Nitrates in water

In my previous question, someones answer among other things, said that boiling the water increases the concentration of nitrates and nitrites. I searched on google and it's true, but how does that happen ? What is the reaction that leads to an increasing level of nitrates? 


Im very courious because nitrates are too dangerous to be high in concentration, i would appreciate your help. 





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> boiling the water increases the concentration of nitrates and nitrites.
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Yes.



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> how does that happen? What is the reaction that leads to an increasing level of nitrates?
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It happens because you boil away the water, and the nitrates and nitrites stay in what's left of the water, thus raising the concentration of those species in the now reduced amount of water. There is no "reaction," per se, just you are concentrating dissolved species by removing the water.


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