2.7 Billion Gallons of Sewage Released into Lake Michigan after Heavy April Storms

April’s heavy rain led to 2.7 billion gallons of untreated wastewater to flow into Lake Michigan, according to the Milwaukee Metropolitan Sewage District.

Discharges began on April 14 at 8:24 p.m. and concluded on April 19 at 2 p.m. for a total duration of 113.6 hours.

Other overflows occurred at these locations:

• West Grant Street and South 77th Street in West Allis 

• 9523 North Broadmoor Road in Bayside  

• North Range Line Road and Milwaukee River in River Hills 

• South 43rd Street and West Lincoln Avenue in Milwaukee  

• South 74th Street and West Oklahoma Avenue in Milwaukee 

• North River Road and West Greentree Road in River Hills 

• North Lake Drive and East Ravine Lane in Bayside 

April was the wettest month in Milwaukee since 2013 with 9.49 inches of precipitation, according to the National Weather Service. Severe storms also featured tornados, hail, flash flooding and thunderstorms.

This was MMSD’s first combined sewage overflow of 2026. By federal law, it is allowed six overflows every year, the Milwaukee Journal Sentinel reported.

The combined sewage system (CSS) collects rainwater, domestic sewage and industrial wastewater into one pipe for sanitation. When excessive water exceeds capacity of CSS, one method is discharging untreated sewage into streams and rivers as a release valve.

To minimize the amount of sewage overflow, about 351 million gallons of wastewater was treated at the Jones Island Water Reclamation facility on April 14.

MMSD is waiting on approval for its 2050 Facilities Plan, a proposal that will comply with the Wisconsin Department of Natural Resources’ wastewater discharges permit and two internal goals including:

• The 2050 Foundational Goals which will “change MMSD from an organization that impacts the environment to one that benefits the environment” by integrating green infrastructure and minimizing MMSD’s financial burden on ratepayers.  

• The 2035 Vision that will integrate watershed management and climate change mitigation while providing an emphasis on energy efficiency. 

Milwaukee’s Deep Tunnel

About $1 billion dollars were invested into the construction of Milwaukee’s Deep Tunnel system in the 80s and has undergone three phases to expand from Jones Island to the city of Glendale. The structure is a massive underground tub that can hold 521 million gallons of rainwater and waste from homes and businesses.

After it began operation, the number of annual overflows went from 50-60 to just two per year.

Since the completion of Milwaukee’s Deep Tunnel, the largest sewage overflow happened during the historic floods in August of 2025. Roughly 5 billion gallons discharged after 14.55 inches of rain were collected in a 24-hour precipitation total, according to MMSD.

The Deep Tunnel was blamed for repeated sewage overflows into Lake Michigan and controversial for taking six lives in the construction process, WISN-12 reported.

But the tunnel has prevented more than 167.3 billion gallons of untreated wastewater from entering the lake since its completion in 1994.