Nitrate is one of the most common contaminants in groundwater. It is a naturally occurring form of nitrogen found in soil. Several sources of nitrate include fertilizers, manure, municipal wastewater, sludge, septic systems, animal feedlots, and N-fixation from atmosphere by bacteria, lightning, or legumes. Fertilizers like potassium nitrate and ammonium nitrate are a primary source of nitrates. When these fertilizers are used in fields, the nitrates leach into the soil and water aquifers. Aquifers contaminated with nitrates are a serious concern. In areas with concentrated belowground sewage disposal, nitrate-rich seepage from septic systems poses a major threat to groundwater pollution. Nitrates do not have the ability to evaporate; therefore, they remain dissolved and leach into the groundwater.

High nitrate levels can cause blue baby syndrome and methemoglobinemia, a condition that is mostly found in children under six months. The stomach acid of an infant is not as strong as that of an older person. This results in an increase in bacteria that can easily convert the nitrate into nitrite. Nitrite is absorbed into the blood and hemoglobin is converted into methemoglobin. Since methemoglobin does not carry oxygen properly, a reduced amount of oxygen is supplied to important tissues like the brain. In an infant, methemoglobin cannot change back into hemoglobin, which usually occurs in elder people. Young animals are affected by nitrates the same way as human babies.

The Maximum Contaminant Level for nitrates has been set as 45 mg/l.

High nitrate levels are often associated with improperly located or poorly constructed wells. People could try to located new wells uphill at least 100 feet away from septic systems, chemical storage facities, barnyards, and feedlots. It would also help to securely and properly cap or seal abandoned well.