Before us is Maseches Eduyos, chapter 7, mishnah 3. This mishnah deals with a mixture of rainwater and spring water, and with the question of whether the status of the mixture follows rainwater or spring water. Let us first explain the fundamentals of the subject, so that we can appreciate what is being said.
Purification is accomplished by means of water, and water can be of two types: living water, which flows from a spring in the ground, or rainwater that has gathered in one place. This subject already came up earlier in the masechta.
The advantages of living water:
Three areas of halachah require living water specifically: the immersion of a zav, the sprinkling upon a metzora for his purification, and the preparation of the mei chatas, the waters of the parah adumah.
The quantity of water: one who immerses in a mikvah of gathered rainwater needs forty se'ah. For spring water there is no minimum requirement at all, and any amount suffices, provided that the item one wishes to purify can be fully immersed in it.
Gathered or flowing: rainwater is valid only when it is gathered and not running, whereas spring water is valid even when it is in motion, emerging from a wall and running downward; in such water too one may immerse a small vessel and purify it.
From here the question arises: a small stream flowing from a spring, whose living waters run through it, and rainwater joins them as well - that is, this little river is a combination of spring water and rainwater - does it have the status of spring water with all its advantages, or not?
The answer is that as long as the majority of the water in that little stream originates from the spring, all the advantages of spring water are preserved in it. That is the idea in brief.
The wording of the mishnah - 'zochalin' and 'notfin':
The wording of the mishnah is liable to mislead, since it is taken from a mishnah in Maseches Mikvaos. Generally, when the word 'zochalin' appears in the context of mikvaos, it refers to rainwater that is running; 'zochel' means flowing, as in the expression "zochalei afar," which refers to small creatures that crawl along the ground, and here too the meaning is water that travels along the surface of the ground.
However, in the context of our mishnah, 'zochalin' refers to a small running stream whose source is a spring, and therefore it has the status of living water. The rainwater, on the other hand, is called in the mishnah 'notfin,' from the word netef, a drop - water that drips, in the manner of rain.
And the wording of the mishnah: "He'id Rabbi Tzadok al zochalin sherabu al hanotfin" - Rabbi Tzadok testified about a small river that is a combination of zochalin, spring water flowing in a stream, which constitutes the majority, and notfin, rainwater, which constitutes the minority, "shehem kesheirim" - the mixture is valid to serve as spring water, as living water, for the preparation of mei chatas, the waters of the parah adumah, and the like.
"U'ma'aseh hayah b'Biras HaPilya" - an incident that occurred in a place called Biras HaPilya, where there was a small river that was a combination of rainwater and spring water, and they wanted to know whether these waters could be used for the purification of a metzora and similar matters. "U'va ma'aseh lifnei chachamim v'hichshiruhu" - the Sages considered the case and declared the water valid, ruling that it has the status of spring water in every respect, since the majority of it is spring water.
In summary: in this mishnah we learned the advantages of living water over rainwater - in the three areas that require living water specifically, in the quantity of water, and in the permissibility of flowing water. We clarified the meaning of the terms 'zochalin' and 'notfin' in the language of our mishnah, and we learned from the testimony of Rabbi Tzadok and from the incident at Biras HaPilya that a mixture whose majority is spring water has the status of spring water in every respect.