How Sewage Overflows Still Reach The Great Lakes Every Year

Tens of millions of people draw their drinking water from the Great Lakes, and the wastewater systems built to protect that supply are among the most advanced in North America. Even so, untreated and partly treated sewage still reaches the lakes every year, and often the reason has nothing to do with a plant breaking down. Some of it flows exactly as engineers a century ago intended; some of it slips through gaps nobody meant to leave open. Researchers testing the water around Lake Michigan keep finding the chemical fingerprints of human waste in rivers, harbors, beaches, and well offshore. The pathways it takes are more varied, and in a few cases stranger, than most people living on the shoreline would guess.

Heavy Rain Can Trigger Combined Sewer Overflows

The most direct route begins under older Great Lakes cities, where one pipe carries household sewage and stormwater together. During ordinary weather, that flow reaches a treatment plant. A cloudburst changes the hydraulics: runoff from roofs, streets, and parking lots can arrive faster than the sewer, pumps, storage tunnels, or plant can handle it. Designed overflow points then release the excess mixture to nearby water rather than forcing sewage back into basements and streets. This is not a rare historical curiosity. EPA's current Great Lakes inventory lists 166 NPDES permits covering 158 communities authorized to discharge from combined sewer overflows.

Milwaukee shows how large a single episode can become. During storms from April 14 to 19, 2026, the regional sewer district reported roughly 2.7 billion gallons of combined sewer overflow. The discharge was diluted with rainwater, but it still contained untreated sanitary sewage and entered waterways connected to Lake Michigan.

Groundwater Can Leak Into Aging Sanitary Sewers

A sanitary sewer is meant to carry wastewater, not rainfall, yet buried pipes are rarely perfectly watertight forever. Groundwater can seep through cracks, failed joints, defective seals, and deteriorated service laterals, a process engineers call infiltration. After prolonged rain or snowmelt raises the water table, that unwanted water may occupy enough pipe capacity to trigger a sanitary sewer overflow. EPA identifies infiltration and inflow as major causes of SSOs, which release untreated or partly treated sewage from systems designed to contain it. The Great Lakes region provides a current example. During Milwaukee's April 2026 storms, the district reported about 11.6 million gallons of sanitary sewer overflows alongside its much larger combined-sewer release. The important distinction is that this water did not originate in a sewer intentionally built to carry storm runoff. Rain and groundwater entered a sewage-only network, increased its hydraulic load, and helped push wastewater out before full treatment.

Downspouts and Sump Pumps Can Feed the Wrong Sewer

Some rain reaches sanitary sewers through deliberate-looking plumbing rather than damaged pipe walls. Roof downspouts, sump pumps, foundation drains, and area drains can be connected to sanitary lines even though their water belongs in a stormwater system or on the landscape. EPA classifies this as inflow and specifically identifies these connections as a cause of sewer overloading. One house may contribute only a modest pulse, but many connections operating during the same downpour can send a sharp surge toward pumps and treatment works.

The mechanism differs from groundwater infiltration because the water has a clear entry point. That matters for repairs. Utilities can smoke-test lines, inspect laterals, disconnect roof leaders, or redirect sump-pump discharge instead of excavating every sewer main.

Research in the Milwaukee watershed has found human sewage markers during wet-weather conditions even outside recognized major overflow events. A 2016 study using human-associated Bacteroides and Lachnospiraceae found storm-driven sewage contamination in urban rivers draining to Lake Michigan, showing how wet-weather flow through imperfect urban sewer networks can move human waste toward the lake.

Blockages and Pump Failures Can Spill Sewage Without a Storm

A sewer can also spill on a dry day. EPA lists blockages, line breaks, power failures, inadequate pump maintenance, equipment failures, and poor system design among the causes of sanitary sewer overflows. Grease, wipes, roots, or debris can reduce the diameter of a pipe until upstream wastewater has nowhere to go. A failed lift station creates a different bottleneck: sewage stops moving uphill through a system that depends on pumps and may escape through manholes or emergency relief points.

Once outside the sanitary network, gravity provides the next connection to the Great Lakes. Wastewater reaching a roadside drain, ditch, urban creek, or river can join the surface-water network without ever visiting a treatment plant. EPA estimates 23,000 to 75,000 SSOs occur annually across the United States, although that figure is national rather than Great Lakes-specific. These failures arise from malfunction, obstruction, deterioration, or inadequate capacity in systems specifically intended to keep sewage enclosed.

Treatment Plants Can Divert Peak Flows Around Biological Treatment

Sometimes the sewer system succeeds and the treatment plant becomes the limiting step. Biological treatment relies on microorganisms and settling processes that operate within particular hydraulic ranges. During exceptional wet weather, incoming wastewater can rise quickly enough to threaten those processes. Hamilton, Ontario, describes a secondary bypass that may be initiated when plant flows exceed capacity and risk washing out the biological treatment system. The diverted wastewater is not equivalent to raw sewage: large solids and grit have already been removed, phosphorus-removal chemicals have been added, and most settleable and floating material has been taken out. From May 15 through October 15, Hamilton also disinfects secondary bypass flow with chlorine before dechlorination. Even so, the water has skipped the normal secondary biological stage before discharge to Hamilton Harbour, which connects directly with Lake Ontario. Sewage reaching Great Lakes waters can therefore range from raw overflow to partially treated wet-weather effluent.

Storage Tunnels and Overflow Tanks Can Eventually Fill

Great Lakes cities have built enormous underground buffers specifically to keep wet-weather sewage out of open water. Hamilton's nine combined-sewer overflow tanks can hold more than 314,000 cubic metres of diluted wastewater. During a storm, they temporarily capture excess flow and return it to the sewer for treatment after conditions ease. The city reports that these facilities have cut the volume of CSOs to receiving waters by about half.

The catch is that storage is finite. If rain continues after tanks and downstream sewers are full, the system still needs a relief route. Hamilton states that an overflow to the harbour can occur once its combined network reaches capacity.

Milwaukee's deep-tunnel system shows the same engineering constraint at a much larger scale. The district's storage and conveyance infrastructure has dramatically reduced overflow frequency over the past several decades, yet the April 2026 storms still exceeded available wet-weather capacity. Additional storage raises the storm threshold that causes an overflow, but every reservoir, tunnel, and pipe still has a measurable maximum volume.

Hidden Cross-Connections Can Leak Sewage for Decades

A wrong connection can turn a storm sewer into a sewage pipe for years without any extreme weather at all. Hamilton uncovered an extraordinary example in November 2022: a hole in a combined sewer was directing wastewater into a large storm sewer that emptied into Hamilton Harbour. City investigators concluded that the connection likely dated to 1996, when a contractor apparently believed the pipes in the area were storm sewers. Hamilton estimated that as much as 337 million litres of sanitary sewage had been discharged over 26 years.

The discovery triggered a risk-based inspection program, which soon found another long-running problem. Eleven properties on Rutherford Avenue had contributed an estimated 59 million litres of sanitary sewage to the harbour over roughly the same 26-year period. Both cases were repaired. They revealed a pollution route that rainfall statistics cannot expose: construction errors and unseen cross-connections can continuously send wastewater into storm drainage that empties toward Great Lakes waters.

Tributaries Can Carry Inland Sewage All the Way to a Great Lake

Many sewage releases reach a Great Lake indirectly. EPA's permit inventory records Akron overflows entering the Cuyahoga River before reaching Lake Erie, while Michigan communities discharge to river systems such as the Rouge that ultimately connect with the Detroit River and Lake Erie. The immediate receiving water may therefore be a creek or river far from open lake water. Microbial testing has confirmed that contamination can travel beyond the original pipe. A peer-reviewed Lake Michigan study around Milwaukee detected human-specific Bacteroides genetic markers as far as 2 kilometres into the lake during sewage-overflow events, with detections occurring between one and nine days afterward. Later research found an urban human-fecal signature extending even farther offshore during overflow conditions. Tributaries are therefore a genuine part of the sewage-delivery network, carrying contaminated water from inland urban systems through harbors into nearshore areas, where currents, dilution, sunlight, and microbial die-off influence how long the signal remains detectable.

Why the Lakes Stay Vulnerable

These eight routes share a single theme: every part of the system has a finite limit, and wet weather keeps finding the weakest point. A combined sewer overflows by design, a sanitary sewer overflows when groundwater and misrouted downspouts fill it, a treatment plant bypasses its biological stage to protect the microorganisms that make it work, and a mislabeled pipe can leak for a quarter century before anyone notices. Billions of dollars in deep tunnels and storage tanks have cut overflow volumes sharply in Milwaukee and Hamilton, yet the April 2026 storms and the Hamilton cross-connections show that reduction is not elimination. As long as older basin cities carry sewage and stormwater in the same pipes, and as long as rainfall keeps intensifying, the question is less whether sewage reaches the Great Lakes than how much, how often, and through which of these eight doors.