How human waste massively pollutes rivers

We often hear about rivers polluted by nitrates from agriculture. But another form of pollution also threatens our rivers: effluents from human excreta, rich in nitrogenous matter and agents pathogens (bacteria faeces). These effluents lead to eutrophication, deplete the water in oxygen and present a significant risk to health. And this pollution is increasingly massive, if we are to believe a new study from the University of California published in the journal Plos One. The researchers analyzed the rate ofnitrogen and pathogens in 135,000 river mouths around the world and found that human effluents discharge 6.2 million tonnes of nitrogen per year on the coasts, or 40% of the amount released by agriculture.

This pollution is however very little studied, regret the authors. “ Unlike the leaks ofhydrocarbons or to plastic, human effluents are invisible, Joleah Lamb rating, a researcher at the University of California. Saw some gorgeous beaches that looked perfectly clean, but when we tested the water quality it contained a large amount of pathogens “.

Because unfortunately, even in countries that have a water treatment system, this pollution goes largely through the cracks. Wastewater treatment plants effectively filter pathogens, but relatively few materials nitrogenous. In addition, they are expensive to build and operate, and therefore rather rare in developing countries. Systems septic tank, which equip around 25% of households in France in rural areas, are less expensive and filter most of the nitrogen, but on the other hand allow faecal bacteria to pass through. Not to mention that in many cases dirty water is dumped directly into the sea. 63% of nitrogen pollution in waste Thus, 5% of the septic tanks and 32% of the effluents directly discharged into the water come from wastewater treatment plants.

Half of pollution comes from just 25 sources

For their estimation, the researchers crossed demographic data, the consumption of protein in each community (responsible for nitrogen discharges), and the presence or absence of filtration. They were thus able to assess the quantities of effluents discharged into 134,846 water mouths flowing into the ocean. A titanic and original work. It shows that the pollution is massively concentrated on a few sources: half of the pollution with nitrogen and faecal bacteria comes from only 25 sources (which are not the same for each). For nitrogen, the Yangtze in China alone accounts for 11% of discharges, followed by the Nile, the Mississippi in the United States, the Paraná in Argentina and the Danube in Europe. Faecal bacteria are mainly found in densely populated areas, such as the Ganges and Brahmaputra deltas in Asia, or the Congo in Africa.

Additional pressure on coastal ecosystems

The researchers also estimated the consequences on coastal ecosystems, and found that 58% of coral reefs and 88% of seagrass beds are affected by human effluents. ” This pollution is added to other anthropogenic pressures such as overfishing, habitat degradation, coastal artificialisation or global warming “, Warn the authors. The study did not take into account pollution caused by phosphate or other chemical agents such as metals heavy, which can also present a danger to the wildlife and aquatic flora as well as human health. In total, all these factors combine to contribute to the degradation of water quality. In France, however, we note a reduction in pollutant discharges in water from industry and urban wastewater treatment plants. ” Releases of organic matter, nitrogen and phosphorus have decreased significantly, thanks in particular to the improvement of treatment systems », Welcomes the Eau France agency. An effort to be continued everywhere on the planet.