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Impervious Surfaces
What Are Impervious Surfaces?
How Do They Contribute to Decreased Water Quality?
Runoff Borne Pollutants
Why Infiltration is a Good Thing

What Are Impervious Surfaces?

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Imperviousness refers to the inability of a surface to allow water to percolate through. A sponge is pervious, a countertop is impervious, cardboard is somewhere in between. Sandy soils are pervious; asphalt is not. On an impervious surface, water is forced to travel downhill until it finds a place it can sink into soil or enter a wetland. As it travels - or runs off - these impervious areas, water can pick up potentially toxic substances (like oil or fertilizer) and carry these materials to the sources of our water.

The important thing to know is what percentage of your watershed is covered by impervious surfaces. These include roads, parking lots, driveways, sidewalks, rooftops, patios, pools, and severely compacted soils (usually from development activities like grading, excavation, and landscaping). You may have noticed that the first three items - and by far the largest contributions to impervious surface area in almost any community - in this list are surfaces built for our automobiles.

In a natural landscape, the maximum amount of runoff occurs after the beginning of a storm or major melt. As impervious surface area increases, the storm and melt water coming off of them increases velocity, quantity, temperature, and pollution load. Any one of these attributes contributes to the degradation of natural hydrology and water quality. Noticeable degradation to water bodies begins when the watershed reaches 10-20% imperviousness.

How Do They Contribute to Decreased Water Quality?

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In urbanized areas, we have been forced to deal with runoff water by building large sewer systems that channel this water directly to lakes, rivers, and other surface water rather than into the groundwater. Because of the toxins this runoff picks up as it travels, expensive water purification systems are often built to cleanse the water before it reenters the natural water cycle. As water runoff increases and is channeled to travel in straight paths, a watershed community will find:
  • Increased erosion
  • Increased toxic load
  • Increased sediment load
  • Increased temperature
  • Increased flooding
  • Diminished groundwater reserves
  • Diminished activity in feeder streams
  • Diminished areas for habitat
  • Diminished fish populations
  • Diminished surface water levels
  • Fewer areas for recreation
  • Diminished overall water quality

Runoff Borne Pollutants

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This table shows common pollutants borne from runoff and their major sources. The impacts of these pollutants vary:

E Coli: High concentrations can prevent swimming, boating, and other recreational activities.

Sediments: Fills in ponds and reservoirs with mud; contributes to decline of submerged aquatic vegetation by increasing turbidity and reducing the light available for photosynthesis. Acts as a sink for nutrients and toxicants and as a source when disturbed and re-suspended. Makes water appear muddy, decreasing its recreational value.

Total Phosphorus: A contributing factor in eutrophication (nutrient over-enrichment) in receiving waterbodies and subsequent algal blooms. Algal blooms contribute to the decline of submerged aquatic vegetation by reducing the light available for photosynthesis, further degrade water quality by decreasing the level of dissolved oxygen, and may cause changes in the composition of plankton and fish species.

Zinc: Toxic to aquatic life and can contaminate drinking water.

Cadmium: Can be bioaccumulated; creates toxic health hazards within the food chain and increases long-term toxic stress for the entire ecosystem.

Copper: An important trace nutrient, it can be bioaccumulated and, thereby, create toxic health hazards within the food chain and increase long-term ecosystem stress.

Why Infiltration is a Good Thing

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The way water moves through soil is called infiltration. Infiltration is the way underground water sources are replenished. Infiltration also helps maintain water quality because many soils and plants filter out certain pollutants as water moves through them. An impervious surface is one that does not allow water to infiltrate to the soil layer.

Infiltration maintains the natural hydrology and biology of the watershed, especially the headwaters.
When infiltration is not allowed to occur, a small stream may carry little or no water when it is not raining because there is so little stored groundwater to provide continuous flow. This means that everyone that counted on this stream for life, recreation, habitat, food, or water must find a new source.
Infiltration recharges water-bearing aquifers.It may take decades and sometimes centuries for rain or snow melt to reach the deep layers of the earth from which many private and municipal wells draw water for human consumption. However, that water does come from the surface. Diversion of surface waters eventually affects the quantity of groundwaters.
Infiltration improves the quality of the water passing through.
Earth, with its wetlands and soils, is the original and still the largest water filtration system around.
Infiltration protects recreational interests.As impervious areas increase, streams become "flashy", which tends to erode stream banks, water levels drop significantly in the summer, water temperature increases thereby reducing the variety of fish. Waters will also likely have higher levels of pathogenic organisms and toxic chemicals. None of this makes for good recreation and tourism.
It protects downstream areas from flooding.When headwater streams become "flashy" so do the next larger order of streams below them, and the rivers below those. This results in more severe flooding. Infiltration allows large quantities of water to be stored in the ground and released slowly - long after the storm passes.

The goal should be to emulate as much as possible the natural stream and groundwater hydrology of your area. This means retaining stormwater runoff on site and provide for vegetative filtering before the runoff reenters the watershed, thereby limiting the pollutants it has picked up.

In general, the more porous the soil, and the more heavily planted the landscapes, the less water runs off and more infiltrates. The less porous the soil and the more hard surfaces exist on the landscape, the more water runs off.

Techniques:
Paved surfaces are easiest to reduce when new construction is planned. When installing a new driveway or sidewalk, consider a gravel bed, bricks and flagstones, bricks or interlocking pavers, crushed shells or stone and bark chips. These porous surfaces encourage some infiltration of runoff. Consider the design and layout of roads and pathways. Roads and pathways can also be designed with small berms, terraces or holding areas to trap storm water --especially on slopes. Designs with curves and designs which reduce standard roadway widths offer water quality benefits.

These design practices can be encouraged through regulations and ordinances as well as thoughtful Site Plan Reviews. Master Plans and Capital Improvement Programming should take into account these factors and cluster new developments in appropriate areas*.

Constructed wetlands are increasingly being used by communities for the treatment of stormwater. A natural wetland may not have the appropriate hydrologic flow to enable it to effectively manage and treat increasing quantities of stormwater from runoff. A jurisdictional wetland (one that requires a DEQ permit review for alteration - generally those adjacent to a surface water body of 5 acres or larger and located within a county with a population greater than 100,000) may require an expensive impact analysis to determine the harmful effects. There are several good examples of municipal constructed wetlands in the State of Michigan (Vermontville and Houghton Lake, for example).
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