A recent study released by the U.S. Geological Survey named Chicago as one of the key contributors to toxins that created an 8,000-square-mile “dead zone” in the Gulf of Mexico, but Chicago’s Metropolitan Water Reclamation District says the study is misleading.

Supplies of nitrogen and and phosphorous contribute to the problem, in which dying algae suck the oxygen out of the water, creating a dead zone that kills fish and other marine life.

Dale Roberts, a research hydrologist with the USGS says Chicago’s Metropolitan Water Reclamation District doesn’t treat the chemicals to the extent that cities such as Minneapolis and Madison, Wisconsin do.

But the MWRD says its discharge of chemicals is within the law, said MWRD spokeswoman Jill Horist.

Roberts says the USGS studied figures of chemical levels from 1992 to 2002, and based its report on that data. The agency is working on a more updated study.

“The City of Chicago is high, but whether it’s number one or four or five (in terms of contributing to the dead zone), it’s still one of the big contributors. This study was trying to use the best models available at the time, but we’re trying to improve on those models,” he says.

Horist says Illinois water quality standards for nitrogen and phosphorous do not apply to the MWRD.

“Standards that apply to natural waterways or lakes do not apply to the District because the effluent is released into the manmade, artificially controlled Chicago Area Waterways System,” she says.

She says the district also keeps monthly tabs on water quality in Chicago.