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Impervious Surfaces and Stormwater Impacts

Surfaces that don't allow for infiltration of stormwater runoff can have impact on our waterways.

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Updated:

August 16, 2024

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Large areas of impervious surface, like this photo from Montgomery County, PA, create runoff when there is not enough soil and vegetation to absorb precipitation. Beth Yount, Penn State

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What are impervious surfaces?

Impervious surfaces are hard, non-porous surfaces that cover the soil and do not permit water to pass through them. These are typically constructed of materials such as concrete, asphalt, brick, stone, or plastics and include roads and streets, sidewalks, driveways, parking lots, roofs, sports courts, swimming pools, decks or patios, and some types of synthetic turf. When soil surfaces are covered by impervious materials, it prevents the infiltration of water into soil and percolation into groundwater. As a result, rainfall and snowmelt flow in sheets across the surface rather than being absorbed into the ground.

What are the watershed impacts of impervious surfaces?

In natural, undeveloped environments, soil and vegetated surfaces are capable of absorbing and infiltrating rainfall and snowmelt, as well as the small amount of runoff that may occur. This precipitation moves through the soil slowly, taking a long and winding path that delays its return to surface waterways. Sediment and pollutants are removed from the water by natural physical, biological, and chemical processes.

Infiltration vs Runoff

When the ground is covered with impervious surfaces, rainfall, and snowmelt can’t soak in. Instead of being absorbed where it falls, precipitation runs across the impervious surfaces until it reaches a point where it can drain. This surface runoff flow picks up nutrients, trash, pollutants, and sediment along its path and rushes quickly into storm drains and nearby waterways, contributing to a large flush of pollutants and a rapid rise in water levels. When more than 10 to 20% of a watershed has impervious cover, that doubles the amount of runoff.

Runoff and nonpoint source pollution

The materials that runoff picks up from impervious surfaces are directly added to waterways through storm drains and other surface entry points. When pollutants enter waterways from multiple, undefined sources, we call this nonpoint source pollution, which is one of the most significant sources of water quality impairment. Stormwater runoff typically contains pollutants such as road salt, fertilizers, motor oil, tire crumbs, pesticides, metals, and soil particles, which pose a hazard to aquatic ecosystems by impairing their ability to function properly. For example, sediment can cause cloudiness that harms aquatic plants and animals by reducing access to sunlight, oxygen, or habitat that they require for survival. Road deicers can elevate chloride levels beyond the threshold where fish and other aquatic animals and insects can live or reproduce, which degrades the entire aquatic ecosystem. Poor water quality conditions reduce overall biological activity, biodiversity, and stream health. When streams have poor water quality, they are reduced to only the most pollution-tolerant species, including invasive species, that can survive under impaired conditions.

Rapid, large volumes of water running off impervious surfaces during storms erode stream banks, destabilizing and undercutting them and adding high sediment loads to the waterway. Flash flooding is an additional hazard associated with runoff.

Water that ponds and pools at low points covered with impervious surfaces can increase vector-borne diseases, such as West Nile Virus because these stagnant areas can allow mosquitoes and other insects to reproduce and thrive.

Heat and Climate

In addition to the role of impervious surface area on stormwater runoff, there is a temperature-based watershed impact from the heat island effect. This is when developed areas with a large proportion of parking lots, roads, sidewalks, buildings, and other impervious surfaces absorb and retain heat, causing a localized heat island. Measured temperatures are elevated in these areas compared to vegetated spaces. Not only does this increase the need for energy to cool buildings in these heat islands, but it also heats up stormwater as it runs across these hot surfaces, leading to increases in waterway temperatures. Elevated temperatures are harmful to many aquatic organisms at various stages of their life cycles. Most aquatic organisms have evolved to withstand a narrow physiological range of temperature, so routinely increasing the temperature of waterways, particularly those that may not have forested banks to help cool them, threatens aquatic life. As we experience a greater number of days with high temperatures, this threat increases.

Lack of Groundwater Recharge

Groundwater recharge is directly linked to the ability of precipitation to soak into the ground and trickle through to the water table. The groundwater reservoir stored in soils and rocks is then slowly released into streams. When the ground is largely covered by impervious surfaces, the opportunity for this recharge is significantly reduced. This leads to reduced stream volume and flow, which is a stream impairment.

What can you do to reduce the impacts of impervious surfaces?

Reduce impervious surfaces on your property

Increasing the amount of impervious surface on your property can cause increased flooding and hydrology issues on your property, as well as any downstream neighbors. Keep in mind that adding a project like a new concrete patio can result in flooded basements elsewhere. As you evaluate any planned projects on your property, avoid adding new impervious surfaces, such as decks, balconies, swimming pools, patios, and sidewalks, that will increase the overall percentage of impervious cover. Consider de-paving and replacing impervious cover with permeable pavers to maintain areas that allow water to pass through. Remove brick, stone, or concrete where appropriate and transition to areas planted with vegetation, including trees and shrubs. Encourage municipalities to incorporate these best practices in municipal codes to improve the stormwater impacts on your community.

Increase water storage and slow the flow

Capture and store precipitation where it falls on your property by adding rain barrels, downspout planters, cisterns, rain gardens, underground storage, and green roofs. Redirect outlets for downspouts or other drains onto vegetated surfaces rather than allowing them to drain onto impervious surfaces.

Minimizing impervious surfaces on your property and providing opportunities for stormwater to infiltrate or be stored onsite is a watershed-friendly practice that improves water quality and flood resilience for you, your neighbors, and your community.