When water falls to the earth as rain or snow some of it seeps into the ground. However, if the ground is saturated, frozen or covered with impervious surface, excess precipitation flows over then land. This storm water runoff runs across the ground and picks up pollutants. This runoff is then depostied into the nearest water body where the water becomes polluted. This type of pollution is called non-point source. So every time it rains or snow melts, water carries pollutants into surface and ground water.