The U.S. Geological Survey yesterday released the results of a study detailing the root causes of phosphorus and nitrogen pollution in the Gulf of Mexico, naming Chicago as the nation’s top offender.

That pollution has helped create an 8,000-square-mile “dead zone” in the Gulf, where marine life suffocates because of an excess of algae.

Chicago’s waste water, treated at the Metropolitan Water Reclamation District, flows out of the city, making its way to the Mississippi River and the Gulf of Mexico.  According to the USGS study, Chicago ranks first in discharging water tainted with phosphorous and nitrogen, chemicals that can accumulate through every day items like laundry detergent and lawn fertilizer.

The study looked at 150 watersheds in the Mississippi River Basin. It was able to determine water sources for each watershed.

While Chicago ranks the highest in the country, Albert Ettinger, a senior attorney with the Environmental Law & Policy Center, says that can be misleading.

“You can be number one and still only be 5 percent of the issue,” he says. “A lot of this nitrogen and phosphorous comes from farmland across the Midwest.”

But, he says, the MWRD contributes by not treating the chemicals in ways other cities do. He says it costs too much.

The MWRD is formulating a response to the city and a critical news release issued by the Environmental Law & Policy Center and other organizations that worry the dead zone is killing the Gulf.

We'll have an update later today. Hopefully it'll answer some remaining questions:

• To what extent does the MWRD treat water for these chemicals?
• What measures are the MWRD and other municipal bodies working to make sure Chicago’s refuse doesn’t pollute areas downstream?
• Will future water permits require water districts to monitor the types of chemicals released into rivers?