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Sewage pollution is a greater threat to rivers than agriculture

A new study led by the University of Oxford is shedding light on a concerning issue that many may have overlooked. The researchers have determined that sewage pollution is more detrimental to rivers than agriculture.

The study was launched ahead of World Rivers Day, which is celebrated on September 24th. 

The researchers found that even when sewage undergoes treatment, its discharge into rivers affects water quality far more than runoff from surrounding land use.

Focus of the study 

The experts took into consideration three pollution sources: treated sewage discharge, agriculture, and urban run-off. 

The team tested four rivers in England both up- and downstream of sewage discharge points across three different months.

While water companies in the UK have the green light to release treated wastewater into rivers, untreated wastewater can also be released during heavy rainfalls, a phenomenon termed as storm overflow. 

This not only leads to significant ecological ramifications but can also jeopardize human health if that water is utilized for drinking, recreational, or agricultural activities.

Key findings from the study

Sewage is a primary pollutant

Sewage, whether treated or untreated, is responsible for the spike in nutrients, algae, and sewage fungus in rivers. 

Moreover, it severely alters the ecosystem by changing the composition of plant, animal, and microbial communities in the water.

Shift in river communities

Downstream of sewage input, rivers exhibited a noticeable shift in macroinvertebrate and algae communities. 

The surge in resilient groups like cyanobacteria and worms is especially alarming. Cyanobacteria are infamous for releasing toxic chemicals that can be lethal to many aquatic species.

Agriculture’s role

Though sewage discharge is a dominant threat, agricultural run-off also contributes to the degradation of water quality, especially harming specific sensitive insect groups such as mayflies, stoneflies, and caddisflies.

Addressing the issue

“Our study highlights the disproportionate impact that sewage discharge has on river quality, presenting an urgent need for a comprehensive action plan targeting the sewage discharge problem,” said study lead author Dr. Dania Albini.

“Improvements to waste water plants should be implemented along with more regulations. These efforts are crucial in safeguarding the integrity and safety of our rivers – fundamental elements of both ecosystems and human wellbeing.”

Ecological state of UK waterways

The revelations from the study are timely, especially considering the deteriorating state of the UK’s waterways. 

With over 90% of freshwater habitats in England’s vital rivers being affected by various pollutants, a holistic strategy addressing pollution is imperative for the well-being of both the environment and its inhabitants.

“There is ongoing debate about the cause of the poor ecological state of many rivers in the UK because it is difficult to disentangle different pollution sources,” said study senior author Dr. Michelle Jackson.

“Here, we show that even treated sewage appears to have a stronger influence on river communities than pollution from the surrounding land. This important information should be used to prioritize the management and conservation of our rivers moving forward.”

Study implications 

James Wallace, CEO of the UK-based charity River Action, said the research demonstrates yet again the damage from unregulated water companies and agriculture. 

“In addition to the catastrophic impact on wildlife from nutrient pollution, the public should be aware that sewage systems do not remove dangerous bacteria such as E.coli and intestinal enterococci from treated sewage,” said Wallace.

“For instance, recent citizen science on the River Thames found that Thames Water’s outflows often have four to five times the safe levels of bacteria, that has likely caused serious illness in swimmers and rowers.” 

“When will the government make water companies and farms clean up their act, especially in places where human lives and sensitive protected habitats are threatened?”

Early detection system

The researchers have also developed a new method to detect potentially dangerous “sewage fungus” outbreaks.

This mix of fungus, algae, and bacteria causes unpleasant smells and severely reduces oxygen levels, which can cause mass fish mortality.

“Rapid identification of sewage fungus pollution events will allow early intervention which would help prevent any potential negative consequences for local wildlife,” said Dr. Jackson.

The research is published in the journal Global Change Biology.

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