Maybe someone can provide more detail than I can but I remember one Passover season reading that people strictly keeping kosher in Israel wouldn't drink tap water from one particular source because people would feed the ducks leavened bread and the bread not eaten by the ducks would stay in the water supply thus people would be eating leavened bread from their water source.
It was a few years ago that I read that and I'm afraid that I don't remember anymore details.
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