Although nitrates can occur naturally from decaying organic material such as on-site septic systems, the greatest source of nitrates in most areas is crop fertilizers. These fertilizers produce excellent crop yields, and conversely, they produce elevated nitrate levels in ground water in many rural areas.
Industry emissions (high levels of nitric oxide) are also a culprit. Emissions are converted to nitrate and introduced to the water supply when it rains.
Because healthy adults can consume high nitrate water with little or no effect, a nitrate problem can go unnoticed for a long time. Very young children, however, especially infants, are susceptible to nitrate poisoning which causes blue baby syndrome.
Boiling water, unfortunately, increases the nitrate level rather than decreases it. Concerned parents who boil their infant's water should be aware of its nitrate level, since boiling two quarts of water down to one quart will double the nitrate level.