrate fauna, composition, abundance and age structure of fish fauna;
If all parameters are of good quality, the river is considered to be in “good ecological status”, which is the goal of the WFD to be reached by 2027 at the latest.
The chemical composition of the aquatic environment is essential for defining water quality. The aquatic ecosystems receive numerous chemical substances resulting from human activities, posing an increasing health hazard not only to the environment but also to human health.
While organic pollution is tackled by the extension of sewage systems and wastewater treatment plants, for hazardous substances, however, there is no easy control over the total number, levels, and the final place of these substances. Some are accumulated in the sludge or sediments; some are taken up by aquatic plants and animals and, thus, get into the food chain. This is how many toxic substances enter the aquatic ecosystems on a daily basis, including detergents, pesticides, persistent organic pollutants, heavy metals, polychlorinated biphenyls, (PCBs), polyaromatic hydrocarbons (PAHs), endocrine disruptors, pharmaceuticals, microplastics, etc.
In recent years, it has been increasingly acknowledged that the river bed, the banks, the floodplains and the underground aquifers form a unitary riverine system, its integrity playing a key role for ensuring a proper habitat for the aquatic communities. The loss of the longitudinal, lateral or vertical connectivity between these components is the main cause of the ecological degradation of rivers. Hydrotechnical constructions, such as hydropower dams, embankments and dikes for flood protection, channelization by cutting meanders and side arms to meliorate navigation, gravel extraction and water abstraction for various human uses (irrigation, industrial or household usage, etc.) and the alteration of river flow severely affect the life of the aquatic communities.
For the Danube River, it is estimated that the construction of the Iron Gates dams has led to a reduction by 55% of the total suspended solids (Teodoru & Wehrli, 2005) affecting the river morphology and intensifying erosion processes along the river and the coastal area of the Black Sea. River construction and other technical impacts have led to a total loss of 65% of the Danube River floodplains losing, amongst others, significant areas of nutrient retention (Sommerwerck et al. 2009). The recent Joint Danube Survey 3 of the ICPDR (ICPDR, 2015) revealed significant impacts to hydromorphological structures: 21% of the Danube River’s length was classified as slightly modified (class 2), 39% as moderately modified (class 3), 26% as extensively modified (class 4) and 14% as severely modified (class 5); no near-natural stretches (class 1) could be found.
Aquatic ecosystems are inhabited by myriads of organisms fully dependent on the quality of water and their habitats. They ensure ecosystem functionality and are grouped into three major classes (producers, consumers, decomposers), each category with distinct roles in the aquatic ecosystems, strongly interlinked with each other.
In the presence of light, through photosynthesis, the primary producers (algae and macrophytes) synthesize organic matter, rendering it available for the higher trophic levels. The primary consumers (zooplankton, benthic macroinvertebrates, juvenile fish, and planktivorous fish) ingest the organic matter created by the producers, becoming, in their turn, food resource for secondary or tertiary consumers (small fish, larger fish, aquatic birds). In aquatic ecosystems, large predatory fish (pike, catfish, sturgeon) or waterfowl feeding on fish (pelicans, cormorants, white-tailed eagles) are the highest ranked consumers (so-called top predators), and therefore, they can represent good ecological indicators of the ecosystem health. The microbial communities play an essential role in recycling the organic matter as they decompose the excreta of the aquatic organisms, the detritus and the decaying bodies, preventing their accumulation in the aquatic systems and mineralizing the nutrients, to be taken up again by primary producers in a new cycle.
Freshwater ecosystems, such as rivers, lakes, and wetlands, are particularly important for biodiversity conservation: while they represent only 0.01% of the world’s water resources, they host almost 10% of the known species (UNEP, 2002, Dudgeon, 2014).
As many human activities rely on freshwater resources, these ecosystems suffered the highest impact, the species living in such habitats declining by 83% between 1970 and 2014 (WWF, 2018).
ASSESSMENT OF ECOLOGICAL STATUS
According to the Water Framework Directive, water bodies should reach good ecological status the latest by 2027. Specific thresholds have been set to evaluate the ecological status of rivers or other types of water bodies:
- high ecological status (class 1): the parameters correspond totally or nearly undisturbed (reference) conditions;
- good ecological status (class 2): slight changes in the parameters compared to the undisturbed conditions;
- moderate status (class 3): the parameters differ moderately compared to the undisturbed conditions.
Waters achieving a status below moderate should be classified as poor (class 4) or bad (class 5).
SIMPLE METHODS FOR DEFINING WATER QUALITY
Amongst many different biological indices, the Saprobic System was developed in the 1970s to evaluate biological water quality (Sladecek 1973). 5 or 7 quality classes, from clean to heavily polluted, are based on benthic bioindicators, i.e. species dependent on (susceptible or tolerant to) oxygen content and concentration of organic carbon. All biota, i.e. aquatic plants and animals are involved, e.g. bacteria, algae, macrophytes, zooplankton, zoobenthos, and fish, but zoobenthos (macroinvertebrates) and algae are most frequently used.
Schmid and colleagues have elaborated a water quality map for the Danube River and tributaries for the first time in 2002 (presented in Bloesch 2009). Nowadays, the Danube water quality is surveyed by the water management authorities of the Danube countries, under the coordination of ICPDR, and published in the Danube River Management Plans (www.icpdr.org).
There are several simple methods of defining the quality of water that can be used in the classroom or in the field with pupils. Here we present some of them.
Simplified biological method based on invertebrates
Loading streams and rivers with nutrients or organic substances leads to changes in the structure of communities of invertebrates that live at the river`s bottom (macrozoobenthos), as every group has different preferences for environmental conditions and rates of resistance against pollution. Therefore, the presence, absence, or relative number of macrozoobenthos can be an excellent indicator of pollution. Furthermore, macrozoobenthos can be sampled in a semi-quantitative way relatively easily (best seasons are spring and fall), and species identification to the genus level is feasible for schools with popular field excursion guides.
Macrozoobenthos are tiny animals that live at the bottom of watercourses. We can catch them with a net (small grid of 1 to 0.5 mm), and they are visible with the naked eye (>1 mm). Large invertebrates are a group that is used in many types of water quality assessments using biological procedures. The analysis of a particular living community is based on a characteristic of living organisms that are indicative of different categories of pollution in a watercourse. Therefore, presence, absence, or a relative number of invertebrates in rivers determine biotic index, that can tell us a lot about the quality of water in a watercourse.
The smell (odour) of water can be an important indicator of pollution. Volatile substances affect the smell of water. This depends upon the temperature and chemical composition of water. The solubility of water-dissolved gases decreases at higher temperatures. Because of this, water with higher temperatures has a stronger smell than water with a lower temperature at equal chemical composition. We can define the strength and type of smell. Considering the type of smell, we can define the smell of decay, soil, manure, rottenness, faeces, fish. We can determine the strength of smell using Ball’s chart of the intensity of smell (see Activity Smell of (Cold) Water; Kompare, 2005).
Undissolved particles that can be removed with different methods (settling down, filtering, etc.) cause blurriness. Remaining undissolved particles, that are not removable in such a manner cause opacity of water. To remove particles smaller than 0.45 micrometers, filtering through membranes has to be used (membrane filters). The clarity of the water is greater when fewer of these substances are present in the water. Opacity can also be determined using an instrument called a “turbidimeter” or the Secchi-Disc used in limnology for lakes and rivers (Schwoerbel 1994).
- Bloesch, J. (2009): The International Association for Danube Research (IAD) – portrait of a transboundary scientific NGO. Environ. Sci. Pollut. Res.: 116-122.
- Dudgeon, D. (2014): Threats to Freshwater Biodiversity in a Changing World. In: Global Environmental Change, pp 243-253
- ICPDR (2015): Joint Danube Survey 3. A comprehensive Analysis of Danube Water Quality. 369 pp.
- Schwoerbel, J. (1994): Methoden der Hydrobiologie. Süsswasserbiologie. 4. neubearbeitete Auflage, UTB 979, Gustav Fischer, Stuttgart, 368 pp.
- Sladecek, V. (1973): System of Water Quality from the Biological Point of View. Archiv für Hydrobiologie, Beiheft 7, E. Schweizerbart’sche Verlagsbuchhandlung (Nägele u. Obermiller), Stuttgart, 218 pp.
- Sommerwerk, N., Bloesch, J., Paunovic, M., Baumgartner, C., Venohr, M., Schneider-Jacoby, M., Hein, T., Tockner, K.: Managing the world’s most international river: the Danube River Basin. Marine and Freshwater Research, 2010.
- Teodoru, C., Wehrli, B. (2005): Retention of sediments and nutrients in the Iron Gate I Reservoir on the Danube River. Biogeochemistry (2005) 76: 539–565
- UNEP [United Nations Environmental Program] (2002): Global Environmental Outlook 3. State of the Environment and Policy Retrospective 1972-2002. Nairobi, Kenya, 466 pp.
- Kompare, B. (2005) in Vahtar M., Zdešar M., Kompare B., Urbanc-Berčič O.: Kako se reka očisti? – Priročnik za učitelje 3. Book collection Vodni detektiv. ICRO Domžale, Domžale, Slovenia, p.32.
- WWF (2018): Living Planet Report – 2018: Aiming Higher. WWF, Gland, Switzerland, 75 pp.