Mikvaot, Chapter One, Mishnah Four
1) If a corpse fell into it or an unclean person walked in it, and a clean person drank of it, he is clean.
2) The same rule applies to the water of pools, the water of cisterns, the water of ditches, the water of caverns, the water of rain flows which have stopped, and mikvehs of less than forty seahs.
3) They are all clean during the time of rain;
4) When the rain has stopped those near to a city or to a road are unclean, and those distant remain clean until the majority of people pass [that way].
Section one: The water that is in the pool is not susceptible to impurity. It became impure in the cases discussed in the previous three mishnayot because someone took out some of the water and then put it back in. In contrast, if a corpse falls in or an unclean person walks in it, none of the water has been taken out and then put back in. Therefore, if a clean person comes and drinks from it, it remains pure.
Section two: The rules that were found in the first three mishnayot and the beginning of this mishnah apply to all bodies of water that are less than forty seahs. [A seah is assumed to be about 12 liters, so we’re talking about 480 liters, or about 126 gallons]. The “water of rain flows which have stopped” refers to water that has flown down from a mountain, but has stopped flowing. If it still flows, then the rule is different (we will see this in mishnah six).
Section three: Rain water does not receive impurity. Therefore, when the rains are still falling, all of the small bodies of water mentioned in section two are pure. Even if someone had defiled this water, the rain waters nullify the impure water.
Section four: If the rain has stopped, then we need to figure out whether the water is pure or not. If the body is close to the city or to a path, then we have to assume that people drank from there and since people might be impure, the water must be treated as impure. But if the body of water is far from the city or the path, then we can assume that it is pure until we know that many people have gone there.