Keeping Our Waterways Clean, One Sample at a Time

July 26, 2022

Storm sewers and grass swales convey runoff from rain events, ultimately discharging to some type of waterway. Often times, runoff will travel thousands of feet in these conveyance systems before reaching its outfall point. Ideally, these conveyance systems should only transport stormwater runoff, but as most of us know that is not always the case. The more publicized events that show up in the news tend to be oil spills or sanitary sewer overflows (SSOs). SSOs occur when a sanitary sewer system cannot keep up with an intense or prolonged rainfall event. Rather than leaving the sanitary sewer system full with the potential to back up into homes, regulatory officials may have to make the decision to bypass flow into adjacent storm sewer or waterways.

Besides sanitary sewer overflows and oil spills, there are other pollutants entering municipal storm sewers and conveyance systems on a daily basis. These pollutants might be smaller in volume in comparison, but they can still have detrimental effects on nearby waterways. As part of state and federal regulations, many municipalities in Wisconsin are required to have Municipal Separate Storm Sewer System (MS4) permits, which are regulated under the Wisconsin Pollutant Discharge Elimination System (WPDES) program. Under this program, municipalities are required to educate and involve the public with stormwater, enforce construction site pollutant control, manage post-construction stormwater (ponds, permeable pavement, etc.), incorporate pollution prevention programs, and implement an illicit discharge detection and elimination (IDDE) program.

The focus of an IDDE program is to eliminate any pollutants that are being discharged to a municipality’s storm sewer system. If a pollutant is discharged to an underground storm sewer, it may never be seen by the public until it reaches its final outfall point. Municipalities are required to inventory their outfalls and prioritize them to field screen during periods of dry weather. Factors such as tributary drainage area, age of infrastructure, history of illicit discharges, and land use are used to determine where an illicit discharge may likely occur. The goal of screening during dry weather is to avoid any excess stormwater in the system. If an illicit discharge is present, it is more likely for it to turn up during a field screening as it would not be diluted by stormwater. There are many parameters to test for, as recommended by the Environmental Protection Agency (EPA) and the Wisconsin Department of Natural Resources (WDNR). Some of these parameters include ammonia, detergents, pH, total chlorine, total copper, phenol, potassium, fluoride, and bacteria.


During annual field screenings, outfalls are inspected for illicit discharges as well as any other maintenance measures that may be required. If an outfall is actively flowing, a sample will be taken by the inspector. That sample will then be tested for the parameters mentioned above. If the sample tests positive for any of the parameters, a further investigation will be required and WDNR staff should be notified. Investigating the upstream conveyance system is crucial in pinpointing the source of the discharge. Sources can be disguised in a number of ways, including residential homes connected to the storm sewer, an industrial business dumping pollutants down the drain, leaks from the sanitary sewer mainline, and others. While investigating the source, additional samples will be collected upstream until the sample no longer tests positive. It is possible that testing measures such as smoke testing, video testing, and dye testing may be required to determine the source of the discharge. Once the source has been identified, the responsible party will have a specific timeframe to eliminate the source.

At the end of the day, the ultimate goal is to keep our waters clean. Measures such as the WPDES program have been implemented and improved upon over the years to help continue the progress of restoring our waterways. Help in doing so can also come directly from the public. If you or someone you know spots a storm drain discharging something other than stormwater, contact your local municipality immediately. WDNR also has a toll free spill hotline at 1-800-943-0003. Together, we can help keep our waterways clean, one sample at a time.

Contact raSmith’s municipal engineering and water resource experts to learn more about how raSmith can assist with your project.

About the Author

Riley Stone, P.E., is a civil engineer in raSmith’s Brookfield, Wis., office. He provides stormwater modeling and analysis and performs erosion control inspections for construction sites. He conducts municipality-wide pollutant loading analyses using WinSLAMM to determine TMDL (Total Maximum Daily Loading) requirements and assists municipalities in preparing MS4 annual reports.

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