Confusion regarding the drinking water standard for nitrates arises because of differences between how NO3 concentrations have historically been reported. It used to be common to report NO3 concentrations in terms of the mass of the entire ion per volume of water. It is now more common to report NO3 concentrations in terms of the mass of nitrogen per volume of water. This simplifies the addition of concentrations of different species (nitrite, ammonia and total Kjeldahl nitrogen being the other common species). Since the molecular mass of NO3 is 62 and the molecular mass of N is only 14, the drinking water standard of 10 mg/l NO3 as N is the same as the drinking water standard of 44 mg/l NO3 as NO3. Does that make sense?
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