Reply #1: "long term" exposure is generally a year or more.

[FBaggins]

When the EPA sets safety levels, they're assuming that you remain exposed to the source all year round. If they say a given level is safe for srinking water, they mean it can stay at that level essentially permanently without appreciable risk.

Does a single dose of I 131 constitute a non-negligible threat for a persons health in the future?

Not at anything close to the level you cite. A picocurie (one trillionth of a curie) is about 2.2 disintegrations per minute... or bit more than 1/30th of a Bq. So 550 of them would be 20 Bq.

That's MUCH higher than I would expect it to be (and it's also much higher than the other readings that have been reported), but it wouldn't do you any harm. I wouldn't want to drink it all year long, but even if the reported level was accurate, it wouldn't BE there all year long.