The fact that Muslims believe that Zam Zam water is holy does not make the presence of arsenic in it trivial, even for them. Imagine that instead of being (otherwise) healthy and clean, Zam Zam water was uniformly dirty and polluted. For example, the Ganges is also considered holy, but it's one of the top five most polluted rivers in the world. Many Indian Muslims as a result ridicule the Hindu practice of bathing in it. Of course it's a theological problem if Zam Zam water were found to be, say, poisonous, or otherwise very unhealthy. You seem to say that even if Zam Zam water were found to be unhealthy, that its religious benefits would likely outweigh the harm of drinking it, at least in the eyes of a Muslim. But it depends. If some Muslim believes that the blessing associated with drinking it is better health, than he is simply wrong, for Zam Zam water was shown, ex hypothesi, to be unhealthy. If, however, he believes that its positive effects lie in some benefit in the afterlife, then he can I suppose argue that drinking it results in a net benefit overall. But then the question is why God would declare Zam Zam water holy but allow it to become dirty and unhealthy to drink. The same question Muslims might level against Hindus for believing the water of the Ganges to be holy.

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