IANAL, but terrorism isn't a well-defined crime in most jurisdictions, this doesn't match any of the various federal definitions in the US, and I can't think of any lesser charges that you could make stick by adulterating water with purer water. But on the other hand, accessing a municipal water supply is likely to involve at least a couple of crimes along the way.
The realistic worst-case scenario for this is something like Portland, Oregon's Mount Tabor reservoir urination saga: you haven't actually done anything dangerous and everyone (except for a few homeopaths, and I think even they would be more likely to treat it as a stunt) knows it, but you've created a very expensive public relations stink for no good reason. Governments tend not to appreciate that.
"Terrorism" is certainly a well-defined crime in the US. See e.g. here, in particular:
(5) the term “domestic terrorism” means activities that ... (B) appear to be intended ... (ii) to influence the policy of a government by intimidation or coercion
(note the word appear to be intended)
The realistic worst-case scenario depends on how pissy the prosecutors are feeling and whether they need a sacrificial goat at this particular moment. See today's Popehat for a sense of how wide the prosecutorial discretion is.
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