Water quality for a particular waterway is the current condition of the water in that ecosystem. Each waterway has different capacities of handling (adapting and compensating) for natural changes in the environment such as heavy rain or leaching from soil, or unnatural changes imposed by pollution.
 

Pollution is generally divided into two categories:

POINT Source Pollution - comes from a specific distinguishable point such as industrial effluent from a pipe.

NON-POINT Source Pollution - comes from an area-wide source such as runoff from rural or urban areas that carries a whole array of pollutants.
 

POINT sources of pollution are regulated by state Pollution Discharge Elimination System (PDES), a permit system for discharge of these pollutants. A PDES permit must be obtained before an owner or operator of any wastewater system can legally discharge sanitary industrial or commercial wastewaters into natural waters. Such a permit contains a list of substances the permit holder may legally discharge in controlled amounts. The amounts are determined by projecting the effects of the discharge. They set limits, which they expect will maintain water quality standards.

Water quality standards depend on the classification of the waterway. Each waterway is classified according to their "best use" and is determined by several factors such as stream flow, existing water quality, and past, present, and desired future uses of the waterway. Standards and classifications differ by state but in general there are classifications for drinking, swimming, fish propagation, and fishing.

For more information on water quality criteria established by the EPA, click on:http://www.epa.gov/waterscience/criteria/
 

NON-POINT sources of pollution are becoming relatively more significant as point sources are brought under control. The type of pollutant associated with certain land uses is outlined in the "Land Use and Possible Pollution Sources" chart. Generally one of the most important categories of non-point source pollution is urban runoff.