Lets take a look at a few pollution sources in our watersheds.
Factories/Industrial Plants - Many metals, oils and chemicals are used to make the things we buy in stores. Almost all factories have waste water that results from cooling and washing products and equipment. Small amounts of the substances used to make the products, clean the products and keep the equipment operating (e.g. oil used to keep parts moving smoothly) often end up in the waste water. This waste water is usually treated to remove the contaminants before it is discharged. The waste water usually ends up in a stream, river or other surface water body as point source pollution.
Wastewater Treatment Plants - The majority of waste water entering a treatment plant is sewage (bathroom waste) that comes from homes, offices, apartments, etc.... The organic matter in the sewage has a lot of salt, potassium, phosphorus and nitrogen in it. These things are nutrients to plants and other organisms; but untreated sewage has more nutrients in it than are "good" for a water body. Sewage also contains pathogens (disease causing organisms) that can make people and animals sick. Treatment plants are designed to remove a lot of organic matter. The more "high tech" systems also treat the water with chemicals to kill the pathogens. After treatment, the waste water is discharged to a surface water body through a pipe.
Unlined Landfills - Landfills, or what we usually call "dumps," are a significant non-point pollution source. For years trash has been dumped in landfills, covered with dirt, and literally left to rot. When rain enters these landfills, moving slowly down through the trash, it causes chemical reactions to take place. Some of these chemical reactions will make new chemicals, and some will change how easy it is for things like metals to dissolve in rainwater. The water will carry some of these metals and chemicals with it as it moves through the watershed.
Lawns and Gardens - Many people put fertilizers and pesticides on lawns and gardens to make them look and grow better. As with farms, a small amount of the fertilizers and pesticides are picked up by rain water and washed into nearby water bodies. The water that soaks into the ground also carries these pollutants down to the groundwater. Whichever way these pollutants travel from our lawns and gardens into our watersheds, it is a non-point source of pollution and one of the most difficult to pinpoint.
Leaking Septic Systems - Septic systems are private, below-ground sewage treatment systems. They typically consist of a holding tank and a leach field. Water from sinks and toilets goes to the settling tank, the solid material settles to the bottom, and the water then flows into the leach field. A leach field consists of a series of underground pipes with holes. The water comes out of the pipes in the leach field and slowly moves through the soil. The settling tank and the soil do the job of a treatment plant by removing excess nutrients and pathogens. This is a good way to treat our sewage unless the septic system isn't working right. In that case, excess nutrients and pathogens enter our watersheds as non-point source pollution.