pollution and eutrophicationReading time:
In its most widely accepted meaning, the term pollution refers to any deleterious man-made change to the natural environment.
More specifically, water is polluted by various types of discharge that can be momentary or diffuse, that injects calories (heat pollution), mineral or organic substances (chemical pollution) or pathogenic micro-organisms (microbial pollution) into the environment.
More specifically, in the case of chemical pollution, this can take the form of:
- either substances that are already present in the natural environment; pollution is then estimated on the basis of an earlier reference level, typically, this is the case of nutrients that accelerate the natural eutrophication phenomenon (see pollution and eutrophication);
- or substances that do not exist in Nature (e.g. some heavy metals and radioactive isotopes; organic micropollutants, see organic impurities).
In many countries, these phenomena are monitored through networks that use sensors and physical and physical-chemical parameter analysers, together with flora and fauna sampling designed to detect the presence of species that are used as indicators of either the satisfactory condition of the medium or, conversely, its deterioration (bio-indicators).
Within the European Union, member States are governed by an outline directive (of the 23rd October 2000), on evaluating, re-instating (by 2015 if possible) and maintaining the ecological quality of water. In France, preparation of water quality evaluation systems (SEQ) has already been in hand since 1992. Those in charge permanently monitor the ecological condition of all continental aquatic environments (waterways, water sheets, groundwater, coasts) on the basis of biological, chemical and physical compounds (a continuous measurement network has been set up on a national scale for this purpose).
Three types of evaluation are undertaken:
- SEQ- Physical for hydrological aspect evaluation (water course);
- SEQ-Water for the physical-chemical aspect;
- SEQ-Biological for the biological aspect.
Water policing on a European scale will henceforth be carried out on the basis of such results.
Initially, this term described a natural lake change phenomenon that can be summarised as follows:
- a lake that is young and deep is termed oligotrophic: this type of lake has water that is blue and transparent; dissolved oxygen is present through to the bed of the lake; its biomass is not very abundant;
- as this lake "ages", it becomes enriched with organic matter as the result of its primary photosynthesis production (algae) and of outside natural contributions; the lake then becomes mesotrophic and then eutrophic (from an etymological viewpoint, "well fed"); the following phenomena occur: the lake’s depth gradually diminishes as it becomes filled in, water becomes coloured (green to brown), water transparency reduces, the biomass increases, planktonic algae proliferate in the upper layers (epilimnion), an oxygen deficiency occurs in the deep layers (hypolimnion) that, however, are enriched by elements in a reduced and dissolved state (iron, manganese, H2S, NH4). The final term is the pond, the marsh, the peat bog…
It takes a very long time to transit from one type to the next: this time can be measured in thousands of years. However, this natural process has accelerated and, in many cases, has become perceptible in a human lifetime. This situation has been caused by bankside activities that introduce organic matter and fertilising compounds into stagnant water: nitrogen and especially phosphorus (agriculture, household or industrial products). This situation sometimes exists upon the creation of the water expanse (e.g. reservoirs filled with water without first removing the pre-existing terrestrial plant life). The appearance followed by the predominance of Cyanobacteriain the photosynthetic population constitutes one of the most customary indicators of a trend to eutrophication.
This artificial eutrophication (likened to pollution as seen earlier) can have disastrous consequences for tourism and fishing; also, this phenomenon will significantly increase the cost of potabilisation treatment due to the equipment and reagents required to eliminate not only the organisms themselves or the product of their metabolism but also the reduced chemical compounds present in hypolimnion water.
Water sheet eutrophication can be fought:
- using remedial measures: oxygenation, destratification, chemical (e.g. using a copper salt as an algicide) or biological fight;
- through preventive measures such as rerouting effluent by using a ring sewer (e.g. Annecy lake) or a bypass;
- through thorough treatment in water treatment plants (reducing suspended solids, removing nitrogen and, above all, phosphorus) that discharge into these expanses of water.
Because it accumulates all that is discharged into it, stagnant water also becomes vulnerable to other forms of pollution. Therefore, it requires special protection.
Although the full definition of eutrophication does not apply to waterways, many waterways suffer from the conditions described above and especially from the excessive development of plankton algae (e.g. in France, the river Loire) occasionally followed by invasive Macrophytes. In the case of major rivers, the problem is examined by subdividing the waterway into sub-basins to which we apply forecasting models that take hydrology, weather conditions, momentary and diffused inputs into consideration. As in the case of reservoirs and lakes, phosphate removal constitutes one of the priority resources for fighting eutrophication.
As far as the river pollution phenomenon is concerned, this can be identified by a great many bio-indicators; in France, after using the saprobic system for many years and then biotic indices, we now use harmonised indicators:
- the standardised global biological index or SGBI (NF T 90-350, December 1992). As in the case of the biotic indices referred to above and from which the SGBI has been derived and completed by an expert system, this index relies on an analysis of the demersal macrofauna (worms, molluscs, crustaceans, insect larvae…); however, it is not very suitable for major rivers. An adapted global biological index (or AGBI ) has been developed for major rivers, this index includes the geographic dispersion of species;
- diatomite biological index or DBI (NF T 90-354, June 2000): this index is an analysis of the demersal diatomite flora (samples taken from the entire submerged support) that is sensitive to chemical changes affecting the medium (organic matter, N and P, mineralisation, pH...).
These indices are expressed by a score of between 0 and 20 and are used to divide water into five grade classifications.
Together with other indices (fish, macrophytes, oligochaeta, birds…), these methods are used to evaluate the biological qualities of a waterway and their changes in time and/or space; they are also used to assess the disturbance caused by pollution through upstream and downstream comparisons of a discharge (disappearance of sensitive species, appearance of tolerant species, reduction in the number of species, increase in the number of individuals within each species), the effectiveness of effluent treatment, a trend towards eutrophication... The SEQ-Bio system referred to earlier uses such bio-indicators.
water table pollution
Contamination that mainly affects relatively shallow water tables (phreatic, alluvial, karstic …). Thanks to the universal presence of underground aquatic fauna (crustaceans, protozoa …) that can go down to several hundreds of metres, a global index specific to this environment is currently being examined.