Makhshirin, Chapter Four, Mishnah Nine
1) If one drew water through a kilon (a pump-beam), it causes susceptibility to uncleanness for three days.
2) Rabbi Akiva says: if it was dried, it at once does not cause susceptibility to uncleanness; but if it was not dried, it causes susceptibility even for thirty days.
Section one: A “kilon” is a pump-beam used to draw water from a deep well. If produce becomes wet through contact with water in the pump the produce is susceptible for up to three days after the pump has been used. For three days we can assume any water in there that was drawn from the well hasn’t dried up and therefore it causes susceptibility. After three days, we can assume that the water pumped from the well dried up and that the water found in there currently came from elsewhere, and does not cause susceptibility.
Section two: Rabbi Akiva says that if we know that the kilon was dried, then even if produce comes into contact with water in there immediately thereafter, we can assume that the moisture was not from water drawn from the well. But if the kilon has not been dried, then the assumption is that the water in it was pumped from the well and it will cause susceptibility.