Semper Fi
Let's assume a lethal dose of that toxin diluted into approximately 1 liter of water to be consumed by each victim. A smaller water volume would require more poison, so it would seem that this is a good worst case.
Let's also assume a small local storage and distribution reservoir of approximately 50,000 acre feet (Oroville is 2 million).
Assume a specific gravity of 1 for ricin.
(50,000 acre ft/reservior)(44,100 cft/acre ft)(6.8 gal/cft)(4 liters/gal)[(60x10e-6 gm ricin/dose)/(1000gm/dose)]= 3,500 liters ricin/reservior (to two sigfigs)
That seems to be a lot of ricin, about 4 tons. It would take ten times as much nicotine.
Once it was delivered it would then have to be dispersed evenly throughout the reservoir and would be subject to various degradative forces. As I said, I am n expert. Do you find an error in this?