Such contamination would be understandable given the hundreds of people putting their hands and fingers into the water. The same concern has been raised in masses where wine is passed from churchgoer to churchgoer. Contamination in such public places would normally raise questions of negligence, though the religious component raises significant constitutional issues.
Even if the water were tampered with by hostile forces, there is a question of the reasonableness of the level of testing or protection of such common resources. There are also assumption issues for those utilizing such common fonts, though they could note that it is a religious practice of the Church.
Do you think that the Church should be found liable in the holy water was tainted?