Mikvaot, Chapter One, Mishnah Seven
Today’s mishnah talks about the next two bodies of water that have greater purity than the pool consisting of renewed rainwater.
1) Superior to such [water] is [the water of] the mikveh containing forty seahs, for in it people may immerse themselves and immerse other [things].
2) Superior to such [water] is [the water of] a spring whose own water is little but has been increased by a greater quantity of drawn water.
a) It is equivalent to the mikveh in as much as it may render clean by standing water, and to an [ordinary] spring in as much as one may immerse in it whatever the quantity of its contents.
Section one: One level superior to the pool of flowing rainwater is the mikveh, a pool that has forty seahs of water that have not been drawn. A person can immerse in a mikveh in order to become pure and he can also immerse things (such as vessels) in a mikveh in order to purify them.
Section two: Greater purity is accorded to spring water. A mikveh does not purify when its water is trickling on the ground. All forty seahs have to be gathered in one place for it to purify. In contrast, a spring generally purifies even if its water is trickling on the ground. However, in the case in our mishnah since the spring waters have been augmented with drawn water, they don’t purify unless they have been gathered together, i.e. they are standing. In this way it is similar to the mikveh.
This spring is similar to an ordinary spring, one whose water has not been augmented with drawn water, in that the fresh spring water purifies even if there are less than forty seahs. In contrast, a mikveh, water that has been gathered (we shall learn how in subsequent mishnayot) only purifies if there are forty seahs.