Chametz on Pesach is assur even a Mashehu, a tiny drop. If so how can we drink water from rivers or lakes. Surely someone poured into it something chametz. Rav Shlomo Zalman Auerbach (Halichos Shlomo 1:4:5) said that there is no problem since the chametz cannot be detected with the senses of a person.
Rav Shlomo Zalman held (D’var Halacha “vuv”) that “Mashehu” is also a shiur and does not mean any microscopic amount. The shiur of Mashehu is if a person were able to isolate the chametz in his cup of water it would be visible or somehow detectable through his human senses. If not, then it is less than the shiur of Mashehu and is mutar on Pesach.
The Orchos Halacha (19) brings that while Rav Shlomo Zalman’s father in law, Rav Aryeh Leib Ruchamkin was still alive, Rav Shlomo Zalman would only drink from the water that was drawn from the well before Pesach since before Pesach Chametz is batel. However after his father-in-law’s petira he stopped keeping this chumra.
Article courtesy of: Revach.net