Rivers are a surprisingly large source of greenhouse gases, and water pollution has repeatedly exacerbated their emissions.
At first glance, one thought the new territories were one of Hong Kong’s greenest places, an area adjacent to mainland China and occupying most of Hong Kong territory, apparently away from busy streets and dense clusters of skyscrapers rising above much of downtown. In contrast, new lands are more rural, with a wide range of farmland, rolling green spaces, wetlands, mountains, parks and rivers.
At first glance, the new world may sound like Hong Kong’s green lung, but the reality is quite uncomfortable. Rivers that pass through this lush landscape release large amounts of greenhouse gases, according to a study of the region’s 15 water canals.
“Chinese rivers are saturated with three major greenhouse gases, namely carbon dioxide, methane and nitrous oxide,” says Derrick Yuk Fu Lai, a professor in the Department of Geography and Resource Management at the University of Hong Kong. Mr. Lai found that the concentration of these gases was sometimes 4.5 times that of barley gases.
The study assessed the impact of water pollution on Hong Kong’s greenhouse gas emissions, noting that the region’s rivers are a persistent source of atmospheric greenhouse gases and can contribute to climate change. “We found that all the rivers we study because greenhouse gas emissions,” Lai says, adding that the more polluted the river, the higher its emissions.
Greenhouse gas emissions from livestock farms, intensification of old buildings and UN irrigated enclosures are the main causes of pollution. In fact, the average saturation levels of carbon dioxide, methane and nitrous oxide in highly polluted rivers are 2.2, 1.5 and 4.0 times higher than fewer polluted rivers, respectively.
Mr Lai said while our rivers emit less carbon than fossil fuels in their daily activities, their contribution to Hong Kong’s global greenhouse gas budget cannot ignored and must be minimized to mitigate future climate change.
Rivers around New Hong Kong are not un-common in this area. Surprisingly, rivers are an important source of greenhouse gases among the world.
Rivers and streams estimated to emit up to 3.9 billion tons of carbon per year (nearly four times the annual emissions of the global aviation industry). This number is huge given the relatively small areas occupied by rivers on earth. In addition, aquatic systems such as rivers and lakes estimated to contribute more than 50% to atmospheric methane, and global N2O emissions from rivers exceed 10% of human emissions.
Sophie Comer-Warner, a biochemist and researcher at the University of Birmingham, says the reason “rivers get a lot of carbon and nitrogen inputs from the landscapes they emit,” he says, “In the past, people thought that rivers only transported these elements to the ocean, but now we know they have a high rate of biochemical reaction.
In other words, different forms of carbon and nitrogen received by microorganisms are broken down in other forms, usually through aerobic or aerobic breathing, which releases carbon dioxide and can release methane and N2O.
A recent study evaluating carbon dioxide, methane and N2O emissions from the urban river system in Cuenca, Ecuador, found a clear trend between water quality and greenhouse gas emissions: the more polluted these sites are, the higher their emissions. In fact, the study found that when river water quality deteriorates, its contribution to global warming increases in large order.
“We estimate that when rivers become polluted, their global warming potential (GWP) can double two to 10 times,” says O Langton, a postdoctoral researcher at Belgium’s University and an author of the study.
He and his team found that increased greenhouse gas emissions from rivers also closely related to changes in land use and land cover around rivers. In particular, the average concentration of CO2 and N2O in places close to urban areas is almost four times higher than in natural places, while methane concentrations are 25 times higher than in natural places. “These findings highlight the impact of land use and land cover on emissions at sites contaminated with sewage discharge and surface water,” Hu says.
After entering the body of fresh water, pollutants such as nitrogen compounds and pollution of human activities by microorganisms converted into greenhouse gases. In particular, when dissolved oxygen in rivers reduced due to pollution, anaerobic bacteria mineralize organic matter, producing carbon dioxide and methane, while bacteria convert nitrate denitrication (NO3) into nitrous oxide (N2O).
“These processes have been validated by the application of machine learning in our study, especially as dissolved oxygen, nitrogen concentration and river flow characteristics have been identified as major factors in river emissions,” Hu says. “
Cleaner the water, cleaner the air
Therefore, the truth is that rivers polluted by human activity, especially in urban areas, which ultimately leads to increased greenhouse gas emissions. More than half of the world’s population lives nearly 3 kilometers (1.8 miles) of surface freshwater bodies, including river networks, the study found. This increase in urbanization has brought a large amount of pollutants to rivers, as more than 80 percent of urban sewage still discharged directly into the environment. Most pollutants come from untreated wastewater, agricultural sewage water and increased sewage accumulation. This continues to make urban polluted water bodies important hot spots for greenhouse gas emissions.
Emissions from rivers expected to increase. “Given the increase in urbanization and the strengthening of agriculture and aquaculture, the share of rivers in future climate change is likely to be much higher than current estimates,” Hu said.
However, river restoration, including pollution reduction, expected to help reduce greenhouse gas emissions.
“Our study shows that the concentration of ch4 found in sites can be reduced by 10 times that water quality in polluted rivers improves well quality,” Hu said, while concentrations of carbon dioxide and N2O reduced fourfold.
Therefore, the results show that improving water quality can have a significant impact. As such, a number of plans have launched to restore and restore rivers by reducing pollution, such as the EU Green Deal in the US, the EU Water Framework Directive and the Clean Water Act.