Kate Katauskas
Communications Intern
When it comes to understanding common terms relating to Illinois water policy, the biggest barrier for many people isn’t a lack of interest—it’s the terminology. Terms like “TMDL” or “effluent discharge” can make a news article or public meeting feel like it’s written in code. But the truth is, these words describe issues that shape our daily lives in Illinois. They affect:
- What comes out of our taps
- How our rivers and lakes look and smell
- Even how much we pay in water bill
The good news? You don’t need a degree in environmental policy to follow along. By breaking down a few key terms, we can all get a clearer picture of the ways in which public health and environmental health work hand in hand.
Below, we define some key terms you will see in our Legislative Agenda, Legislative Report, action alerts, and other resources.
Watershed
A watershed is the boundary, or divide, that separates where water flows into different drainage basins. In everyday use, people often use “watershed” to mean the drainage basin itself, the land area where all the water drains to a common point such as a river, lake, or aquifer. Rain that falls on your street eventually flows somewhere, whether into the sewer or a local stream, and from there it travels farther downstream.
The North Branch of the Chicago River is the main watershed for three major tributaries: the Skokie River, the Middle Fork, and the West Fork. The NBCR is unique because together with the South Branch of the Chicago River, it forms the Chicago Area Waterway System, which connects the Great Lakes to the Mississippi River. This means the stormwater runoff from Chicago’s streets, or farmland in central Illinois, eventually makes its way down the Mississippi to the Gulf of Mexico. Local actions on land and water here can ultimately influence ecosystems and communities far downstream.
Effluent
Effluent is the wastewater that leaves a factory, sewage treatment plant, or industrial site and enters rivers and lakes. Sometimes it’s treated to remove pollutants, and sometimes it’s not. Policies and permits regulate what can legally be discharged and in what amounts.
Effluent from wastewater treatment plants flows into waterways like the Des Plaines River or Chicago River. If treatment plants fail to meet standards, it can increase pollution levels and harm aquatic life. The Illinois Environmental Protection Agency (IEPA) sets effluent limits through permits that facilities must follow.
Point and Nonpoint Source Pollution
Human activities such as farming, manufacturing, and energy production release pollutants into the environment, which are categorized as point source or nonpoint source pollution.
Point source pollution comes from a single, identifiable place. This can be a smokestack, discharge pipe, or wastewater treatment plant. It can introduce chemicals, heavy metals, nutrients, and microbes into air and water.
Nonpoint source pollution is harder to trace. It comes from many spread out sources at once. Stormwater runoff carrying oil, trash and fertilizers from urban areas, or sediment, pesticides, and acid from rural landscapes, is one example. Emissions into our air that lead to acid rain are also considered nonpoint source pollution. Both types of pollution harm ecosystems, but policies like the Clean Air Act and Clean Water Act have reduced their impacts, resulting in cleaner air and water today compared to much of the 20th century.
In Chicago, point source pollution comes from industrial and municipal discharge pipes, while nonpoint source pollution is primarily driven by urban runoff that enters the waterways through storm drains. The Chicago River system and Lake Michigan are affected by both types of pollution.
TMDL (Total Maximum Daily Load)
This term sounds intimidating, but think of it like a pollution “limit” or cap for rivers. A total maximum daily load, or TMDL, is a tool created under the Clean Water Act. In Illinois, the Illinois Environmental Protection Agency develops TMDLs to calculate the maximum pollutant load a waterbody can receive and still meet water quality standards.If a lake has too much nitrogen or phosphorus, for example, a TDML requires communities and industries to reduce their contributions until the water stays healthy.
A TMDL report for the Vermilion River Watershed identified water quality impairments linked to heavy agricultural runoff from corn and soybean fields, along with other sources. While TMDLs establish enforceable reduction targets for point sources such as wastewater treatment plants, they only provide guidance for addressing nonpoint sources like farm runoff. In practice, this often means promoting voluntary strategies such as septic upgrades, improved stormwater management, and landowner education to restore watershed health. The report emphasizes that most reductions must come from these nonpoint sources.
Nutrient Pollution
Nutrient pollution occurs when excess nitrogen and phosphorus from activities like agriculture, wastewater, stormwater runoff, fossil fuel burning, and household products enter air and water, fueling harmful algae blooms and other environmental damage.
While these nutrients are naturally part of ecosystems, too much of them harms water quality, depletes oxygen, kills fish, and can release toxins dangerous to people, pets, and wildlife. Nutrient pollution also contaminates drinking water, contributes to air pollution and acid rain, and creates “dead zones” where aquatic life cannot survive.
Its impacts extend to human health, causing rashes, respiratory issues, neurological problems, and risks to infants exposed to nitrates in water. Nutrient pollution also harms the economy through higher water treatment costs, tourism and recreation losses, damage to fisheries, and declining waterfront property values. Found nationwide in rivers, lakes, coastal waters, and groundwater, nutrient pollution is a widespread issue with serious environmental, health, and economic consequences. In Chicago, the city’s extensive network of impermeable surfaces like roofs, streets, and parking lots prevents rainwater from being absorbed into the ground. Instead, it runs off and picks up pollutants, including excess phosphorus from yard waste like leaves and pet waste, before flowing into the sewer system and eventually the river.
Combined Sewer Overflow (CSO)
In older cities like Chicago, sewer systems were designed to carry both sewage and stormwater in the same pipes. During heavy rains, these systems can’t handle the volume, so they overflow, dumping untreated sewage directly into rivers or lakes. This is called a Combined Sewer Overflow, or CSO.
Chicago has invested billions in the Tunnel and Reservoir Plan (TARP), better known as the “Deep Tunnel,” a massive underground tunnel and reservoir system designed to capture and store combined sewer overflows (CSOs) before they reach Lake Michigan and other waterways. The project has greatly reduced flooding and pollution, but during increasingly frequent extreme storms the system can still be overwhelmed, leading to untreated discharges. This highlights both the importance of continued infrastructure investment and the challenges cities face as climate change intensifies rainfall and stresses aging systems.
National Pollutant Discharge Elimination System (NPDES) Permit
An NPDES permit (National Pollutant Discharge Elimination System) is required under the Clean Water Act for anyone discharging pollutants from a point source such as a pipe, ditch, or confined system into waters of the United States.
These permits set discharge limits, require monitoring and reporting, and ensure water quality and public health are protected. They can be issued by the Environmental Protection Agency or authorized states, typically last up to five years, and may be site-specific (individual permits) or broader (general permits) covering multiple similar dischargers. Facilities must comply with permit conditions, self-monitor their discharges, and report results, while regulators and even the public can review, comment, and help enforce compliance. In essence, NPDES permits translate the Clean Water Act’s general protections into enforceable, location-specific requirements.
The IEPA Permit No. IL0045012 is a NPDES permit authorizing the City of Chicago’s Department of Water Management to discharge combined sewage from up to 184 Combined Sewer Overflows (CSOs) into various local waterways during heavy rainfalls. The permit includes requirements for compliance with water quality standards, control of excess water entering the sewer system, implementation of Nine Minimum Controls (NMCs), maintaining a Pollution Prevention Plan, and submitting operational and monitoring data.
Source Water
This is exactly what it sounds like: the original source of the water we drink. It can be a river, lake, or aquifer. Protecting source water is critical because the cleaner it is, the easier (and cheaper) it is to treat and deliver to homes.
Chicago and its suburbs rely on Lake Michigan for drinking water, while much of central and southern Illinois depend on groundwater. Over-pumping and contamination of these aquifers pose risks for the future.
Why Language Matters
Understanding these terms isn’t just about decoding technical reports. It’s about building the confidence to engage in discussions, attend public meetings and advocate for solutions. When you hear about nutrient pollution or a new NPDES permit, you’ll know what’s at stake.
Water protection and policy affect every Illinoisian, whether it’s reducing flooding, keeping drinking water safe, or restoring rivers for recreation. By cutting through the jargon, we can all have a stronger voice in shaping clean, affordable, and sustainable water projects, programs, and policies.
About the Author
Kate is the Illinois Environmental Council’s Communications Intern, creating content to inspire Illinois’ environmental community to take action. Her past work highlights sustainability, environmental policy, and environmental justice. She is passionate about inclusive scientific communication and strives to turn complex issues into actionable advocacy.