Geography of Sewage Contamination in the Ocean

Elizabeth Borneman


Untreated sewage pollution spells environmental disaster.

Three Biggest Threats to the World’s Oceans

Many environmental managers focused on the world’s oceans will tell you that climate change, overfishing, and ocean pollution are the three biggest threats to the marine environment today.

The consequences of overfishing are widely understood and managed by networks of local, state, federal, and international fishing regulations, and how climate change impacts the world’s oceans is a focus of much research.

Finally, while ocean pollution is known to be detrimental to the environment, less attention has been to a major source of ocean pollution: untreated sewage.

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80% of the World’s Sewage Enters the Ocean Untreated

The world’s sewage has to go somewhere, and unfortunately 80% of the sewage produced by the global population makes its way into the world’s oceans untreated.

Inconsistent or nonexistent public sanitation efforts, a lack of political will, or simple disinterest keep the problem out of sight and out of mind of many. 

This image depicts a sewage overflow in Roswell, Georgia, near the Chattahoochee River.
Heavy rainfall on September 21, 2009 near Atlanta, Georgia resulted in an overloading of several integrated storm/sewage systems with overflow spilling out and flowing directly into the Chattahoochee River. Photo: USGS, public domain.

Humanity has treated our oceans as a common dumping ground for decades.

Pollution in all forms is detrimental to delicate marine habitats, like seagrass beds, coral reefs, and near-coastal environments. When these habitats are healthy they are carbon sinks, oxygen producers, and provide a safe location for marine flora and fauna to thrive. 

Global Problem – Global Consequences of Untreated Sewage

Untreated sewage pollution is a global problem with global consequences.

In a recent study, the most detailed of its kind, researchers mapped 130,000 watersheds around the globe to discover the inputs and impacts of human waste on the marine environment.

These watersheds were studied to determine if they carried human waste into the marine environment and, if so, how much sewage was being released. There were 25 watersheds that contributed a significant amount of pollution to the ocean and many other watersheds carrying lesser, although still significant, levels of pollution.

Satellite image of an algae bloom off the coast of New York and New Jersey, August 3, 2015.
Every day, approximately 1.7 billion gallons of treated sewage are emptied into the ocean via the inlet connecting Sandy Hook, New Jersey and Rockaway, New York, contributing to algae blooms. This algae bloom occurred on August 3, 2015. Image: Landsat 8, NASA, public domain.

Untreated human waste can cause environmental degradation in the form of algal blooms and eutrophic, or dead zones, that smother aquatic life. Widespread algal blooms due to high levels of nitrogen pollution can kill aquatic plant life and decrease the amount of oxygen available in the water.

The report showed that 88% of seagrass beds and 58% of coral reefs, both highly important marine habitats, have already been exposed to the nitrogen in untreated human sewage.

Some of the watersheds that carried the most pollution to the ocean included the Yangtze, the Danube, the Nile, the Mississippi, and the Parana River.

Large populations of people throughout history settled near major water sources, making these locations unsurprising to many who are involved in this project.

The research shows that untreated sewage pollution happens on every continent, regardless of economic status or GDP, and poses particular risks to coastal areas.

Maps showing where A) coral reefs and B) seagrass beds are heavily impacted by sewage entering the oceans.
Impact on (A) coral reefs and (B) seagrass by sewage in the oceans. Maps: Tuholske, C., Halpern, B. S., Blasco, G., Villasenor, J. C., Frazier, M., & Caylor, K. (2021). Mapping global inputs and impacts from of human sewage in coastal ecosystems. PloS one, 16(11), e0258898. CC BY 4.0

Sanitation and Public Policy

According to the study, only 25 watersheds generate 46% of worldwide nitrogen imports into the ocean via wastewater. About half of the fecal pathogen pollution came from 25 sources, some of which overlapped

Understanding where pollution is coming from, where it’s going, and how to manage it better is essential for protecting our fragile marine ecosystems.

The data gathered in this study have implications far beyond just cleaning up the ocean; the implications for public policy, sanitation, and efforts to give people access to clean water are just a few of the ways in which the world can be changed because of this research.

These data are integral for environmental managers to understand how much pollution is being released into the ocean, where it’s being released, and if that pollution is primarily nitrogen, pathogens, or both.

A picture of water flowing from pipes at a sewage treatment plant.
A sewage treatment plant. Photo: Montgomery County, CC BY 2.0

This knowledge helps policy makers and sanitation workers implement the most appropriate measures to mitigate sewage pollution in the most efficient and cost-effective ways possible. 

A policy decision that sanitation managers have to make is whether to treat sewage in a wastewater plant or using a septic system. There are benefits and downsides to both options; wastewater treatment can be expensive, but is able to remove much of the pathogens from untreated sewage. Septic systems are cheaper to install but are not as beneficial for removing pathogens, although they are good at removing nitrogen from untreated waste.

Agricultural runoff is another major contributor of coastal pollution. Nitrogen from fertilizers and pathogens from livestock waste enter into watersheds and have similar impacts to human waste in the ocean. Unlike oil slicks or plastic pollution, pollution in the form of human and animal waste can be invisible in the marine environment and detected only by taking water samples. 

Future Implications for Sewage Management

Researching sewage pollution may not be every student’s dream job, but it’s an important area that has long been overlooked. More importantly, environmental managers and others working tirelessly on this issue know that it is a problem with implementable solutions. 

While people work to filter plastic out of the oceans, study the impact of agricultural fertilizer, others are studying the global impacts of untreated sewage on the world’s oceans and how we can work to improve not only peoples’ quality of life, but the quality of the environment around us at the same time.


Tuholske, C., Halpern, B. S., Blasco, G., Villasenor, J. C., Frazier, M., & Caylor, K. (2021). Mapping global inputs and impacts from of human sewage in coastal ecosystems. PloS one16(11), e0258898.

Wear, Stephanie. “More than 80% of the world’s sewage is discharged into the environment untreated. We can fix this.” The Nature Conservancy. 15 November, 2020. Available at

“Researchers Map Impacts of Human Sewage Along the World’s Coasts.” The Earth Institute. 15 November, 2021. Available at

Ogasa, Nikk. “Half the World’s Coastal Sewage Pollution Flows From a Few Dozen Places.” Scientific American. 12 November, 2021. Available at


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About the author
Elizabeth Borneman
My name is Elizabeth Borneman and I am a freelance writer, reader, and coffee drinker. I live on a small island in Alaska, which gives me plenty of time to fish, hike, kayak, and be inspired by nature. I enjoy writing about the natural world and find lots of ways to flex my creative muscles on the beach, in the forest, or down at the local coffee shop.