Not with 100% certainty, though probabilistically, the isotope signature would be different. :-)
But I admit that's irrelevant to the point you were trying to make. The thing is, your point is incorrect.
You can no more distinguish a molecule of baseflow water from meteoric or one in your bathtub, but it's funny how we can call some "floodwaters"... just like the source and location matter for carbon dioxide.
This is a well established concept; though perhaps you are trying to institute a more liberal approach, I just don't see the legal support for it.
And it's not just federal. Look at state regulations. For example, New Jersey's technical regulations for site remediation (N.J.A.C. 7:26E) very explicitly note that "contamination" in groundwater can be natural, and "contamination" is any amount, even if below groundwater quality criteria or standards. If natural, it is "background contamination." Are they trying to outlaw nature and its realities?
Are you a liberal who has wondered on in here by mistake?
I'm a conservative who is sticking with the law as it has been passed by the legislative branch (and signed by President Bush), interpreted by the judicial branch (SCOTUS), and enacted by the executive branch (USEPA). I am "wondering" why I should change to your liberal, non-Constitutional interpretation for which you have provided no legal basis.
Lawyerly parsing verging on sophistry placemarker.
Just because the Supreme court says it’s constitutional does not mean it is. This is why liberals try very hard , and way too often succeed , in getting collectivists who rule by whim. The carbon dioxide decision is a glaring example of that. Decisions like that ultimately hang on the commerce clause. No honest interruption supports any of the massive intrusions which have been built on that clause. In point of fact it was a threat to pack the court that intimidated the court into making a farmer growing wheat on his own land for himself subject to the commerce clause. In most civilized lands agreements made under coercion are not valid.