system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 What prevents poisoning the Quabbin (Large reservoir) like in the movie "Dreamcatcher?"

In the movie dreamcatcher an alien tries to poison the Quabbin reservoir (A man made reservoir used to supply Bostons drinking supply) I live 5 minutes away from said Reservoir and was just wondering, out of all the times I've been there I see no security or anything. Is it just too large of a pool of water where poison of some sort would become too diluted? Or are there too strong chemicals dumped in the water? (Which I can't imagine is the case due to a lot of fishing going on there)  


So yeah just curious if anyone knows how it is prevented. Used chemistry flair since it seemed most appropriate.
gpt
There isn't really a substance that exists which could be practically used to render such a large volume of water toxic, and have it remain so long enough to poison someone. 

There's plenty of poisonous substances easily available, but you'd need truckload quantities of them to achieve a toxic level in something like a reservoir. 

That's ignoring the fact that any toxins would then need to withstand degradation in the aquatic environment and whatever water purification process is used before it goes into the municipal water system.