Please submit your name and email to access this content

    Why Stormwater Requires Treatment

    By: Jeff Urban, VP, Enterprise Account Management

    Stormwater is the natural water runoff after rain events. In rural areas, a large volume of the water from rain events absorbs directly into the earth allowing for natural processes like transpiration to occur. As cities and towns develop, more impervious surfaces are created, preventing stormwater from absorbing into the earth. The impervious surfaces: parking lots, buildings and roads are beneficial for urbanization but compound nutrients and sediment that facilitate stormwater pollution. Without plants and grasses, stormwater flows over concrete and asphalt collecting trash, debris and pollutants. When stormwater is not slowed by vegetation and infiltration, it collects in large quantities as it flows downstream and eventually to drinking water sources and recreational waters.

    The top stormwater pollutants are nutrients, sediment and metals. These pollutants are harmful to the environmental and human health. The top polluting nutrients: nitrogen and phosphorus are found in household products and agricultural practices. Pollutants like sediment and metals in stormwater are often due to construction activities, agricultural and industrial practices. The EPA classifies sources of pollution into two categories: non-point and point sources. Point source pollution describes pollution that can be traced back to an identifiable source, often a sewer overflow or illegal discharge. Properties found guilty of polluting stormwater are labeled as point sources and can be held legally and financially responsible for the pollution and all repairs. Once point sources are identified, the pollution is stopped, and the ecosystem can be repaired. Unfortunately, most pollutants are generated from non-point sources which means the source is unidentifiable, and therefore it is difficult to designate accountability and eliminate the pollution.

    To regulate this, the federal government founded the Environmental Protection Agency (EPA). The EPA creates laws and regulations related to human health, specifically the Clean Water Act (CWA) which includes water regulations for surface waters, drinking water, stormwater and wastewater. The CWA establishes the National Pollutant Discharge Elimination System (NPDES) that is intended to control the discharge of pollutants into the waters of the U.S. and is divided into two phases. Phase 1 targets municipal storm sewer systems (MS4s) serving over 100,000 people and holds local governments accountable for stormwater discharges. Also included are stormwater management expectations for any construction activity that disturbs over five acres of land. Phase 2 of the CWA was released in 1990 and regulates stormwater practices of MS4s serving populations of 50,000 or more and any construction activity that disturbs one or more acres of land. The NPDES requires site-specific stormwater pollution prevention plans, sediment and erosion control plans for construction sites and stormwater management programs for MS4s.

    MS4s are often, but not exclusively municipally owned. MS4s may also include universities, military bases, prisons and more. Through the NPDES, these MS4s have the power to establish regulations of stormwater discharges in their jurisdiction including required maintenance, inspections and civil penalties.

    While stormwater is initially regulated on the federal level, states and local governments are allowed to establish more stringent guidelines. Federally, stormwater management and pollutant expectations are largely established for various industrial practices from agricultural, forestry, mining and construction as well as municipalities. States have also set stormwater management and pollutant level expectations that are more specific to the climate and state practices. There may also be local regulations established by the municipality in your area. If you are interested in the allowed pollutant levels in your area, check out our state and local compliance pages.

    Stormwater management is a relatively new topic with new technologies emerging constantly. Much of the stormwater infrastructure that was originally implemented around the 1970s, is aging out of compliance. Stormwater systems have an expected life of 50-70 years with proper maintenance.

    Many states and municipalities have created a “Stormwater Best Management Practices Manual” to guide engineers, builders and property owners through local stormwater management. Stormwater control measures (SCMs), also called best management practices (BMPs), that are most appropriate for the local climate are outlined in these manuals as well as water quality and quantity expectations per asset. Engineers are aware of these local regulations and therefore design sites to comply by incorporating the most applicable SCMs.

    In areas that have been designated high risk, regulations may be more stringent to protect the local ecosystems. During the pre-construction phases, engineers and builders are aware of these regulations and build each site to comply. Post construction maintenance of stormwater systems becomes imperative as SCMs are designed to collect pollutants; they must be regularly cleaned of the pollutants in order to function effectively.


    Sediment is the most common stormwater pollutant in many states, by volume. Due to construction activities, loose soil and erosion sediment is suspended in stormwater, often measured as Total Suspended Solids (TSS). Additionally, sediment carries with it many pollutants. SCMs are designed to collect sediment through filter systems and settling techniques. Especially on construction sites, erosion and sediment controls are implemented to prevent extensive sediment pollution.


    The presence of nitrogen and phosphorus in stormwater is often due to excess fertilization of vegetation from both residential and industrial practices. These nutrients are beneficial in many areas, but concentration levels quickly compound with negative effects. Nitrogen is the hardest pollutant to remove from stormwater as it is water soluble. However, plants naturally remove nitrogen from ground water and release it into the atmosphere through a process called denitrification, making green infrastructure the ideal choice for stormwater management.

    As nitrogen and phosphorus encourage plant growth, using vegetation to collect the stormwater before the pollutants reach downstream bodies of water is crucial. Excessive nitrogen and phosphorus in water bodies feeds algal blooms that can be toxic to humans and animals. Algae also blocks sunlight from reaching the bottom of ponds, and during its decomposition process, decreases dissolved oxygen in the water, killing beneficial underwater vegetation and potentially aquatic life.


    Metals in stormwater have become an increasing concern of the EPA. Urbanization has increased the concentration of metals in stormwater as runoff travels through industrial sites. Clay and organic matter are often utilized in SCMs as they can retain dissolved metals removing them from stormwater, this is referred to as the cationic exchange capacity (CEC) of the soil. Perfluoroalkyl and polyfluoroalkyl substances or PFAS are widely used chemicals that break down slowly over time. Learn more about PFAS regulations in this webinar focusing on the EPA’s proposed regulations.


    Eventually the pollutants compound inside the SCM and impede the asset’s ability to collect additional pollutants. Regularly maintaining the SCMs on your property prevents the assets from failure due to inundation.

    If you are interested in learning more about stormwater management and regulations in your area, request a FREE consultation today. AQUALIS is the leading provider of sustainable water management and engineering including stormwater, wastewater and drinking water. Positioned to provide nationwide assistance with management, inspections, design and compliance for your water assets.