Improving water quality with the Marquette

By Kevin L. Shafer, PE

Most everyone I know in the Milwaukee area jumps in a car at some point during their busy days to get to some destination. I know I do.

Automobiles have maintained their hold as the favorite mode of transportation, but few people understand what vehicles do to our region’s environment. Recently, high gas prices and hybrid vehicles have reinforced our understanding of the impacts automobiles have on dwindling oil supplies and maybe even air pollution, but few know about their impacts on our region’s waterways and Lake Michigan.

Fuels, oils, bacteria and chemicals from our vehicles are getting into our waterways as one form of nonpoint pollution. And nonpoint pollution is the leading source of pollutants in our waterways.

In fact, recent studies have reported the first flush of water off a roadway can be more polluted than a combined-sewer overflow. While it is very hard to prevent water pollution from nonpoint sources, some very innovative steps are being taken through the reconstructed Marquette Interchange to reduce these pollutants.

Currently, a portion of the runoff from the Marquette Interchange flows into the city of Milwaukee’s combined-sewer system and then into the Milwaukee Metropolitan Sewerage District’s regional system. During small to moderate rainfall events, this flow network allows the MMSD to collect and treat some of
the nastiest pollutants that come from our automobiles.

But during very heavy rainfalls, this flow network can help to overwhelm the MMSD system and lead to combined-sewer overflows. While up to six combined-sewer overflows per year are legal, nobody wants them, and we are doing everything possible to minimize them in the future.

So, the quandary that faced the design engineers from the Wisconsin Department of Transportation, the city of Milwaukee and the MMSD was, “How do you maintain the beneficial capture of pollutants while limiting the amount of storm water that flows into the MMSD system?” Their answer: first-flush diversion chambers.

A first-flush diversion chamber is a non-mechanical device that allows low flows to continue through the existing flow network to the MMSD sanitary system, where they are treated. During a heavy rain event, the low flows would still go to the MMSD sanitary system, but the larger volumes of storm water that run off the interchange would flow through a diversion pipe and go directly to the waterways.

Since the first flush of flow off the pavement is the dirtiest, this chamber allows the runoff from the majority of our storms to be treated. Only during very heavy storm events would there be a diversion to the river, and these flows are much cleaner. This holistic design strategy will provide a model to future highway projects in Wisconsin and throughout the United States.

For the Marquette Interchange project, this strategy is projected to reduce the annual volume of combined-sewer overflows by 9 percent, reduce the annual fecal coliform loading by 41 percent and reduce suspended solids by 38 percent.

While this is definitely a success story, it is not the panacea for controlling all nonpoint pollution. It is just another weapon in our arsenal to attack water pollution.

So, the next time you jump in your car, think about the impacts of that action. But also, if you happen to be driving through the Marquette Interchange reconstruction project, think about how regional cooperation on this effort is making Lake Michigan a little bit cleaner.