Many people in the water pollution field classify stormwater pollution as nonpoint source pollution, pollution that occurs from diffuse sources, such as rainfall on an urban business district or a farmer's field that runs off over a wide area into streams. In other words, nonpoint source pollution originates from everyday activities that take place in residential, commercial, industrial and rural areas and is carried by runoff to streams.

The federal stormwater regulatory program, however, views stormwater pollution as point source pollution. Usually point source pollution is considered to be pollution that enters streams from a specific point, such as a discharge pipe from an industrial process or a sewage treatment plant. Although stormwater pollution originates from diffuse sources, industries, businesses and cities frequently discharge stormwater through pipes and channels. These pipes and channels, as viewed by the federal program, are point sources for pollutants entering streams, rivers, lakes and oceans. Thus stormwater pollution originates from nonpoint sources and is frequently discharged as point sources through pipes and channels.

It is easy to think that since stormwater pollution is from nonpoint sources, many of which seem to be "natural," that this pollution is not harmful. On the contrary, the runoff from urban, commercial, industrial, residential, and rural areas contributes significant amounts of pollutants to our streams, rivers and lakes. These pollutants can have a large impact on the quality of a water body and can make it unfit for recreation or as a source for drinking water.

Development of land changes the land use from one use, such as forest or farmland, to another use such as residential, industrial or commercial use. The type of activities taking place with the new land use will change the type and amount of pollutants generated in the area and will alter the volume and rate of flow of stormwater runoff from that area. Impervious surfaces, such as roads, roofs and parking lots, collect pollutants and allow them to be more easily carried by runoff. The net effect is that pollutant export from a developed site can be increased many times over that which came from the site in its natural state. If stormwater pollution is not controlled, this increased pollutant loading can cause severe degradation of streams and can prevent normal uses of the water far downstream.

The following sections list common pollutants found in stormwater from urban and developing areas. Each section describes a type of pollutant, the pollutant sources, impacts on the receiving water, indicator parameters, methods to reduce the amounts of the pollutant entering stormwater and methods for removal. (Indicator parameters are certain characteristics of water or a water body that can indicate the presence of a pollutant without having to do a laboratory test for that specific pollutant. A good example of an indicator parameter is a rainbow-colored slick on the water's surface. The slick indicates that oil and grease are contaminating the stormwater. A follow-up laboratory test will confirm the oil and grease in the water.)

The impact of raindrops falling on exposed land surfaces dislodges pollutants, making them readily available for transport by flowing runoff. Flowing runoff may dislodge more pollutants or dissolve certain pollutants and carry them in its flow. Once pollutants are exposed to runoff, they are transported in two ways or phases: the dissolved or soluble phase and the sediment-bound or solid phase.

Soluble pollutants, those that are dissolved in water, are carried within the runoff water by a weak attraction to water molecules. These pollutants are the most mobile because they essentially become part of the runoff water and flow where the water flows. Generally, soluble pollutants can be removed only by chemical treatment (which is expensive) or through biological uptake by plants and soil organisms. Biological uptake will occur in ponds with extended holding times or permanent pools of water, and in soil when stormwater infiltrates into the soil.

Pollutants transported in the sediment-bound or solid phase are either attached to or are part of soil or dust particles, organic matter, or are solids themselves. For example, iron (a metal) can be transported while attached to soil particles or it can be transported as individual granular particles. The mobility of sediment-bound pollutants depends on the transport of the sediment particles.

Sediment particles themselves have two methods of transport. Smaller sediment particles are held up or "suspended" in water (cloudy or muddy water). Larger sediment particles roll or hop along the land surfaces or stream bottoms from the force of the moving water. Thus, sediment-bound pollutants may be either up in the water with the small suspended particles or in the bottom sediment with the larger particles.

The type of transport processes available will directly affect pollutant mobility--how far and how fast a pollutant moves. Pollutants that are dissolved or are associated with suspended particles will be transported much farther than those associated with larger particles.

The effectiveness of an actual stormwater management system is difficult to determine. In fact, the effectiveness of a single stormwater control device already in place is difficult to obtain by direct measurement. Needless to say, estimating the effectiveness of a not-yet-built system with accuracy is very difficult because each site has widely varying hydrological characteristics that affect the removal efficiency of each stormwater control practice. Because the effectiveness of each practice varies, the overall effectiveness of the system will also vary from site to site.

One of the best methods to estimate the overall effectiveness of a stormwater management system is to simply observe it when it is functioning. Logs of the observations and of other efforts such as source reduction and reduced exposure to rain help to evaluate the management system. All systems need to be inspected regularly basis. Inspection and maintenance are the keys to a properly functioning pollution control system.

For systems that are in the design phase, some literature is available as a guide to estimating the effectiveness of a stormwater management system. Estimates of pollutant removal are available for various practices; their actual effectiveness will vary depending on location, design, and construction of device.

For systems where the stormwater flows through the devices in series, the overall effectiveness can be estimated by combining the percentage removal for each device in the system. Keep in mind that the pollutant removal of one device influences the effectiveness of the next device. For stormwater control devices that stand alone, such as infiltration devices, the estimated removal effectiveness for that device can be used.

Another alternative, especially for a complex or expensive stormwater management system, is to construct a pilot scale model of the actual system and take measurements on the model during operation. This has inherent limitations in that small-scale models cannot accurately duplicate the conditions in the full-size system. However, a pilot-scale model can yield information which may save money and time in the long run.

Estimating the effectiveness of an already constructed and in-place system is also difficult. One technique is to sample the incoming and outgoing stormwater. The difference in the pollutant concentrations is the removal effectiveness. The various devices in the system can be observed and logs kept of the observations. The observations and records can help determine whether the system is functioning as designed. For instance, the records may show that a detention basin is holding stormwater too long so that new runoff enters before the pond has returned to its dry-weather state. (This condition lowers the volume of runoff captured by the pond.) The depth of sediment in detention and retention ponds should be checked regularly. When the depth of sediment reaches the volume set aside for sediment storage (sediment pool), the sediment should be removed. A stake set at the correct elevation of the top of the sediment pool provides an easy visual indicator of total sediment accumulation. Rapid sediment accumulation may indicate that the detention or retention pond is working better than expected by removing more sediment, or that the area draining to the pond is creating more sediment than expected. The drainage area should be inspected for disturbed areas, eroded channels, etc. To prevent too much sediment accumulating in the pond, the rate of sediment accumulation should be monitored. A laboratory analysis of the sediment in a stormwater control pond may give some indication of the removal of sediment-bound pollutants.

Visual inspection of filter strips and channels can determine the degree of erosion taking place. Sediment from runoff should be retained in the filter strip. If the sediment stains extend over the entire width of the strip, then sediment is flowing through and past the strip.

Good housekeeping is an inexpensive means to greatly improve the effectiveness of a pollutant removal system. Recycling programs may be used as an alternative to disposal of wastes.

A local expert with design experience is often the best resource for designing a stormwater management system. An experienced professional engineer should be able to choose the most cost-effective measures for the overall system that will meet the requirements for pollutant removal effectiveness.

Return to Table of Contents