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Analysis of approximately 135,000 watersheds shows that a large number of key pollutants come from human wastewater, not just agricultural runoff

All over the world, sewage flows from pipelines into rivers and oceans, threatening the health of humans and aquatic ecosystems. Cascade Tuholske, a geographer at Columbia University’s Earth Institute, said that although it has long been known that some individual locations are the main source of coastal pollution, “we have never had a global understanding of the severity of the problem.” He and his colleagues studied this problem extensively by counting the amount of fecal pathogens and nitrogen. These pathogens and nitrogen can promote the proliferation of harmful algae and cause hypoxic death zones. These pathogens and nitrogen are distributed with human wastewater in nearly 135,000 locations around the world. Washed into the ocean. They found that they could attribute about half of the nitrogen pollution to 25 locations, and about half of the pathogens also came from 25 sources, in some cases the same source.

Researchers say that these findings, published this week on PLOS One, can inform international cooperation and help policymakers choose the most effective wastewater treatment strategies for contaminated areas.

Tuholske said that many scientists studying the impact of humans on coastal ecosystems are focusing on agricultural runoff, because the fertilizer washed into the sea carries a lot of nutrients and pathogens that may harm the marine environment. But Jolia Lamb, a marine scientist at the University of California, Irvine, who was not involved in the study, said that the impact of human sewage has received much less attention. Part of the reason may be that, unlike garbage or oil spills, sewage is invisible in the water. "I was taken to the beach that looked beautiful and clean," Lamb said. "But then we started testing the water and found that the water contained a lot of human pathogens."

Tuholske explained that there are many ways to treat sewage, and each has advantages and disadvantages. For example, wastewater treatment facilities thoroughly filter pathogens, but they are less effective in removing nitrogen. Their construction, operation and maintenance costs are also high. Septic tank systems are cheaper and can capture most of the nitrogen, but they are less effective in preventing pathogens from entering the environment. Tuholske said that identifying where nitrogen and pathogens are separated or merged in wastewater can help decision makers determine the most effective solutions.

To address this information gap, Tuholske and his colleagues analyzed demographic data of urban and rural communities around the world. They also studied the ways these communities have access to different types of wastewater treatment, as well as national statistics on protein consumption, which helps them calculate how much nitrogen people’s waste may contain. The researchers used this information to construct a global grid showing where the nitrogen and pathogens in the sewage came from and how the sewage was treated. Then, they combined this grid of sewage sources with the map boundaries of the watershed (the areas that flow into rivers and other water bodies) and the locations of pollution-sensitive coral reefs and seagrass beds.

The team found that on a global scale, the nitrogen content of sewage entering the ocean is about 40% of the global agricultural runoff pollution. This figure shows that although sewage is not obvious, it causes a lot of nutrient pollution. In addition, researchers have shown that nitrogen in human sewage can reach about 58% of coral reefs and 88% of seagrass beds in the world. Both of these coastal ecosystems are important wildlife habitats that can help mitigate climate change by sequestering carbon.

Tuholske and his colleagues also found that sewage pollution is a highly concentrated problem. Half of the analyzed watersheds hardly flushed nitrogen or pathogens from sewage into the ocean. Only 25 watersheds—almost across all continents and across multiple countries—contribute about 46% of wastewater nitrogen. The same number of watersheds contribute 51% of sewage-borne pathogens in the ocean. The main sources of sewage nitrogen include the Yangtze River Basin in China (11% of the total), the Nile River in North Africa, the Mississippi River in the United States, the Parana River in Argentina and the Danube River in Europe.

The researchers pointed out that the contributions of nitrogen and pathogens are not always related. For example, they found that a large number of sewage pathogens were washed into the Yarlung Zangbo River in Central and South Asia, but compared with the Yangtze River, the nitrogen contribution of the former was much lower than expected. The team stated that the differences in existing wastewater treatment practices in the two regions may partly explain this.

Lamb said the results of this study are "very exciting because we can begin to pinpoint where sewage nitrogen and pathogens enter the environment." "This helps conservationists, wastewater managers, and health experts to jointly develop these interventions to reduce nitrogen or pathogens entering the environment, or both. It separates these issues and shows where you will be Get the most benefit at all."

Nikk Ogasa is a science journalist living in California. He loves the environment, earth and space. He is an editorial intern of "Scientific American." Follow Nikk Ogasa on Twitter

Sara Schonhardt and E&E News

Asher Lawson and Hemant Kakkar | Viewpoint

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