Nitrite (as Nitrogen):


Source: Enters environment from fertilizer, sewage, and human or farm-animal waste. Toxicity results from the body's natural breakdown of nitrate to nitrite. Causes "blue baby disease," or methemoglobinemia, which threatens oxygen-carrying capacity of the blood.
This inorganic chemical is used in fertilizers and is found in sewage and wastes from humans and/or farm animals and generally gets into drinking water as a result of those activities. While excessive levels of nitrite in drinking water have not been observed, other sources of nitrite have caused serious illness and sometimes death in infants under six months of age. The serious illness in infants is caused because nitrite interferes with the oxygen carrying capacity of the child's blood. This is an acute disease in that symptoms can develop rapidly. However, in most cases, health deteriorates over a period of days. Symptoms include shortness of breath and blueness of the skin. Clearly, expert medical advice should be sought immediately if these symptoms occur. EPA has set the drinking water standard for nitrite at 1 part per million (ppm) to protect against the risk of these adverse effects. EPA has also set a drinking water standard for nitrate (converted to nitrite in humans) at 10 ppm and for the sum of nitrate and nitrite at 10 ppm. Drinking water that meets the EPA standard is associated with little to none of this risk and is considered safe with respect to nitrite.

References
http://www.dep.state.fl.us/water/drinkingwater/inorg_con.htm
http://water.usgs.gov/edu/groundwater-contaminants.html