Wow, that's one big pipe. Though, you'd think the pressure inside the pipe would be greater than outside it, nullifying any immediate risk of contamination from other water sources.
MWRA is activating its emergency water supplies such as the Sudbury Aqueduct, Chestnut Hill Reservoir and Spot Pond Reservoir. THIS WATER WILL NOT BE SUITABLE FOR DRINKING, but can be used for bathing, flushing and fire protection.
From here: http://www.mwra.com/updates/leak.html