Nitrate is one of the most frequent groundwater pollutants in rural areas. It needs to be regulated in drinking water basically because excess levels can cause methaemoglobinaemia, or "blue baby" disease. Although nitrate levels that affect babies are not dangerous for older children and adults, they do indicate the possible presence of other more serious residential or agricultural pollutants, such as bacteria or pesticides.
The origin of nitrate in groundwater is primarily from fertilizers, septic systems, and manure storage or spreading operations. Fertilizer nitrogen not taken up by plants, volatilized, or carried away by surface runoff ends up in the groundwater in the form of nitrate. This makes the nitrogen unavailable to the plants, and can also raise the concentration in groundwater above the admissible levels for drinking water quality. Nitrogen from manure can be similarly lost from fields, barnyards, or storage locations. Septic systems remove only half of the nitrogen in wastewater, leaving the other half to leach to groundwater, this way raising groundwater nitrate concentrations.