In the US the EPA's limit of 10 mg/L refers to nitrate-nitrogen (NO3-N), not nitrate (NO3). So the limit is actually 45 ppm, not 10. NO3-N is only a measurement of the nitrogen atoms within the nitrate ions. It doesn't measure the other 3 oxygen atoms. This makes NO3-N 4.42 times less than NO3 so:justarn said:I've seen 45ppm out the tap here in kent UK. Our legal limit is much higher than the USA at 50ppm!
10 ppm NO3-N x 4.42 = 44.2 ppm NO3.
It's rounded up to 45 ppm nitrate for whatever reason.
*On annual water reports and water tests nitrate is almost always listed as nitrate-nitrogen (NO3-N). This means you need to multiply your listed nitrate number by 4.42 to convert to nitrate. California does not list nitrate as nitrate-nitrogen on water reports.