Although some kinds of water pollution can occur through natural processes, it is mostly a result of human activities. We use water daily in our homes and industries, about 150 gallons per day per person in the United States. The water we use is taken from lakes and rivers, and from underground (groundwater); and after we have used it-- and contaminated it-- most of it returns to these locations.
The used water of a community is called wastewater, or sewage. If it is not treated before being discharged into waterways, serious pollution is the result. Historically, it has taken humanity quite a bit of time to come to grips with this problem. Water pollution also occurs when rain water runoff from urban and industrial areas and from agricultural land and mining operations makes its way back to receiving waters (river, lake or ocean) and into the ground.
Stormwater is a point source that contributes to water pollution. Because impervious surfaces (parking lots, roads, buildings, compacted soil) do not allow rain to infiltrate into the ground, more runoff is generated than in the undeveloped condition. This additional runoff can erode watercourses (streams and rivers) as well as cause flooding when the storm water collection system is overwhelmed by the additional flow. Because the water is flushed out of the watershed during the storm event, little infiltrates the soil, replenishes groundwater, or supplies stream baseflow in dry weather.
Pollutants entering surface waters during precipitation events is termed polluted runoff. Daily human activities result in deposition of pollutants on roads, lawns, roofs, farm fields, etc. When it rains or there is irrigation, water runs off and ultimately makes its way to a river, lake, or the ocean. While there is some attenuation of these pollutants before entering the receiving waters, the quantity of human activity results in large enough quantities of pollutants to impair these