Climate change is putting pressure on water quality in rivers worldwide. That is the conclusion of a study in Nature Reviews Earth & Environment. “Weather extremes in particular are decisive,” says Michelle van Vliet, associate professor of water quality at Utrecht University and lead author of the publication. “Climate change will make such weather extremes more common in the future.”
Van Vliet, together with a group of international scientists, examined 965 cases to analyze how water quality in rivers worldwide changes during various weather extremes, such as droughts, heat waves, heavy precipitation and floods. “As far as water quality is concerned, we looked at, among other things, the water temperature and the oxygen and salinity,” says Van Vliet. “But also to concentrations of metals, medicines, plastics and nutrients.”
Precipitation and evaporation
River water quality is determined by so-called hydroclimatic factors: the amount of precipitation, evaporation and water discharge. Geographic factors such as soil structure and land use also play a role. Finally, the final water use also has an influence. Van Vliet: “To make it even more difficult, these processes are also influenced by each other.”
During hydrological droughts – very low water discharge in rivers – and heat waves, water quality decreased in 68 percent of the rivers studied, the study concludes. For example, the quality decreases sharply in rivers where pollution flows into the water from point sources, such as in industrial areas. “This is because there is simply less water to dilute medicines, metals and other chemicals, for example,” says Van Vliet. “Then those concentrations quickly become much too high.”
Lower river discharge also results in salinization – a higher salinity – because relatively more water evaporates. In addition, in deltas, such as in the Netherlands, seawater can penetrate further into the delta due to a lower water level. “Such salinization, for example, hinders the irrigation of agricultural areas because many crops cannot tolerate high salt levels.”
Also read this report: Salt water enters fresh rivers. How can that be prevented?
Van Vliet and her team also saw extreme algal blooms during hydrological droughts and heat waves in many studies, mainly due to higher water temperatures. Due to the growing amount of algae, the oxygen concentration in the river decreases, to the detriment of other life. For example, during a hydrological drought between 2018 and 2019, around 5 million fish died in Australia’s Darling River due to a lack of oxygen.
Algal blooms do not often occur in flowing water, but the construction of dams and large reservoirs is changing this, the study states. This not only affects local ecosystems, but can also pose risks to people. For example, certain types of algae, so-called cyanobacteria, produce toxic substances that can end up in drinking water.
Agricultural pesticides
Not only hydrological droughts and heat waves affect water quality in rivers. Due to heavy rainfall and flooding, the water quality in 51 percent of the rivers studied decreased substantially, Van Vliet concludes. Heavy rainfall results in greater runoff of contaminants from the land to the river, for example in the form of pesticides from agricultural areas or high concentrations of heavy metals from the soil.
With regard to weather extremes, Van Vliet and her team mainly looked at historical water quality measurement data. “But to determine how water quality will change in the future, we also looked at future climate change using computer models,” says Van Vliet. “In addition to the increasingly common weather extremes, there are also gradual changes in water quality.” For example, the water temperature rises in the majority of rivers, resulting in lower oxygen concentrations. In addition, the amount of water in rivers will also decrease worldwide in the future, which makes the dilution of waste increasingly difficult.
“It is clear that the impact of climate change on water quality in rivers is substantial,” she says. “And many waters in the Netherlands already do not meet European guidelines, so climate change adds even more to that.”