The standard measure of acidity is pH, and a pH value of 7 represents a neutral condition. Many biological processes, such as reproduction, cannot function in acidic or alkaline waters. Acidic conditions also aggravate toxic contamination problems because sediments release toxins such as metals in acidic waters. Common sources of acidity include mine drainage, runoff from mine tailings, and acid rain. It should be noted, however, that rain water is normally slightly acidic due to interactions of rain water with atmospheric carbon dioxide.
Hydrologic modifications alter the flow of water.
Examples of hydrologic modifications include channelization, draining, damming, and dredging. Other pollutants Other pollutants include salts and oil and grease. Fresh waters may become unfit for aquatic life and some human uses when they become contaminated by salts. Sources of salinity include irrigation runoff, brine used in oil extraction, road deicing operations, and the intrusion of sea water into ground and surface waters in coastal areas. Crude oil and processed petroleum products may be spilled during extraction, processing, or transport or leaked from underground storage tanks.