Water naturally contains less than 1 milligram of nitrate-nitrogen per liter and is not a major source of exposure. Higher levels indicate that the water has been contaminated. Common sources of nitrate contamination include fertilizers, animal wastes, septic tanks, municipal sewage treatment systems, and decaying plant debris.
The ability of nitrate to enter well water depends on the type of soil and bedrock present, and on the depth and construction of the well. State and federal laws set the maximum allowable level of nitrate-nitrogen in public drinking water at 10 milligrams per liter (10 parts per million). These laws apply to all city and village water supplies and are used as an advisory for private wells.