Cn recent decades, the requirements for the quality of drinking water have become much more stringent. In France, for example, controls are striving to find an increasing number of molecules in watercourses, obviously giving priority to the substances that are the most dangerous for human health and the environment.
However, what is released into the watershed can be found in rivers, and humans have significantly disrupted the cycling of elements naturally present in rivers (such as nitrates, phosphates or organic matter). Elements from the chemical industry also need to be taken into account - potentially more than 100,000 substances from human activities. Most of them are pesticides, but also hydrocarbons, plasticizers, drugs and many other compounds with potential endocrine disrupting effects, which may be neurotoxic, carcinogenic, or affect fertility and the genome.
These residues generally take the form of micropollutants, i.e. toxic substances in very low doses (of the order of one microgram per litre, the equivalent of two pinches of salt in an Olympic swimming pool). Unfortunately, they are not all sought after and their long-term effects in the presence of a cocktail of other molecules are little, if at all, known.
Pesticides on top
In French rivers, pesticides and their degradation products are the first substances detected. Not surprising in a country where the utilised agricultural area represents 29 million hectares, or more than half of the territory.
Their concentration varies according to the period when the plant protection products are applied (generally in spring). It also depends on the crops present in the catchment area: vines, orchards or cereals, for example, require numerous treatments. Herbicides are mainly detected in rivers because their properties make them more sensitive to runoff. Thus, glyphosate and its degradation product AMPA are the molecules most often detected, as they are widely used before cultivation.
But this presence also depends on the persistence of pesticides: the presence of atrazine (a herbicide that has been banned since 2003) and its degradation product, deethylatrazine, can be explained by a rather slow transfer to watercourses. Although they spread more rapidly during rainy periods, a large proportion of pesticides migrate diffusely to groundwater, only to be returned to surface water several years later.
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In 2014, 389 pesticide residues (as well as their degradation products) were found at least once in French rivers, when 484 different pesticides were used the same year by farmers. These figures show the potential for these substances to be exported to watercourses.
When a pesticide becomes too prevalent in the natural environment, it is removed from the market, leaving farmers to switch to other products. With the progressive ban on pesticides, this choice is being restricted to an increasingly limited number of products without addressing the underlying problem of pollution. For example, for weeding corn, the use of S-Metolachlor more than doubled between 2013 and 2017 in Seine-et-Marne. Since applications are mainly made in spring, average monthly concentrations have risen from 0.010 to 2 µg/L from April to June.
This seasonal dynamic is generally observed in all French rivers.
Poorly filtered urban waste
In addition to pesticides, another major source of contamination is urban discharges from industry and individuals.
After draining cleaning water (detergents, plasticizers, solvents), during showers (musks, parabens, insecticide repellents) or even by excretion via toilet flushing (medicines, hormones, caffeine), the products of our daily lives finally reach the waterways. Despite the increasing efficiency of wastewater treatment plants (whose main role is to remove organic matter), a large number of compounds are still found in rivers.
To this wastewater are added the discharges of rainwater. As it falls, it carries with it airborne particles and pollutants from the air. In urban areas, air contaminants come from industrial discharges, road traffic, district heating (especially in winter) and the progressive diffusion of material components. The atmosphere and then rainwater gradually become laden with pollutants: hydrocarbons, furans, dioxins, etc.
Finally, runoff on roofs and impermeable pavements carries hydrocarbons and heavy metals (cadmium, lead, zinc). Some accidents can also produce exceptional pollution: the fire at Notre Dame de Paris, for example, released large quantities of lead from the roof, and it is likely that this disaster will soon be detectable in the Seine.
Contamination difficult to measure
As part of a research program entitled PIREN Seinewe study these compounds of agricultural and urban origin, as well as atmospheric deposition related to the water cycle as a function of climatic hazards (dry years vs. wet years) and the hydrological regime (annual variations in flow between floods and droughts). lowlands). This enables us to understand the mechanisms of contaminant transfer and their dynamics from the upstream of the Seine or Marne rivers, which are very agricultural, to the downstream of Paris, which receives urban pollution.
Some of these compounds bind to the particles suspended in the water and sediment at the bottom of the Seine before being remobilized during the movements caused by the floods. Between diffuse pollution and the re-suspension of sediments, contamination can thus be observed several years after the use of a substance.
It is up to the water agencies to put in place strategies to achieve good ecological status in rivers. In recent years, in the face of the increasing number of new substances generated by the chemical industry, the number of micropollutants they are looking for has grown steadily. In total, nearly 900 parameters are monitored, including chemical substances and their degradation products. This monitoring represents a considerable cost and yet cannot be exhaustive, especially since a large number of these substances are not detected. Other approaches are therefore currently being studied.
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Aquatic organisms are the first warning factors for stream contamination. Even if it is generally not visible, contamination has an impact on living beings present in the water by selecting species resistant to pollution (modification of biodiversity) or by generating malformations in individuals. If it is not possible to search for all the substances that may be present, the search for biomarkers (indicators of the effects of pollution on certain organisms such as fish, freshwater mussels or gammares) would make it possible to measure the overall impact on organizations.
The environmental quality standards s set by Europe allow a more consistent estimate of contamination with regard to potential impacts on aquatic organisms. However, it is still not possible to identify the potential impacts of the many substances that are not yet investigated (because their existence or harmfulness is unknown), or that cannot be detected using current analytical techniques.
Hélène BlanchoudLecturer in Environmental Chemistry, EPHE, Sorbonne University
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