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Ammonia in Groundwater, Runoff, and Streams

Nitrogen is an essential nutrient that is required by all plants and animals for the formation of amino acids. In its molecular form (N2), nitrogen cannot be used by most aquatic plants, therefore it must be converted to another form before it can be utilized. One such form is ammonia (NH3). Ammonia may be taken up by plants or oxidized by bacteria into nitrate (NO3) or nitrite (NO2).

Ammonia (NH3) is a colorless gas with a strong pungent odor. Ammonia will react with water to form a weak base (NH4OH ––> NH4+ + OH). The term ammonia refers to two nitrogen species which are in equilibrium in water, the un-ionized ammonia (NH3) and the ionized ammonium ion (NH4+). Tests for ammonia usually measure total ammonia (NH3 plus NH4+) content. The toxicity of ammonia is primarily attributable to the un-ionized form (NH3), as opposed to the ionized form (NH4+). In general, more NH3 and greater toxicity exist at higher pHs.

When dissolved in water, normal ammonia (NH3) reacts to form an ionized species called ammonium (NH4+):

NH3 + H2O ↔ NH4 + + OH

This is a shorthand way of saying that one molecule of ammonia reacts with one molecule of water to form one ammonium ion and a hydroxyl ion. From the double-headed arrow, we can tell that the reaction can go either way and hydroxyl ions and ammonium ions could combine to form ammonia and water. This is precisely what happens as the pH of water increases, that is, the water becomes more alkaline. You may recall that alkalinity is caused by an increase in hydroxyl ions. An increase in hydroxyl ions (or alkalinity) pushes the equilibrium to the left and more un-ionized ammonia is formed.

At any given time, there will be both ammonia molecules and ammonium ions present. The quantity of each species is dependent on both pH and temperature.

To Recapitulate: Ammonia exists in two forms in the water:

NH3 (this is called unionized ammonia)

NH4+ (this is called ionized ammonia or the ammonium ion)

Together, these two forms of ammonia are called TAN which means Total Ammonia Nitrogen.

NH3 is the principal form of toxic ammonia. It has been reported toxic to freshwater organisms at concentrations ranging from 0.53 to 22.8 mg/L. Toxic levels are both pH and temperature-dependent. Toxicity increases as the pH and temperature increase. Plants are more tolerant of ammonia than animals, and invertebrates are more tolerant than fish. Hatching and growth rates of fishes may be affected. During structural development, exposure to ammonia may alter or change normal tissue development in gills, liver, and kidneys. Toxic concentrations of ammonia in humans may cause loss of equilibrium, convulsions, coma, and death.

Ammonia levels in excess of the recommended limits may harm aquatic life. Ammonia toxicity is thought to be one of the main causes of unexplained losses in fish hatcheries. Although the ammonia molecule is a nutrient required for life, excess ammonia may accumulate in the organism and cause an alteration of metabolism or increases in body pH. Different species of fish can tolerate different levels of ammonia but, in any event, less is better. Rainbow trout fry can tolerate up to about 0.2 mg/L while hybrid striped bass can handle 1.2 mg/L.

Fish may suffer a loss of equilibrium, hyperexcitability, increased respiratory activity and oxygen uptake, and increased heart rate. At extreme ammonia levels, fish may experience convulsions, coma, and death. Experiments have shown that the lethal concentration for a variety of fish species ranges from 0.2 to 2.0 mg/L. Trout appear to be most susceptible of these fish and carp the least susceptible.

At higher levels (> 0.1 mg/liter NH3), even relatively short exposures can lead to skin, eye, and gill damage. Slightly elevated ammonia levels falling within the acceptable range may adversely impact aquatic life. Fish may experience a reduction in hatching success, reduction in growth rate and morphological development, and injury to gill tissue (i.e., hyperplasia), liver, and kidneys. Hyperplasia - the gill filaments are swollen and clumped together, reducing the fish's ability to 'breathe.’

Elevated levels can also lead to ammonia poisoning by suppressing normal ammonia excretion from the gills. If fish are unable to excrete this metabolic waste product there is a rise in blood-ammonia levels resulting in damage to internal organs. The fish response to toxic ammonia levels would be lethargy, loss of appetite, laying on the pond bottom with clamped fins, or gasping at the water surface if the gills have been affected. Unfortunately, this response is similar to the response to poor water quality, parasite infestations, and other diseases.

Experiments have shown that exposure to un-ionized ammonia concentrations as low as 0.002 mg/L for six weeks causes hyperplasia of the gill lining in salmon fingerlings and may lead to bacterial gill disease.

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