About nitrate ions: Nitrate nitrogen is found in fertilizer. It is also a chemical formed in the decomposition of waste materials, such as manure or sewage. If infants less than six months of age drink water (or formula made with water) that contains more than 10 mg/L nitrate nitrogen, they are susceptible to methemoglobinemia. This disease interferes with the blood's ability to carry oxygen. Recent studies also suggest that high nitrate water may be linked to birth defects and miscarriages, so pregnant women should avoid drinking high nitrate water. High nitrate levels also suggest that other contaminants might be present.

Nitrate and Water | So, what are acceptable concentrations of nitrate?

The nitrate level in freshwater is usually found in the range of 0.1 to 4 mg/L NO3--N. Unpolluted waters generally possess nitrate levels BELOW 1 mg/L. The effluent of some sewage treatment plants may have levels in excess of 20 mg/L.

About the image transformation: I finally have my nitrate sensor working! Here I am experimenting with using formal 2D geometries to map water quality. The more green you see, the healthier the water is. However, the any great concentration or buildup of red shapes, blue shapes, or yellow shapes indicates that there is a problem with the water quality and that the water is likely polluted.

Source: http://www.uwsp.edu/cnr/etf/Interpre.htm