Nitrates
Nitrates are chemical units of nitrogen and oxygen most commonly used in fertilizer. Possible sources of nitrates include livestock manure, human feces and densely-fertilized agricultural runoff.
The Environmental Protection Agency (EPA) has set Maximum Contamination Limits (MCLs) for nitrates at 10,000 parts per billion (ppb) and nitrites at 1 parts per million (ppm).
For more information about nitrates and nitrites, see the EPA's fact sheet on nitrates.