Impervious Surfaces
As more land is covered by impervious surfaces, more polluted runoff enters our rivers, streams and the Bay.
Impervious surfaces are paved or hardened surfaces that do not allow water to pass through. Roads, rooftops, sidewalks, pools, patios and parking lots are all impervious surfaces. Impervious surface data is used to measure the rate of
development
across the watershed, as well as to identify high-growth areas and potential sprawling development patterns.
How are impervious surfaces a pressure on the Bay?
Impervious surfaces that replace natural, vegetated areas do not allow precipitation to soak into the soil. Instead, water runs off the hardened surfaces and into sewers, local rivers, streams and the Bay.
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As water runs across impervious surfaces, it picks up pollutants like
nitrogen,
phosphorus
and
chemical contaminants
that can harm aquatic life.
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Impervious surfaces increase the amount and speed of runoff flowing into storm drains and local streams, which can alter natural stream flows. Fast-moving runoff can also
erode
stream banks and beds, sending more water-clouding sediment into streams, rivers and the Bay.
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Stormwater that is diverted into storm drains usually bypasses wetlands and riparian forest buffers that naturally slow and filter runoff.
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Increased runoff from impervious areas can also cause flooding.
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Hardening surfaces limits the amount of rain that is able to seep into the soil and replenish groundwater supplies, which are an important source of drinking water in many communities.
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Impervious areas that replace plants and soil are not able to absorb and break down airborne pollutants.
As more land is covered by impervious surfaces, more polluted runoff enters our rivers, streams and the Bay. When as little as 10 to 15 percent of a small drainage area is impervious, there are measurable impacts on
water quality
and aquatic species. As development and urbanization have increased,
stormwater has become the fastest growing segment of pollution in the Bay watershed.
How has impervious cover increased in recent years?
Between 1990 and 2000, impervious cover increased by nearly 250,000 acres—about 41 percent, or the size of five District of Columbias. During that same time period, the Bay watershed's
population grew by just 8 percent. This increase in impervious cover indicates that our personal footprint on the landscape is growing.
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Since the 1970s, the number of people per household has steadily decreased, while the average home size and number of vehicles per household has risen.
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“Big-box” stores and shopping centers common to urban and suburban lands have also created large areas of impervious cover. A typical “big-box” store consumes about 2 acres of land, with an additional 7 to 9 acres for parking.
At the current rate of change to impervious cover—nearly 25,000 acres per year—the watershed is estimated to have about 1.1 million acres of impervious surface by 2010.