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World Water Day: How Cities Cause “Dead Zones”

WRI identifies 13 new eutrophic areas around the world.

World Water Day this year focuses on “Water for Cities,” but what about water from cities? Urban runoff is one of the biggest threats to water quality around the world, with serious impacts on economies and people. However, it’s a problem that most cities are only starting to address.

Nutrient Pollution and Urban Runoff

Eutrophication occurs when water bodies are polluted with nutrients (for example, chemicals from fertilizer and sewage) that wash into surface waters from farms and urban areas that can cause oxygen depletion, fish kills, and ecosystem collapse. These are often called “dead zones” – because of the impact on fish and other sea life.

These issues can be especially problematic in urban areas. When it rains, nutrient pollution from lawns, pet waste, and vehicle exhaust washes into nearby waterways. This sewage (sometimes treated, sometimes not) is often discharged into nearby bodies of water.

Eutrophic Areas Around the World

In January, the World Resources Institute (WRI) and the Virginia Institute of Marine Science (VIMS) identified 534 low-oxygen “dead zones” and an additional 228 sites worldwide exhibiting signs of marine eutrophication. Thanks to responses from readers, WRI has since discovered 13 additional sites that are already eutrophic and in danger of becoming dead zones, bringing the total number of coastal areas around the world known to be suffering from nutrient pollution to 775.

<p>Explore our Interactive Map of Eutrophication & Hypoxia</p>

Explore our Interactive Map of Eutrophication & Hypoxia

Some of the newly recorded sites have symptoms caused by urban runoff:

  • Halifax, Canada: Due to the growth of urban populations, Halifax Harbour and Bedford Basin receive high concentrations of urban waste that are high in nitrogen, phosphorus and other organic matter. Compounding the problem, municipal sewage is entering Bedford Basin from neighboring Bedford and Sackville, and a recent failure of the Halifax treatment plant resulted in high levels of fecal coliform pollution in the Inner Harbor. As a result, people can no longer safely swim or fish in certain areas. As the urban area around Halifax has grown since with 1960s, there have been more severe symptoms of eutrophication, including phytoplankton blooms and fish-kills.

  • Algeciras, Spain: The nearby Palmones River Estuary is located in a small area with a high population and a mixture of agricultural, urban and industrial land. Symptoms of eutrophication in the estuary have been observed since the early 1990’s, caused by high phosphorous concentrations from urban runoff, organic sewage from nearby towns, and waste from both a paper mill and nearby industrial park. Recent reports indicate the system is highly eutrophic and already many shellfish species have been diminished or depleted.

<p>Development in the City of Algeciras exerts tremendous pressure on the bay. Photo credit: Wikimedia/Falconaumanni</p>

Development in the City of Algeciras exerts tremendous pressure on the bay. Photo credit: Wikimedia/Falconaumanni

In addition to sea water, fresh water sources often suffer eutrophication. In some extreme cases, local rivers and lakes can become so polluted by urban runoff that they are unsuitable for drinking water or even industrial uses. One striking example of this is Tai Lake in China, where urban runoff, combined with sewage and industrial discharge, led to a massive toxic blue-green algae bloom in May 2007. The bloom rendered the water in the lake too polluted for human, agricultural or industrial uses, and residents were forced to import water from other locations.

States and Cities Taking Action

Some regions are starting to take steps to reduce urban runoff and address wastewater issues:

  • In New Jersey, in an effort to reduce the nutrient load to Barnegat Bay, a bill was recently passed that will limit the nutrient content of lawn fertilizers in the state.

  • In Maryland, a June 2000 bill imposed strict standards for enhanced nutrient removal on all major wastewater treatment plants, in an effort to control pollution entering the Chesapeake Bay.

  • Around the Great Lakes, where eutrophication is a growing problem, New York, Michigan and other surrounding states have enacted phosphorus bans for detergents.

  • Some cities, like Portland, OR, have begun to manage urban runoff through the use of “green infrastructure” such as forest lands, rooftop gardens, rain gardens, wetlands, ponds and trees planted along stream banks to intercept runoff and cycle nutrients before it can reach surface waters.

WRI has also released the full data set available for 775 eutrophic sites worldwide. We hope that by making this data set widely available, we can help advance the critically important research and policy discussions to address the problems associated with eutrophication.

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