Water quality benefits of Green Stormwater Infrastructure
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Water quality benefits of Green Stormwater Infrastructure

This page provides information on the water quality benefits of green stormwater infrastructure (GSI) practices ( best management practices). The water quality benefit of a practice is defined by its ability to attenuate pollutants from stormwater runoff and prevent them from reaching receiving waters. All GSI practices provide water quality benefits since that is their primary function.

These benefits vary between each practice, primarily as a result of the mechanism by which pollutants are attenuated.

Constructed ponds ( wet ponds) and wetlands ( stormwater wetlands) remove pollutants through sedimentation. This removes medium- to large- diameter particles and pollutants attached to those particles, though effectiveness varies with design of the practice. See Calculating credits for stormwater ponds. For more information on sedimentation processes, link here.
Filtration practices include bmps that have an underdrain ( biofiltration, permeable pavement, tree trench, swales, green roofs, and media filters) or bmps that trap sediments from flowing water (vegetated filter strips, swales, green roofs).
Infiltration practices remove pollutants by capturing runoff and infiltrating it vertically into underlying soil, the vadose zone, and groundwater. Attenuation occurs primarily through adsorption and filtering, though dilution in groundwater may also be a mechanism for reducing pollutant concentrations.

Practice	Water quality benefit	Notes
Bioretention and infiltration	●
Tree trench and tree box	●
Green roof	◔
Vegetated swale	◔
Vegetated filter strip	◔
Permeable pavement	◕
Constructed wetland	◑
Rainwater harvesting	◕
Level of benefit: ◯ - none; ◔; - small; ◑ - moderate; ◕ - large; ● - very high

1 Green infrastructure and multiple benefits
2 Pollutant removal percentages for bmps
3 Infiltration practices
4 Bioretention
5 Tree Trench
6 Permeable Pavement
7 Green Roofs
8 Water Re-use and Harvesting
9 Constructed Ponds and Wetlands
10 Swale with Check Dam (with and without underdrain)
11 Sand Filter
- 11.1 References

Green infrastructure and multiple benefits

Green infrastructure (GI) encompasses a wide array of practices, including stormwater management. Green stormwater infrastructure (GSI) encompasses a variety of practices primarily designed for managing stormwater runoff but that provide additional benefits such as habitat or aesthetic value.

There is no universal definition of GI or GSI (link here fore more information). Consequently, the terms are often interchanged, leading to confusion and misinterpretation. GSI practices are designed to function as stormwater practices first (e.g. flood control, treatment of runoff, volume control), but they can provide additional benefits. Though designed for stormwater function, GSI practices, where appropriate, should be designed to deliver multiple benefits (often termed "multiple stacked benefits". For more information on green infrastructure, ecosystem services, and sustainability, link to Multiple benefits of green infrastructure and role of green infrastructure in sustainability and ecosystem services.

Pollutant removal percentages for bmps

The adjacent table provides a summary of estimated pollutant removal for stormwater bmps. However, pollutant removal is a function of many factors, including design, construction, and maintenance of the BMP; quality of incoming stormwater; time of year; rainfall and watershed characteristics; and so on. The user is encouraged to read the section called Factors affecting pollutant removal.

Median pollutant removal percentages for several stormwater BMPs. Sources. More detailed information and ranges of values can be found in other locations in this manual, as indicated in the table. NSD - not sufficient data. NOTE: Some filtration bmps, such as biofiltration, provide some infiltration. The values for filtration practices in this table are for filtered water.
Link to this table

Practice	TSS	TP	PP	DP	TN	Metals¹	Bacteria	Hydrocarbons
Infiltration²	³	³	³	³	³	³	³	³
Biofiltration and Tree trench/tree box with underdrain	80	link to table	link to table	link to table	50	35	95	80
Sand filter	85	50	85	0	35	80	50	80
Iron enhanced sand filter	85	65 or 74⁶	85	40 or 60⁶	35	80	50	80
Dry swale (no check dams)	68	link to table	link to table	link to table	35	80	0	80
Wet swale (no check dams)	35	0	0	0	15	35	35	NSD
Constructed wet ponds^{4, 5}	84	50 or 68⁵	84	8 or 48⁵	30	60	70	80
Constructed wetlands	73	38	69	0	30	60	70	80
Permeable pavement (with underdrain)	74	41	74	0	NSD	NSD	NSD	NSD
Green roofs	85	0	0	0	NSD	NSD	NSD	NSD
Vegetated (grass) filter	68	0	0	0	NSD	NSD	NSD	NSD
Harvest and reuse	Removal is 100% for captured water that is infiltrated. For water captured and routed to another practice, use the removal values for that practice.

TSS=Total suspended solids, TP=Total phosphorus, PP=Particulate phosphorus, DP=Dissolved phosphorus, TN=Total nitrogen
¹Data for metals is based on the average of data for zinc and copper
²BMPs designed to infiltrate stormwater runoff, such as infiltration basin/trench, bioinfiltration, permeable pavement with no underdrain, tree trenches with no underdrain, and BMPs with raised underdrains.
³Pollutant removal is 100 percent for the volume infiltrated, 0 for water bypassing the BMP. For filtered water, see values for other BMPs in the table.
⁴Dry ponds do not receive credit for volume or pollutant removal
⁵Removal is for Design Level 2. If an iron-enhanced pond bench is included, an additional 40 percent credit is given for dissolved phosphorus. Use the lower values if no iron bench exists and the higher value if an iron bench exists.
⁶Lower values are for Tier 1 design. Higher values are for Tier 2 design.

Infiltration practices

Bioretention

A rain garden in a residential development. Photo courtesy of Katherine Sullivan.

Bioretention is an excellent stormwater treatment practice due to the variety of pollutant removal mechanisms, including vegetative filtering, settling, evaporation, infiltration, transpiration, biological and microbiological uptake, and soil adsorption. Bioretention can be designed as an effective infiltration / recharge practice, particularly when parent soils have high permeability (> ~ 0.5 inches per hour). Bioretention designed for infiltration ( bioinfiltration) removes 100 percent of pollutants for the portion of runoff water that is infiltrated, although there may be impacts to shallow groundwater. Bioretention designed as filtration ( biofiltration) employs engineered media that is effective at removing solids, most metals, and most organic chemicals. Removal of phosphorus depends on the media (link here). Links to water quality information for bioretention - [1]; [2]

The following design considerations can improve the water quality function of bioretention practices.

Maximize infiltration by designing with the maximum ponded depth that can be infiltrated in 48 hours, up to 1.5 feet (to protect vegetation). Where space allows, surface area can also be increased. Utilize multiple bioretention practices in series. On lower permeability soils where an underdrain is used, raise the underdrain to the maximum extent possible, allowing water stored in the bioretention media below the underdrain to drain in 48 hours. Use an upturned elbow in underdrained systems.
For bioinfiltration (bioretention without an underdrain), use a high organic matter media to maximize pollutant removal
For biofiltration (bioretention with an underdrain), use a media mix that does not export phosphorus or use an amendment to attenuate phosphorus.

See Calculating credits for bioretention

Tree Trench

Photo of the completed tree system for the Central Corridor Light Rail Transit project, St. Paul, Minnesota. Image courtesy of the Capitol Region Watershed District.

Tree trenches and tree boxes are an excellent stormwater treatment practice due to the variety of pollutant removal mechanisms including vegetative filtering, settling, evaporation, infiltration, transpiration, biological and microbiological uptake, and soil adsorption. Tree trenches and tree boxes can be designed as an effective infiltration / recharge practice, particularly when parent soils have high permeability (> ~ 0.5 inches per hour). Link to water quality information for tree trench/tree box - [3]

The following design considerations can improve the water quality benefits of tree trenches.

Maximize infiltration by designing with the maximum ponded depth that can be infiltrated in 48 hours, up to 1.5 feet (to protect vegetation). Where space allows, surface area can also be increased.
Utilize multiple bioretention practices in series.
On lower permeability soils where an underdrain is used, raise the underdrain to the maximum extent possible, allowing water stored in the engineered media below the underdrain to drain in 48 hours. Use an upturned elbow in underdrained systems.
For bioinfiltration (bioretention without an underdrain), use a high organic matter media to maximize pollutant removal.
For biofiltration (bioretention with an underdrain), use a media mix that does not export phosphorus or use an amendment to attenuate phosphorus.

See Calculating credits for tree trenches and tree boxes

Permeable Pavement

Example of a new retrofit permeable parking lot at the University of Minnesota

Permeable pavement are an excellent stormwater practice that allows for the absorption and infiltration of rainwater and snow melt onsite. It can reduce the concentration of some pollutants either physically (by trapping it in the pavement or soil), chemically (bacteria and other microbes can break down and utilize some pollutants), or biologically (plants that grow in-between some types of pavers can trap and store pollutants)(5). Permeable pavement functioning as an infiltration practice (no underdrain) effectively treats most pollutants, including dissolved pollutants. When an underdrain is employed, permeable pavement is effective at removing solids and pollutants attached to those solids. Additionally, permeable pavements can reduce the need for road salt (6).

The following design considerations may improve the water quality benefits of permeable pavement.

Ensure the subgrade is flat. Since roads are typically sloped, utilize terracing in the subgrade to achieve flat slopes. See page 5 of the North Carolina design guidance.
Incorporate signage into the design to ensure maintenance activities do not affect the infiltration properties of the pavement.
Design to maximize retention time and prevent short-circuiting. Storage may be increased by use of geotextile subgrades. An example is presented by Nnadi et al, (2014).
Plan for the expected loading on the permeable pavement and ensure capabilities and reduce compaction or clogging
Use in conjunction with other treatments to establish a treatment train or reuse water on site
Some research has been conducted into use of geotextiles and other amendments for enhancing water quality treatment. See Ostrom and Davis (2019) and Nnadi et al. (2014).

See Calculating credits for permeable pavement

Green Roofs

Green roofs have the ability to improve water quality, though the source of water should be taken into consideration, which can affect the effluent quality. Much of the water green roofs receive is in the form of precipitation which is less polluted compared to urban stormwater runoff. That being said, green roofs receive relatively clean water and are less likely to receive stormwater runoff and make a large impact on improving water quality. Nevertheless, green roofs can have a positive impact on water quality. A New York City (NYC) study monitored 4 green roofs over a period of 23 months and 100 storm events found that the pH runoff from green roofs was consistently higher than that from the roofs and precipitation with observed average pH’s equal to 7.28, 6.27, and 4.82 for the green roods, control roofs, and precipitations, respectively. As a result, green roods neutralized the acid rain. One thing to consider with green roofs is that influent Phosphorus (P) concentrations are lower than that of urban stormwater runoff. As a result, it is possible for green roofs to introduce P into their effluent rather than remove P since rainwater naturally has very low concentrations of P. The NYC study showed higher P concentration in green roof runoff than control roof runoff¹. The probable source of phosphorus to runoff from the green roofs is fertilizer and soil².

Water Re-use and Harvesting

Water re-use and water harvesting facilities can help improve water quality by capturing stormwater runoff and reduce offsite discharges into the storm sewer system and nearby water resources. The nature of stormwater re-use facilities vary widely, thus there is great variability in their effectiveness to remove storm water pollutants. Most water re-use projects have been developed to meet non-potable water demands, such as agriculture, landscape, public parks, irrigation, etc. In these cases, the stormwater that contains pollutants (phosphorus, sediment, excessive nutrient, metals, etc.) is captured for re-use and is reused for irrigation as opposed to being discharged to a body of water, possibly leading to the deterioration of that water body. Moreover, water re-use systems can be used to create or enhance wetlands and riparian (stream) habitats for streams that have been impaired or dried from water diversion. These systems are likely to remove more pollutants than those system which are used for the non-potable uses above.

Additional recommended reading

-Rainwater Harvesting

Constructed Ponds and Wetlands

Stormwater Ponds and wetlands are BMPs designed and constructed to settle out pollutants and nutrients (nitrogen, phosphorus, sediment, etc.) that are readily found in stormwater runoff. The removal of these pollutants helps prevent the degradation of downstream water bodies. It’s important to consider the timely and proper maintenance of stormwater ponds so that they function as designed. The lack of a proper maintenance plan for stormwater ponds can compromise their efficiency to remove pollutants and improve water quality. If not maintained properly, excess pollutants in ponds and wetlands may become sources of water quality issues as a poor water color/clarity/odor, low dissolved oxygen can lead to plant die off, and prevalence of algal blooms. Excess sediment can also clog structures within the BMPs impacting their overall effectiveness.

Additional recommended readings

-Stormwater Wet Pond and Wetland Management Guidebook

Swale with Check Dam (with and without underdrain)

Vegetated swales that are designed and constructed with an impermeable check dam with/without an underdrain can improve water quality through treatment of polluted stormwater runoff through sedimentation, filtration, and infiltration. A study performed by the university of Maryland found a reduction of mass pollutant loads for Total Suspended Solids (TSS) by 38% - 62%, nitrate by 92% - 85%, and nitrite by 54% - 71%⁷. For swales with underdrains, the soil mix design should be carefully selected to ensure that there is no net increase in the export of pollutants, primarily nitrogen and phosphorus. For example, filtration media mixtures that contain a large percentage of compost can potentially export nutrients, such as nitrate and phosphorus, instead of retaining them.

Additional recommended reading

-Tennessee Permanent Stormwater Management Design Guidance Manual: Vegetated Swale

-Guidelines for Evaluating and Selecting Modifications to Existing Roadway Drainage Infrastructure to Improve Water Quality in Ultra-Urban Areas

Sand Filter

Sand filters are an effective BMP for removing several common pollutants in stormwater runoff and can increase water quality to downstream water resources. Pollutants are removed through sedimentation and filtration processes. The highest pollutant removal efficiencies include sediment, biochemical oxygen demand (BOD), and fecal coliform bacteria. There is a moderate removal efficiency for total metals, a lower removal efficiency for nutrients⁸. Lab effectiveness studies have shown a reduction in 99.98% protozoan, 90% - 99% bacterial, and 80% - 98% E. coli removal rates, respectively⁹.

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