Water quality benefits of Green Stormwater Infrastructure

Water quality benefits are defined by the effectiveness of green stormwater infrastructure (GSI) practices ( best management practice) to attenuate pollutants that are in stormwater runoff. All GSI practices provide water quality benefits. These benefits vary between each practice, primarily as a result of the mechanism by which pollutants are attenuated.

• Constructed ponds and wetlands remove pollutants through sedimentation. This removes medium- to large- diameter particles and pollutants attached to those particles.
• Filtration practices include bmps that have an underdrain (biofiltration, permeable pavement, tree trenches, 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

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.

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.

Tree Trench

Tree trenches provide improvements to water quality by capturing, storing, and filtering stormwater runoff from storm events. Trees help slow down and temporarily store runoff and reduce pollutants by taking up nutrients and other pollutants from soils and water through their roots. As a result, trees transform pollutants into less harmful substances. The growth of tree roots helps increase soil infiltration capacity and rate allowing stormwater runoff to infiltrate into the ground rather than runoff into the adjacent storm sewer system or nearby water resources. Additionally, trees take up trace amounts of harmful chemicals, including metals, organic compounds, fuels, and solvents from the soil. This is called Phytoremediation. Inside the tree, these chemicals may be transformed into less harmful substances, used as nutrients and/or stored in roots, stems, and leaves³.

Bioretention

Bioretention BMPs (raingardens, bioswales, etc.) can have a profound impact on water quality. They allow water to pool for a period of time and then drain through infiltration. They effectively trap silt and other pollutants such as Total Suspended Solids (TSS), metals and hydrocarbons, Total Phosphorus (TP) & Total Nitrogen (TN), and bacteria, which are normally carried in runoff from impermeable surfaces. A study of two bioretention facilities at the University of Maryland campus were monitored and showed water quality improvements. Several storm events, all of the runoff flow was attenuated by the bioretention media and no flow exited the cells, resulting in zero pollutant discharge⁴. In all cases, the median pollutant output was lower than the input, indicating successful water quality improvement through the bioretention media. Median values for percent removals based on combined data sets were 47% for TSS, 76% for TP, 57% for copper, 83% for lead, 62% for zinc, and 83% for nitrate. It should be noted that the soil media used to build bioretention facilities should be taken into consideration. Some media can contain high levels of phosphorus and lead to a low performance for phosphorus removal. Therefore, it critical to make a proper media selection in order to achieve high phosphorus sequestration.

Permeable Pavement

Permeable pavement 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. Many of the pollutant concentrations that can be reduced from permeable pavement include Total Phosphorus, Dissolved Phosphorus, Total Suspended Solids, and Chlorides. Additionally, permeable pavements can reduce the need for road salt⁶.

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.

References

1. https://cfpub.epa.gov/si/si_public_record_report.cfm?dirEntryId=291092
2. https://www.sciencedirect.com/science/article/pii/S0925857408002024
3. https://www.epa.gov/sites/production/files/2015-11/documents/stormwater2streettrees.pdf
4. http://online.liebertpub.com/doi/abs/10.1089/ees.2006.0190
5. https://www.usgs.gov/science/evaluating-potential-benefits-permeable-pavement-quantity-and-quality-stormwater-runoff?qt-science_center_objects=0#qt-science_center_objects
6. https://www.epa.gov/soakuptherain/soak-rain-permeable-pavement
7. https://drum.lib.umd.edu/bitstream/handle/1903/9237/Jamil_umd_0117N_10107.pdf?sequence=1&isAllowed=y
8. https://nepis.epa.gov/Exe/ZyNET.exe/200044AG.TXT?ZyActionD=ZyDocument&Client=EPA&Index=1995+Thru+1999&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D%3A%5Czyfiles%5CIndex%20Data%5C95thru99%5CTxt%5C00000015%5C200044AG.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h%7C-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=hpfr&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results%20page&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL#
9. https://www.cdc.gov/safewater/sand-filtration.html