It's plausible, but not very possible. There have been a number of studies on the topic, especially since 9/11. See this paper for reference and a quick summation.
The filtration, purification, and safeguards in place for most large population center's water supply would prevent most contaminates from ever making it to the public at large. Not to mention the sheer logistical implications.... Think about how many thousands of gallons of contaminate would have to be added to a reservoir to make the water consumed at the end of the supply chain potent....
It's just not a likely scenario. I'd be inclined to worry about more imminent and plausible threats.
The filtration, purification, and safeguards in place for most large population center's water supply would prevent most contaminates from ever making it to the public at large. Not to mention the sheer logistical implications.... Think about how many thousands of gallons of contaminate would have to be added to a reservoir to make the water consumed at the end of the supply chain potent....
It's just not a likely scenario. I'd be inclined to worry about more imminent and plausible threats.