Most people believe that water pollution is caused by pipes dumping toxic industrial waste into a river. But this type of pollution, called point source pollution, has largely been controlled by legislation such as the Clean Water Act, which was passed by Congress in 1970. In reality, a large amount of water pollution does not come from a single point. This type of pollution is called non-point source pollution.
Point source pollution is pollution that comes from a single source, such as a factory or wastewater treatment plant. The Clean Water Act put restrictions on how much and what kind of pollutants industries can dispose of in rivers and lakes. While this has not eliminated industrial or domestic waste from entering our waters completely, it has reduced what once was our biggest source of water pollution.
Many people incorrectly believe waste that is dumped into the storm sewer is treated at a waste water treatment plant. In fact, Denver has separate storm and sanitary sewer systems. Sanitary sewers collect wastewater from homes and businesses and treat it before discharging it into the river. Storm sewers, on the other hand, are a direct connection to the city's waterways. Anything dumped into a storm grate or gutter discharges - untreated - to a stream or lake at an outfall. Each outfall is considered to be a point source.
Denver's Wastewater Management Division is responsible for administering the city's municipal stormwater discharge permit, which sets goals for the reduction of pollutants discharging from stormwater outfalls. The Wastewater Management Division has instituted a series of programs to help reduce the discharge of pollutants from stormwater outfalls. These programs include reducing polluted runoff from municipal facilities and city streets, placing screens over stormwater outfalls, reducing pesticide and fertilizer use by city agencies and the public, and reducing polluted runoff from municipal and private construction sites.
Non-point source pollution does not have one specific source, such as a factory. Non-point source pollution comes from the cumulative effect of a region's residents going about their everyday activities, such as fertilizing a lawn or driving a car.
One type of non-point source pollution is fertilizer. Fertilizer contains nitrogen compounds called nitrates. When fertilizer is applied excessively or just prior to a rainstorm, it washes off the lawn and into the gutter, where it makes its way through the storm sewer system and into a river or lake. Once in the water, these nitrates have the same effect on algae as they do on lawns - they make it grow! Overgrown algae can have devastating effects on a lake or stream, consuming all the oxygen and suffocating fish and other aquatic wildlife. This is called
eutrophication. Increased awareness of alternative practices such as xeriscaping and using native grasses can help cut down on fertilizer use.
Other types of non-point source pollutants include pesticides, pet waste, motor oil, and household hazardous wastes. Again, any of these pollutants which get either washed or dumped into the storm sewer flow directly to a stream or lake without treatment.