It is the EPA MCL (maximum contaminant limit).  If you google around you can learn lots about it and how it was derived.  I learned this in water treatment engineering class 20 years ago.  There is also an MCLG (maximum contaminant limit goal) which is 2mg/L.

It’s been a while but the real issue with methemoglobinemia is nitrites, nitrate levels are used as a proxy, kind of, for nitrites.  This is a condition where your blood cannot carry enough oxygen around.  More for babies.

Environmental risk is otherwise generally categorized by acute toxicity (LD50, or lethal dose required to kill 50% of the sample population) or lifetime cancer risk.  Both of which are calculated by load, which is volume times concentration.  Nitrites and nitrates are totally water soluble, excess is going to leave your body in a liquid form.

I would look into some sort ofkitchen sink filter, a little bit of ice or an evening glass of water out of the bathroom sink is small potatoes.