In a natural landscape, soil and vegetation hold and clean rain and stormwater. In developed areas, however, much of the land has been paved over, and the soil itself is often compacted and impervious. Rainfall flows from our roofs to gutters and downspouts, over compacted lawns and driveways into roads, and down storm drains picking up pollutants along the way. Researchers have found runoff from developed land to be the leading cause of water pollution in urban areas (Loizeaux-Bennet 1999). In many older cities stormwater can overwhelm sanitary sewers, sending raw sewage as well as runoff into nearby waterways.
Common sources of stormwater pollution
Water running off developed landscapes is the leading cause of water pollution in urban areas (Loizeaux-Bennet 1999). Common sources of stormwater pollution include:
- Fertilizers, herbicides and insecticides –Excess nutrients from overuse of fertilizer can cause algal blooms when the fertilizer runs off with stormwater. When algae die, they sink to the bottom and decompose in a process that removes oxygen from the water. Fish and other aquatic organisms can’t exist in water with low dissolved oxygen levels. Insecticides and herbicides present in stormwater can poison aquatic life. Land animals and people can become sick from eating contaminated fish and shellfish or drinking pesticide-contaminated water. Homeowners can reduce the risk of these pollutants by minimizing the use of fertilizers and pesticides. Any excess fertilizers spread on driveways, sidewalks, streets or other impervious surfaces should be collected and properly disposed of before it has an opportunity to pollute stormwater.
- Animal waste – Feces contain bacteria and other nutrients that pollute our waterways and can harm human health. Animal waste should be removed from vegetative areas surrounding waterways or other areas where it may contaminate stormwater runoff.
- Road salt – Rock salt can leach into the soil, changing its chemical composition, and flow into local waterways where it can poison fish and aquatic organisms. It can also harm sensitive plants. Salt is highly corrosive to paved surfaces, buildings, and metal. Alternatives to rock salt include materials that increase traction, such as kitty litter and sand. For situations where a product that actually melts ice is required, look for rock salt substitutes such as those made with beet juice extracts, a byproduct of beet sugar production that would normally be disposed of as waste. Magnesium chloride, which is safer to use near plants than rock salt but not as effective in very cold conditions, is another possibility.
- Fluids from automobiles such as oil and gas and particles from brake linings, tires and engines – Cars, trucks and other automobiles deposit pollutants onto paved surfaces. The pollutants are washed off by rainfall and transported into storm sewers,often ending up in local waterways. Landscapes that capture and reuse the stormwater minimize the spread of pollutants and provide a valuable water source to the garden.
- Sediment from improperly managed landscapes – Sediment can cloud waterways and carry pollutants. Landscapes can minimize sediment pollution by preventing erosion and capturing stormwater and sediments on-site.
- Building materials such as copper and zinc roofs or gutters, galvanized materials and treated lumber – When replacing roofs, homeowners should consider materials such as wood shingles, slate or baked enamel painted metal products that typically release fewer pollutants. Green roofs are also an attractive option for buildings that can support the additional weight.
- Coal tar sealants used on paved roads – Coal tar sealants, better known as blacktop, the shiny black material applied to many driveways, are high in polycyclic aromatic hydrocarbons (PAHs), which contribute to pollution of many of the nation’s urban lakes. PAHs are toxic to aquatic life and several are suspected carcinogens. Blacktop is also impermeable. Sustainable landscapes use less toxic materials such as crushed seashells, which would otherwise be disposed of as a waste, or locally obtained gravel.