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Where do nitrates come from?

Cycle of conversions of nitrates in aquatic environment

All aquatic organisms excrete wastes and aquatic plants and organisms eventually die.  These activities create ammonia.  Some bacteria in the water change this ammonia to produce nitrite which is then converted by other bacteria to nitrate. Nitrates (NO3-) are an oxidized form of nitrogen and are formed by combining oxygen and nitrogen.

Nitrates also come from the earth.  Soil contains organic matter, which contains nitrogen compounds.  Just like the ammonia in water, these nitrogen compounds in the soil are converted by bacteria into nitrates.

Although nitrates occur naturally in soil and water, an excess levels of nitrates can be considered to be a contaminant of ground and surface waters.  Most sources of excess nitrates come from human activity.  The source of excess nitrates can usually be traced to agricultural activities, human wastes, or industrial pollution.  

Nitrogen fertilizers have been applied to fields, yards, and golf courses to promote the growth of plants. Rainwater can wash nitrates in the fertilizers into streams and rivers or the nitrates can seep into the ground water. This runoff problem tends to be most serious when the fertilizer is animal waste or manure.

In addition to animal waste, untreated human sewage can contribute to nitrate levels in surface and ground water.  Leaking or poorly functioning septic systems are a source of such nitrates. City sewage treatment plants treat sewage to make it non-hazardous, but treatment plants still release nitrates into waterways.  In addition, industrial plants that produce paper or munitions are potential sources of nitrate pollution.

Although having excess nitrates is usually associated with some type of human activity, excess nitrates can come from natural sources. A good example of this is the presence of large numbers of birds that might live in or around a body of water.  Bird excretions that get into the water can create large amounts of nitrates. 

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