Sources. Nutrients enter stormwater from a variety of sources, including runoff from fertilized lawns and gardens, agricultural fields, leaks from sanitary sewers and waste water from septic tank systems. Intensively landscaped areas such as golf courses, industrial and commercial parks, and cemeteries can contribute large amounts of nutrients to stormwater.

Impacts. Excess plant nutrients in water cause a process called eutrophication. This process occurs as plant nutrients accumulate in a water body, causing increasingly heavier growths of plants and algae and reducing the amount of oxygen in the water available to fish and other animals. As eutrophication progresses, water bodies can experience algal scums and blooms, cloudy and discolored water, strong odors, lack of oxygen due to decay of the plant material, fish kills, production of toxins (poisons) by algae and bacteria, and disagreeable tastes and odors in drinking water. Although eutrophication occurs naturally at a slow rate, it can be greatly accelerated by stormwater pollutants, making the water body unfit for recreation and as a source of drinking water. Eutrophication is most readily seen in water bodies where the water does not flow through quickly, such as lakes and bays.

Indicator parameters. Overgrowth of algae and aquatic plants in lakes and streams may mean that nutrient levels are too high for that water. Bright green growth and long, stringy algae growing in channels or ponds may indicate high nutrients. If the conditions become extreme, algae blooms, unpleasant odors and tastes and fish kills will occur.

Source reduction. Nutrients in stormwater runoff can be reduced by limiting or eliminating the use of fertilizers on lawns, gardens and landscaped areas. Fertilizer should be applied only at the proper rate and at times during the growing season to reduce the amount lost to stormwater. If commercial landscaping services are employed to maintain an area, they can be instructed (possibly in the contract) to minimize the use of fertilizers and to apply at the proper rates and times of the growing season.

The installation of sanitary sewers in an area will eliminate nutrients leaching into water bodies from septic tank systems. Leaks in sanitary sewers and malfunctioning septic tank systems should be fixed promptly to prevent contamination of stormwater.

Removal. Nitrogen and phosphorus compounds occur in a variety of chemical forms. Under some conditions, certain chemical forms of nutrients are not dissolved and are bound to soil particles. Thus, some portion of nutrients can be removed by settling out the soil particles in a dry detention basin or a wet retention pond. However, most plant nutrients are in the dissolved phase in the stormwater, making them especially difficult to remove in detention basins and retention ponds.

Another removal method, which is effective for dissolved nutrients, is biological uptake by plants. Biological uptake occurs in wet retention ponds where aquatic plants consume the nutrients as part of the normal growing process. The plants should be harvested periodically to keep the nutrients from reentering the water when the plant dies.

Infiltration techniques also are effective at removing sediment-bound and dissolved nutrients. The filtering action of soil particles and plants and biological activities in the soil take up the nutrients as the stormwater moves through.

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