[gabaymkj] poisoning through backflow

the government recommendation that homeowners install backflow preventers on their garden hose spigot so as not to contaminate the water supply seems to communicate an obvious way to attack a water supply.  central water treatment facilities are typically well guarded; home spigots are not.

attack a particular house by tampering with an unprotected spigot on the outside of the house.  or, launch the attack from any house upstream of the target.  attack an entire neighborhood, maybe more.  can one look up what is upstream and downstream in a municipal water supply?  are there backflow preventers built into a municipal system?

diffusion theoretically goes both directions but will probably be overridden by flow.

what poisons would be effective?  again, we can consult government research: LSD (thanks MkUltra).  there are a whole bunch of famous waterborne diseases that spread through contaminated water.  lead poisoning (thanks Detroit/Flint) is intriguing: a slow attack.  Detroit/Flint is additionally intriguing because the attack "poison" could be purified water: the lack of corrosion inhibitors caused existing lead pipes to become lead poison.  where else relies on corrosion inhibitors to prevent lead poisoning?

is it surprising that an attack like this hasn't already been done?  perhaps it is more difficult than it seems.  while an attack like this might not kill that many people, it destroys trust (in the municipal water supply), great for terrorism.