The trend for most of us when it comes to human wastewater is out of sight, out of mind. We rarely consider what happens after we flush the toilet or turn off the tap.
However, researchers at UC Santa Barbara have turned their attention and considerable computing power to the subject and its impacts on global coastal ecosystems. The results aren’t pretty, but they are enlightening.
“The motivation behind this research was the desire to have a fine-grained understanding of the impact of sewage on coastal waters around the world,” said Cascade Tuholske, lead author of an article in the journal PLOS A. While research on land-based threats to coastal marine ecosystems often focuses on agricultural runoff and what happens when fertilizer and livestock waste ends up in the ocean, he said, few Studies investigate what happens when human wastewater does the same.
“This is not the first study to produce a global model of wastewater, but it is the first study to map nitrogen and pathogen inputs from wastewater to 130,000 watersheds across the planet” , Tuholske said. “And that’s important because there are trade-offs in the intervention space.” Information from this model, he added, could make those trade-offs clearer and management decisions easier to make.
The scale of the problem
The majority of human wastewater is discharged into the ocean around the world in a variety of treated and untreated states from sewage, septic systems and direct inflow sources. Not surprisingly, the main sources of human wastewater are also places with dense human populations, which tend to cluster around major watersheds.
“We estimate that 25 watersheds contribute about 46% of global nitrogen inputs from sewage to the ocean,” said Tuholske, postdoctoral researcher at Columbia University who conducted the study as a student. graduated from UC Santa Barbara. “Almost half the amount of nitrogen comes from wastewater compared to agricultural runoff globally,” he added, “which is a huge fraction.” Coasts around the world are being affected by the increase in nitrogen, according to the article.
Tuholske and an interdisciplinary group of fellow UCSB scientists — Ben Halpern, Gordon Blasco, Juan Carlos Villasenor, Melanie Frazier and Kelly Caylor — created a data visualization that globally maps the sources and destinations of nitrogen, a common element in both agricultural and human wastewater which causes eutrophication. It is a phenomenon in which excessive nutrients create phytoplankton blooms just offshore which produce toxins and deprive the waters of the area of oxygen. These so-called “dead zones” not only suffocate marine life unfortunate enough to be trapped there, but can also cause problems in the food chain, including for humans.
“Many coastal ecosystems, such as coral reefs and seagrass beds, are particularly sensitive to excess nutrients, even if you don’t have a dead zone,” said Halpern, a professor at the Bren School of Environmental Science & Management. and director of the National Center for Ecological Analysis and Synthesis at UCSB. “The whole ecosystem can tip into a highly degraded state when nutrient levels get too high. Coral reefs can be converted into algal fields that overgrow and kill the corals below. Our work here helps map where nutrients from sewage likely put these ecosystems at greatest risk.”
For Tuholske, whose research focuses on food systems, the model highlights the impact of modern diets on coastal ecosystems.
“What was really surprising about this research was how diets switching to animal-based protein are impacting marine ecology,” he said. The wealthier countries get and incorporate more meat into their food systems, he explained, the more nitrogen appears in wastewater, on top of the already high levels generated by agriculture.
“The more burgers people eat, the more nitrogen goes into the ocean,” he said.
Two targets
Excess nitrogen isn’t the only problem with the increasing amount of human wastewater dumped into the ocean; where the wastewater goes, so do the pathogens. But removing nitrogen or pathogens can require very different methods, which can make it difficult for policymakers with limited resources and varying priorities to weigh their options between improving public health and protecting. coastal ecosystems.
With the fine-scale estimates of nutrient and pathogen inputs provided by this model, the goal is to provide information that can lead to local solutions that together can solve a complex global problem.
“These fine-resolution top-down hotspot maps can be combined with bottom-up approaches, and we can transfer knowledge between geographies,” Tuholske said. “Adaptation and mitigation really come from the bottom up, and having a global map helps to focus priorities and share knowledge.
“While we map the scale of this problem, there is something we can do,” he added. “We can protect both public health and coastal ecosystems.”
