Pollution can come from point sources, which include industrial drain pipes or discharge pipes from sewage treatment plants. Another type of pollution, which is difficult to control are from non-point sources. These include agricultural fields or lawns. The pollution from these areas comes from eroded sediments, fertilizers, pesticides, and animal wastes washed from farm fields and city lawns. Water can come into contact naturally with the land and interact with substances that dissolve in it like sodium, calcium, magnesium, potassium, bicarbonate, and chloride.
Water can also come in contact with runoff water from urban developments.
The flood waters after a rainstorm are collected into storm-sewer pipes and is drained directly into nearby surface waters. In some urban areas, as in large cities, water pollution results from inadequate sewer systems. Storm sewer pipes collect runoff water to prevent flooding, and channel it to the nearest river, lake, or ocean. During a rainstorm, pollutants such as fertilizers and pet droppings are carried into the storm-sewers and deposited to surface waterways. Most cities have a second and separate system of pipes which carry wastewater from homes and commercial buildings.