

Mirroring what others have said - at a nearby university that has (had? sigh) a large foreign student population, some folks actively feed the squirrels. For several weeks at the beginning of the school year, you could very easily spot new students by who was out taking photos and getting mobbed by these squirrels that are way, way too comfortable getting close to humans.
Everything you said is valid, and in my experience mailings easily take a week to orchestrate.
If you have to send out 5,000 letters, you have to first print 5k letters — assuming the local water department already has a robust template in place, and it doesn’t wind up dragged on by reviews and approvals.
If they haven’t made generic prints to keep in stock, they have to have their own print facilities, or have an on-call printer capable of doing all other work to deal with emergencies, or possibly work outside of business hours.
Even then, it’s a minimum turnaround of a day. The mail has to go into the system, be sorted and sent to local post offices, then given to mail carriers. The few times I did direct mail, they estimated a minimum of 3 days to deliver, even when dropping off first thing in the morning and the addressee was in the same city.
Even if they managed to get next day delivery, they’d still have a 24h delay in which people could be drinking contaminated water.