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Stormwater Unit :: What is Stormwater?

What is Stormwater?
Why be Concerned about Stormwater?
Controlling Stormwater
Types of Pollutants Commonly Found in Stormwater Runoff
How Stormwater Is Carried to Surface Waters
Public Awareness of the Potential Impacts of Stormwater Runoff
Existing Programs to Control Stormwater Runoff
Types of Activities that Citizens be involved with to Help Control Stormwater Pollution

What is Stormwater?

Stormwater: The flow of water that results from precipitation and which occurs immediately following rainfall or as a result of snowmelt.

When a rainfall event occurs, several things can happen to the precipitation. Some of the precipitation infiltrates into the soil surface, some is taken up by plants, and some is evaporated into the atmosphere. Stormwater is the rest of the precipitation that runs off land surfaces and impervious areas.

Stormwater discharges are generated by precipitation and runoff from land, pavements, building rooftops and other surfaces. These hardened surfaces are called ‘impervious surfaces’ and they do not allow rainfall to infiltrate into the soil surface like natural vegetation, so more of the rainfall becomes stormwater runoff. Storm water runoff accumulates pollutants such as oil and grease, chemicals, nutrients, metals, and bacteria as it travels across land. Heavy precipitation or snowmelt can also cause sewer overflows that may contaminate water sources with untreated human and industrial waste, toxic materials, and other debris.

Why be concerned about Stormwater?

Why be concerned about Stormwater? Stormwater runoff can have a number of impacts. As development and imperviousness increase in an area, the natural capacity of the soil and vegetation to infiltrate and take up rainfall decreases, and more rainfall becomes stormwater runoff. This can produce negative impacts by causing erosion of land areas and stream banks, by causing or increasing flooding and also by carrying pollutants to surface waters. As North Carolina grows, development increases. When more houses, roads and businesses are constructed, water has nowhere to go and can cause serious drainage, pollutant, and sanitation problems. The following diagram demonstrates how continued development causes:

Increased Imperviousness
Increased Runoff
Increased Pollutants
Impact to Streambanks
Erosion/Sedimentation

Natural Runoff

Built Up Development Runoff

Control Measures Applicable in Managing Potential Pollutant Loading from Stormwater Runoff

Mechanisms for controlling stormwater runoff impacts can be grouped into a couple of categories of activies:

Preventative Measures – these measures work to reduce the impacts of stormwater runoff through changes in design, operation, or management to minimize or prevent the generation of runoff and the contamination of runoff from pollutants. Preventative measures include land use management practices and source reduction practices. Land use management practices use methods to best plan the way to locate land uses within a jurisdictional area or project site to avoid environmental impacts. Source reduction practices focus on locating the sources of pollutants and implementing design and operational changes that minimize or completely remove these sources. Preventative measures can be very efficient and effective since they are implemented to keep pollutants from ever getting into stormwater.
Control Measures – these are devices that are put in place to capture stormwater flows and provide pollutant removal through filtering, infiltration, detention, or some related process. These measures may be limited in their ability to efficiently remove some pollutants and may be fairly costly. Control measures also require commitment to long-term operation and maintenance to assure that the measures continue to function properly.

Stormwater Control
Stormwater Wetland

Stormwater Control
Sand Filter

Stormwater Control Diagram
Bioretention Plan

Stormwater Control
Constructed Bioretention Area

Pollutants Commonly Found in Stormwater Runoff and Their Impacts

The Table below summarizes common stormwater pollutants and also provides information on potential sources of these pollutants and types of impacts they may cause.

Sediment	Sediment is often viewed as the largest pollutant load associated with stormwater runoff in an urban setting. The loadings have been shown to be exceptionally high in the case of construction activity. Sediment is associated with numerous impacts in surface waters including increased turbidity, effects on aquatic and benthic habitat and reduction in capacity of impoundments. A number of other pollutants often attach to, and are carried by, sediment particles.
Nutrients	The nutrients most often identified in stormwater runoff are phosphorus and nitrogen. In surface waters, these nutrient loads can lead to heavy algae growth, eutrophication (especially in impoundments) and low dissolved oxygen levels. Nutrients enter the urban system in a variety of ways, including landscaping practices (commercial and home), leaks from sanitary sewers and septic systems, and animal wastes.
Organic Matter	Various forms of organic matter may be carried by stormwater in urban areas. Decomposition of this material by organisms in surface waters results in depleted oxygen levels. Low levels of dissolved oxygen severely impact water quality and life within surface waters. Sources of organic matter include leaking septic systems, garbage, yard waste, etc.
Bacteria	High bacterial levels may be found in stormwater runoff as a result of leaking sanitary systems, garbage, pet waste, etc. The impacts of bacteria on surface waters may affect recreational uses and aquatic life as well as impose health risks.
Oil and Grease	Numerous activities in urban areas produce oil, grease, and lubricating agents that are readily transported by stormwater. The intensity of activities, including vehicle traffic, maintenance and fueling activities, leaks and spills, and manufacturing processes within an urban setting contribute heavily to the level of these pollutants present in adjacent surface waters.
Toxic Substances	Many toxic substances are potentially associated with urban stormwater including metals, pesticides, herbicides and hydrocarbons. Toxic compounds may affect biological systems, and accumulate in bottom sediments of surface waters.
Heavy Metals	Heavy metals such as copper, lead, zinc, arsenic, chromium and cadmium may be typically found in urban stormwater runoff. Metals in stormwater may be toxic to some aquatic life and may accumulate in aquatic animals. Urban sources of metals in stormwater may include automobiles, paints, preservatives, motor oil and various urban activities.
Temperature	Stormwater runoff increases in temperature as it flows over impervious surfaces. In addition, water stored in shallow, unshaded ponds and impoundments can increase in temperature. Removal of natural vegetation (such as tree canopy) opens up water bodies to direct solar radiation. Elevated water temperatures can impact a water body’s ability to support certain fish and aquatic organisms.

How Stormwater Is Carried to Surface Waters

Stormwater runoff may be carried through natural or manmade drainage ways or conveyance systems. In some cases stormwater runoff leaves a site spread out over a large dispersed area as “sheet flow.” It may also be conveyed through natural ditches, swales and natural drainage features. In most developing and urbanizing areas, stormwater is conveyed through a system of catch basins and pipes commonly referred to as a storm sewer system.

Public Awareness of the Potential Impacts of Stormwater Runoff

Public awareness is an important part of stormwater pollutant reduction. Unfortunately not everyone is currently aware that the decisions they make can have an impact on stormwater pollution. As an example, some people assume that stormwater runoff that enters a storm sewer system is being routed to some type of treatment process before entering our surface waters. In North Carolina, there is no pre-treatment of stormwater. Storm sewer systems are designed simply to capture the stormwater and convey it to the nearest surface water.

Existing Programs to Control Stormwater Runoff

In North Carolina there are a number of programs directly tied to the management and control of stormwater runoff in urban and developing areas. These include both federal and state programs to control stormwater runoff from new development activities near sensitive water areas. These sensitive areas include:

The Twenty Coastal Counties – to protect sensitive coastal waters
Water Supply Watersheds – to protect waters that are used as drinking water supply sources
High Quality Waters and Outstanding Resource Waters – to protect waters that have unique water quality or unique resources.
Nutrient Sensitive Waters – to reduce the inputs of nutrients from urban and developing areas.

There is also a federal program (the National Pollutant Discharge Elimination System (NPDES) Stormwater Program) that requires stormwater permits for point source discharges of stormwater from certain industrial activities and from large municipalities (population 100,00 or larger). These requirements were modified (November 1999) nationally to expand the municipal coverage to a large number of smaller local governments across the state as well.

Activities that Citizens can be Involved with to Help Control Stormwater Pollution

Many of our daily activities have the potential to cause stormwater pollution. Any situation where activities can contribute more pollutants to stormwater runoff is an area that should be considered in attempts to minimize impacts. The list below is certainly not all inclusive, but it gives an idea of things citizens can do to help control stormwater pollution.

Maintain buffer areas around stream segments to protect stream banks and to provide a mechanism for pollutant removal.
Minimize impervious areas to reduce runoff.
Design all new construction to prevent or minimize runoff and stormwater pollution – a major component here is planning up front in the design process to consider and manage potential stormwater problems.
Practice “good housekeeping” by keeping areas clean of potentially harmful pollutants. This also may involve changing activities or practices if they have potential impacts.
Use lawn care practices that protect water quality – minimize the use of fertilizers and pesticides, and when used, do so in a safe manner. When possible incorporate native plant species since they are best adapted to the local growing conditions and tend to be naturally pest resistant.
Properly use and store household materials and be aware of and make use of local recycling and collection centers to handle household wastes.
Remember that any materials that are poured or placed on the ground, streets, driveways, etc. can be picked up and carried by stormwater runoff to our surface waters
Report any pollution, illegal dumping, or soil erosion that you see to the appropriate authorities.
Get involved with local efforts for public education, water quality monitoring, stream cleanup, recycling, etc.

Bibliography

Controlling Urban Runoff: A Practical Guide for Planning and Designing Urban BMP’s, Metropolitan Washington Council of Governments, 1987.
How to Create a Stormwater Pollution Prevention Campaign, Environmental Health Coalition, 1992.
Site Planning for Stormwater Management, NC Division of Water Quality, 1998
Stormwater Best Management Practices, NC Division of Water Quality, 1999
Stormwater Fact Sheet for NPDES Municipal Stormwater Permits, NC Division of Water Quality, 1994.
Stormwater Management Fact Sheets, Land-of-Sky Regional Council, 1994 & 1995.
Stormwater Management in North Carolina: A Guide for Local Officials, Land of Sky Regional Council, 1994.