Non-point source Large land area such as crop fields and urban areas that discharge pollutant into surface and underground water over a large area. See point source. Point source A single identifiable source that discharges pollutants into the environment. Examples are smokestack, sewer, ditch, or pipe. See non-point source. Source Any process or activity that releases a greenhouse gas, an aerosol, or a precursor of a greenhouse gas into the atmosphere.