[MaxMax]

It seems to me that it would take tons of any agent to do any damage.
And the biological stuff would be killed by the Chlorine. I cannot imagine what
a terrorist could use to contaminate a large water supply and be effective.

[daniel1212]

They use Ozone for this H2o supply, not Chlorine.

[daniel1212]

More specifically,

MWRA, like all water suppliers, disinfects its water to protect against harmful bacteria (from birds, animals, and even humans) that can enter into reservoirs. MWRA uses ozone to kill any harmful bacteria at the CWTP in Marlborough. Ozone leaves no taste or odor, and in fact improves the natural taste of the water. To protect the water as it travels through the miles of pipe to get to your house, MWRA uses an mild disinfectant called mono-chloramine that has very little taste or odor. Occasionally those with a sensitive palette may find their water has the odor of chlorine. It usually doesn’t last long, and is related to the normal adjustment of the mono-chlorine treatment. This is more likely in the summer when water is warmer. Chilling the water usually eliminates it. You can leave a pitcher of water in the refrigerator. - http://www.mwra.state.ma.us/04water/html/qual4concerns.htm

What advantages does ozone have over chlorine as a primary water disinfectant?: http://www.aces.edu/waterquality/faq/faq_results.php3?rowid=3742