Au Québec, il existe près de 30 000 exploitations agricoles. Qui plus est, au-delà de 40 000 personnes travaillent dans ce secteur. C’est donc dire que l’agriculture occupe une place prépondérante dans notre économie. Malheureusement, elle occupe aussi une place importante dans le palmarès des sources de contamination de nos cours d’eau. Mais comment l’agriculture a-t-elle un effet dommageable sur les lacs et rivières, exactement?
Water samples
Many agricultural lands in Quebec have direct access to watercourses. While consulting the regional profile of the biofood industry in Quebec, we notice that several are close to the St. Lawrence River or Lac-Saint-Jean, for example. This phenomenon can be explained by historical reasons and by the fact that agriculture requires large quantities of water.
In fact, agriculture requires more water than any industry: globally, water for agricultural irrigation purposes monopolizes 70 % à 80 % of all the fresh water consumed on the planet. The animal products sectors are the most voracious. Quebec is also fertile ground for the pork industry: it provides 6 % of all pork consumed in the world. In 2019, this represented a production of 7.1 million animals! Watering the cereals used to feed the animal, hydration, cleaning of facilities: when we consider all of Quebec's animal production, we understand why water consumption is increasing so quickly.
Where does the water taken come from?
This water, just like drinking water, is drawn from groundwater or simply surface water (in lakes and rivers). It even happens that water is drawn from certain wetlands.
Some farmers manage to make do with rainwater collected in a well to irrigate their land; their actions are therefore not taken into consideration in the calculation of the levy. On the other hand, with climate change and periods of drought and flooding, these farmers risk turning to sources other than rainfed and inflating the current figures.
Consequences of water withdrawals
When a significant quantity of water is withdrawn, this risks creating low water (the lowest water level) and modifying the natural flow of the watercourse. In the event of a weakened flow, the contaminants are therefore less well diluted. During low water periods, nautical and swimming activities are affected.
The most worrying thing is that drinking water and agricultural water are, in a way, in competition : we will have to make a choice considering that we plan to reach a population of 9.6 billion humans in 2050, that global food demand will double, that climate change will alter the quantity of available water by generating droughts and floods, and that certain regions already have drinking water shortage issues.
Contaminants: phosphorus, nitrogen and fecal coliforms
Agricultural production has several other impacts on waterways. Among other things, it causes a high level of phosphorus and nitrogen in the water as well as a higher presence of fecal coliforms.
According to many experts, the presence of phosphorus in water is not alarming in itself. It is even essential to the productivity of freshwater ecosystems. On the other hand, due to human activities such as agriculture, there is far too much phosphorus in our waterways. Indeed, according to researchers in a study from the University of Montreal and McGill University studying data between 1985 and 2011, 19 of 23 Watershed étudiés ont franchi le seuil des 2,1 tonnes par kilomètre carré. Selon leurs estimations, cela prendrait des centaines (peut-être même des milliers) d’années pour revenir sous un seuil acceptable.
The nitrogen contamination level is less important than for phosphorus, but it should still not be neglected. In 1981, « 80 % des terres agricoles au Canada se retrouvaient dans la catégorie de risque “très faible” pour le risque de contamination de l’eau par l’azote ». En 2016 ce sont seulement 49 % qui étaient à « très faible » risque. Sans surprise, les régions ayant subi une densification plus intense depuis les dernières années sont celles où le risque est le plus grand. La carte suivante démontre quelques régions des plus à risque par une couleur plus foncée.
Consequences of contaminants
Phosphorus and nitrogen are in fact found in several fertilizers, which sometimes also include potassium, and in manure. During spreading periods, farmers cover their land with a layer of fertilizer and manure. Phosphorus and nitrogen then become fixed in the soil, sometimes saturating it. When there is rain, or any other water discharge, the precipitation runs off and flows, carrying contaminants with it until they reach surface and groundwater.
Similarly, during erosion, saturated soils contaminate banks and water. Then, present in too large quantities in water, nitrogen can be harmful to human health while phosphorus rather causeseutrophication. Remember that agricultural land is often located near watercourses; the streaming drops therefore have a higher concentration than if the drops took a longer trip.
Moreover, in addition to generating phosphorus, spreading practices also contaminate water with fecal coliforms. Indeed, fecal coliforms are bacteria contained in excrement. By the same principles of runoff and erosion, manure contaminates waterways. Thus, it becomes impossible to swim or navigate in water with too high levels of fecal coliforms.
Cumulative effects
Avec ces formes de contamination, on remarque souvent un effet cumulatif. Puisque les fermes sont souvent assez proches les unes des autres, les contaminants émis par chacune d’elles sont additionnés aux autres. Autrement dit, en ayant plusieurs fermes plutôt qu’une ferme, on augmente le volume total de phosphore émis dans un rayon de 10 km. D’ailleurs, il n’y a actuellement aucune obligation de caractériser l’emplacement de toutes les fermes du Québec. Les municipalités régionales de comté (MRC), elles, vont le faire dans leur plan de développement de la zone agricole. Ainsi, les permis octroyés ne prennent pas en considération l’activité du voisinage.
Cet effet de proximité a aussi un effet sur les prélèvements, car s’il n’existe qu’une seule ferme dans un rayon de 10 km, ses prélèvements ne risquent pas d’affecter les cours d’eau. Cependant, une région ayant une centaine de fermes dans un rayon de 10 km peut avoir de sérieuses répercussions. Contrairement aux contaminants, les prélèvements d’eau sont déclarés au Ministère de l’Environnement et de la Lutte contre les changements climatiques (MELCC) et sont donc comptabilisés.
In addition, between them, the different contaminants can have a cumulative effect. Indeed, the presence of fecal coliforms will, for example, deprive water of oxygen just like phosphate. Thus, when several different contaminants are found, eutrophication can be even more rapid.
Let us understand clearly: agriculture is essential to meeting the needs of society, but in the interests of environmental protection and social justice, its practices deserve to be reviewed and better supervised. Reducing exports, diversifying our industries for greater autonomy, limiting the quantity of large farms and prioritizing human-scale farms that promote local agriculture are avenues to explore.
Main photo: Hindrik Sijens
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