A high-resolution mapping analysis, the first of its kind, estimates the amounts of nitrogen and pathogens released to coastal ecosystems from human sewage sources worldwide. Cascade Tuholske (now affiliated with the Columbia Climate School) and colleagues at the University of California, Santa Barbara present this research in the open access journal PLOS ONE November 10, 2021.
Human sewage can introduce pathogenic pathogens and nitrogen into the ocean, potentially impacting human health as well as coastal ecosystems and communities that depend on them for purposes such as fishing. However, most research on human impact on coastal ecosystems has focused on agricultural runoff, while human wastewater investigations have been limited.
To better understand the impact of sewage on coastal ecosystems, Tuholske and his colleagues conducted a new analysis in which they estimated and mapped nitrogen and pathogen inputs to the ocean from sewage for approximately 135,000 watersheds around the world at 1 kilometer resolution. The assessment used newly available high-resolution data on global human populations and modeled how sewage plumes entering the ocean would overlap with different ecosystems.
The analysis suggests that sewage from human sewage introduces 6.2 teragrams of nitrogen into coastal ecosystems per year – for comparison, this is about 40% of estimated inputs from agriculture. Sixty-three percent of nitrogen comes from sewage systems, 5 percent from septic systems, and 32 percent from untreated direct inputs.
Of the watersheds that appear to release the most nitrogen from wastewater, most are located in India, Korea and China, with the Chang Jiang (Yangtze) River contributing 11% of the global total. Researchers have also identified hotspots for coral reef nitrogen exposure in China, Kenya, Haiti, India and Yemen. Hotspots of seagrass exposure have been found in Ghana, Kuwait, India, Nigeria and China. The Chang Jiang and Brahmaputra rivers contain the most pathogens.
Further research will be needed to refine the model and its estimates. Nonetheless, this work provides a new resource that could play a key role in efforts to mitigate damage to ecosystems and human health, for example by highlighting where trade-offs between managing nitrogen levels and pathogens are particularly important to consider.
The authors add: “The scale of the amount of sewage impacting coastal ecosystems around the world is staggering. But because we map wastewater inputs to the ocean through more than 130,000 watersheds, our results identify priority target areas to help marine conservation groups and public health officials. working together and reducing the impacts of wastewater on coastal waters across the planet.”
To note:
The researchers created a visual representation of their research, available at: http://www.globalwastewatermodel.com/webmap.html
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