Al Quaida DOES have credible plans for poisoning water supplies and just such a plan was busted in Italy. But it won't be done by putting stuff in reservoirs.
In theory: Preferably, I would hijack a single truck full of nuclear waste material, probably of the type generated by a hospital, then I wouldnt bother with trying to access a reservoir and simply dump it in a small lake, or lake system that has plenty of residential development around it. Being from Michigan, I see plenty of candidates for such a thing.
Now, lets assume that the end result of this doesn't "really" poison anything seriously (ie residents on the lake will have a 1 in 10,000 chance of getting mildly ill if they continue thier normal lake activities for a period of 2 years)....
Is that really relevent in the overall effectiveness of this scheme to terrorize?
Remember, we are a nation afraid of second hand smoke, afraid to let pilots carry a gun, afraid of someone smoking a joint...
Think about it, we let our fear of boxcutters KILL 3000 PEOPLE.
Heck, if one is a suicidal terrorist, you could keep a city in terror for weeks simply through a random handgun killing everyday before you got caught, especially if you targeted a specific class of people like children or the elderly.
Another venue is simple arson.
Actually, if you had powdered plutonium, I know of a fairly easy and far better place to put it than in the water, but I won't say it.