Nitrates are the plant nutrient and fertilizer that enters water supply sources from fertilizers, animal feed lots, manures, sewage, septic systems, industrial wastewaters, sanitary landfills, and garbage dumps.

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Other /More definition:
Nitrates are Inorganic compounds that can enter water supplies from fertilizer runoff and sanitary wastewater discharges. Nitrates in drinking water are associated with methemoglobinemia, or blue baby syndrome, which results from interferences in the bloods ability to carry oxygen.