“I wonder how they even found out about this?”
Well, I just googled “Boston water supply”, and the first link mentions Quabbin in the first paragraph. Doesn’t seem to be a well guarded secret.
As for how law enforcement caught them in the act, that's something to ponder. I only skimmed the article. Do we know why LE just happened to be out driving in a rural, secluded area at that precise time of night? And when i say precise, that means a very narrow window of time. Curious. Maybe they were under surveillance.