Research aims to illuminate the natural pollution-fighting qualities of river ecosystems

By John Butcher

A Duke Kunshan professor is undertaking research into river ecosystems aimed at shedding light on ways to reduce environmental damage caused by pollution.

Rivers have a natural ability to improve water quality, which could be harnessed, says Chuanhui Gu, associate professor of environmental science, who has begun a four-year study looking at the impact of microbes found in river sediment on nitrogen pollution levels.

Chuanhui Gu, associate professor of environmental science at Duke Kunshan, during fieldwork

“This study aims to understand the natural potential of rivers themselves to remove nitrogen from the water, and boost that potential, to help mitigate the problem,” said Gu.

“It is of great importance, because it provides evidence of the extent to which tidal rivers can help prevent coastal eutrophication — the excessive growth of algae and aquatic plants fueled by too much nitrogen.”

The use of synthetic fertilizers in agriculture causes nitrogen runoff from the land into rivers, which is carried to bays and estuaries where it can cause toxic algal blooms dubbed “red tides”. These blooms use up oxygen in the water and block sunlight from underwater fish and plants. Once the algae dies, it floats to the surface as a toxic rotting green or red scum which further depletes water oxygen levels.

The process, called eutrophication, is now a common sight, according to Gu, creating “dead zones”, where marine life is suffocated and dies, that spread out from river mouths affecting fishing and shrimp industries.

Gu’s research will look at how rivers can prevent this kind of environmental damage through a natural process of denitrification.

Toxic algal blooms dubbed red tides can cause severe environmental damage

“A healthy river with lots of native plants on the riverbank and an unlined natural channel has great potential to absorb the nitrogen rather than sending it downriver,” said Gu.

This is because they have a fine layer of sediment beneath the river and to the sides which are home to myriad microbes that act to denitrify the water. Water flows through this sediment layer, driven by changes in the river such as ocean tides. Starved of oxygen, the microbes that populate the sediment instead use nitrogen for respiration, in the process converting nitrate to dinitrogen gas.

Gu aims through his research to better understand this process, including factors that lead to higher or lower levels of denitrification.

In doing so it will provide “evidence of the extent to which tidal rivers can help prevent coastal eutrophication,” he said.

The research, which is at its beginning stages, will focus on the Yellow River Delta area, with team members measuring the rate of water exchange between river and sediment, using piezometers (small wells), at more than 20 locations. In the process they will gauge water quality, using real-time sensors and data loggers, taking regular water and sediment samples to quantify denitrification potential at different times and locations.

“The collected data, spanning over at least two years, will determine whether the locations chosen are hot or cold spots for groundwater-surface water interaction and nitrate retention as well as how the conditions change over time,” said Gu.

Using the data, Gu intends to develop a computer model that can predict where denitrification hot and cold spots will occur, from which he can produce a map.

Chuanhui Gu and a member of his research team at one of more than 20 test locations

“The study findings will guide best river management and restoration practices for mitigating eutrophication,” he said.

This could include advice to avoid some activities such as sealing riverbeds, which could prevent denitrification and encourage others, such as tree planting by riverbanks and allowing natural flooding onto floodplains, which help to promote it. Carbon from leaves and wood falling into rivers stimulates microbes to denitrify faster, while floodplains provide a broader area for the process to take place.

There is a long way to go before the study is complete, but the next step is already in sight. Gu hopes to expand the research to look at the impact of groundwater-surface water interaction on other pollutants, including phosphorous, metals and microplastics.

“These pollutants threaten our coasts just like nitrogen does,” he said. “Unravelling the transport mechanism behind these pollutants from tidal rivers to estuary is of great significance for protecting the coastal environment.”

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