Improving Stormwater Quality

Introduction

Stormwater is rainfall or snowmelt that flows across land surfaces and constructed areas such as rooftops, driveways, parking lots, and landscaped grounds. As water travels across these surfaces, it can pick up soil particles, nutrients, bacteria, and other pollutants before moving into storm drains or waterways. In many locations, stormwater is not treated in a wastewater facility but instead enters streams, lakes, wetlands, or groundwater directly. Managing stormwater quality involves capturing, slowing, filtering, or storing runoff in ways that reduce pollutant movement while maintaining the natural hydrologic function of a site as much as feasible.

Pollutants of Interest in Stormwater

The types and concentrations of pollutants in stormwater vary with land cover, maintenance practices, climate, and surrounding activities. Sediment is one of the most common pollutants because exposed soils and disturbed areas readily erode during rainfall. Nutrients such as nitrogen and phosphorus are frequently transported from fertilized lawns, gardens, agricultural areas, and eroding soils. These nutrients contribute to algal blooms and impaired aquatic habitat in our water bodies. Bacteria from pet waste, wildlife, and failing sewage systems can affect swimming and recreation conditions. Metals such as zinc, copper, and lead originate from vehicle wear, building materials, and atmospheric deposition. Chloride used in de-icing remains a persistent pollutant of concern in colder climates, and microplastics and tire wear particles are now more commonly detected and monitored.

Fundamental Processes That Improve Stormwater Quality

Improvements in stormwater quality occur through multiple interacting processes whenever runoff is stored, slowed, or brought into contact with soil, engineered media, vegetation, and microbial communities. These processes do not act independently; rather, they overlap and reinforce one another depending on site conditions, design, and the amount of time runoff remains in contact with treatment surfaces. Each of these mechanisms is shown below in Table 1 and briefly explained.

Stormwater Control Measures

Stormwater Control Measures (SCMs), sometimes referred to as green infrastructure systems, Best Management Practices (BMPs), or low impact development (LID) practices, are designed to create environments where the pollutant removal mechanisms described above can occur. Bioretention systems and rain gardens guide runoff into a shallow depression filled with engineered soil and vegetation that encourage filtration, adsorption, microbial activity, and plant uptake. Permeable pavements allow water to pass through the surface into a gravel reservoir, where it can infiltrate into the underlying soil if conditions permit. Green roofs store rainfall in growing media and vegetation, reducing runoff volumes and delaying peak outflows. Constructed wetlands and retention ponds provide storage areas where sediments can settle, and biological processes can develop in water and soils over time. Sand and media filters treat runoff by routing water through layered media where filtration and chemical interactions occur, particularly where infiltration is limited by soil or site constraints. Cisterns and rainwater harvesting systems reduce the total volume of runoff by capturing precipitation for later use in irrigation or other non-potable applications.

Considerations Based on Site and Context

The performance of stormwater systems depends on site conditions such as soil permeability, groundwater depth, available space, slope, and the presence of utilities or structural features. Sites with well-drained soils may be suitable for infiltration-based systems, whereas sites with compacted clays, high groundwater levels, sensitive underlying geology, or contaminated soils may employ filtration, storage, or reuse practices that limit infiltration. Selecting an appropriate system involves evaluating these conditions carefully and designing the system to function reliably under expected storm patterns.

Maintenance and Long-Term Function

Stormwater systems require ongoing maintenance to sustain pollutant removal processes. Sediment and organic matter accumulate in forebays and inlets and should be removed periodically to maintain capacity. Vegetation must be monitored and managed to ensure that desired species remain established and hydraulic flow paths are preserved. Filter media may need to be restored or replaced after extended use. Underdrains, outlets, and overflow structures must remain clear to function as intended. Regular inspections allow for the early detection of clogging, erosion, or vegetation decline, thereby preventing reductions in system effectiveness.

Flow Management and System Performance

Stormwater systems must be designed to handle both the frequent storms targeted for treatment and the larger storms that exceed the system’s storage capacity. Designs typically include defined overflow structures that allow excess stormwater to pass safely without damaging the system or causing erosion. Providing stable conveyance, proper grading, and clear outlet structures ensures that treatment systems continue performing effectively even during variable conditions.

The following plots illustrate the removal efficiency of various SCMs in treating nutrients, suspended solids (sediments), and metals. These results are based on long-term monitoring data compiled under the International Stormwater BMP Database, a national research program supported by the Water Research Foundation (Clary et al., 2020). The database aggregates field performance data (influent and effluent concentrations) from hundreds of monitored SCM installations across the United States and Canada, submitted by universities, municipalities, state agencies, consulting groups, and research organizations. The SCMs evaluated were Detention Basins (DB), Retention Ponds (RP), Wetland Basins (WB), Wetland Channels (WC), Grass Swales (GS), Grass Strips (GS), Bioretention (BR), Media Filter (MF), High Rate Biofiltration (HRBF), High Rate Media Filtration (HRMF), Hydrodynamic Separation Devices (HDS), Oil/Grit Separators and Baffle Boxes (OGS), Permeable Friction Course (PF), and Porous Pavements (PV).

The monitoring results demonstrate that no single SCM consistently removes all pollutants; rather, each SCM performs best for particular pollutant types and under conditions that allow adequate storage time and contact with soil or engineered media. Practices that retain or infiltrate water through soil or media, such as bioretention systems, permeable pavements with infiltration capacity, and gravel wetlands, generally show the highest removal of suspended solids and particle-bound pollutants, including particulate phosphorus and many metals. This is because filtration, sedimentation, and adsorption processes become more effective as water moves slowly through media with high surface area.

Nutrient removal, especially for nitrogen, is more variable. Systems with sustained saturation zones that promote microbial transformation, such as gravel wetlands or bioretention cells designed with internal water storage, tend to show greater nitrate removal through denitrification. In contrast, SCMs that only provide surface-level filtration or short residence times may show limited nitrogen reduction. Phosphorus performance depends strongly on media composition, with engineered soils containing iron-, aluminum-, or calcium-based amendments often showing better results than unamended mixes.

Practices that primarily provide storage or open-water settling, such as wet ponds and constructed wetlands, tend to perform well for suspended solids and metals but show inconsistent nutrient removal, and in some cases can re-release nutrients during warm seasons or turnover events. Green roofs contribute most to runoff volume reduction and peak delay, but their nutrient removal capabilities are limited, and leaching may occur if the media is not properly selected or aged. Dry detention basins show low pollutant removal overall and are best described as conveyance or pretreatment systems unless retrofitted with media or filtration components. Overall, practices that maximize contact time, subsurface storage, and media interaction consistently demonstrate the most pollutant removal.

Though a great deal of variability is shown in the data reported herein, it is clear that those LIDs that bring the stormwater in contact with the soil were most effective in removing pollutants from the stormwater runoff. The improvements in water quality were somewhat less when the stormwater came in contact with soil-like media or mulch. Collecting the stormwater into water-based systems, such as wetlands and wet basins, was less effective, often because of limited contact time with the microorganisms present. The least effective methods of stormwater pollutant removal were those treatment LIDs that did not expose the stormwater to soil, media, or a wet environment.

Summary

Stormwater runoff can carry sediment, nutrients, bacteria, metals, and other pollutants into nearby waterways, especially where natural infiltration has been reduced by development. Improving stormwater quality depends on slowing runoff and providing time for filtering, settling, adsorption, plant uptake, and microbial processing. Stormwater control measures (SCMs), such as bioretention areas, permeable pavements, constructed wetlands, swales, and filtration systems, create conditions that allow for the occurrence of physical, chemical, and biological processes. However, each SCM performs differently depending on site conditions, design, and maintenance, and no single practice addresses all pollutants or objectives. Effective stormwater management involves selecting measures that fit the landscape and using them in combination to reduce pollutant loading, support runoff storage and infiltration where appropriate, and ultimately help protect watershed health over time.

References

Clary, J., Leisenring, M., Poresky, A., Earles, A., & Jones, J. (2020). International Stormwater BMP Database: 2020 Summary Statistics. Water Research Foundation.

Green Roofs: Benefits and Design Considerations. Penn State Extension.

International Stormwater BMP Database (2020). The Water Research Foundation.

Microplastics. Penn State Extension.

Rain Barrels: Information and Guide. Penn State Extension.

Rain Garden Resources from Penn State Extension. Penn State Extension.

Rain Gardens (BioRetention Cells) - a Stormwater BMP. Penn State Extension.

Roadside Guide to Clean Water: Porous and Permeable Paving Materials. Penn State Extension.