Mikvaos, Chapter 6, Mishnah 10. Our chapter has been occupied from the start with a single subject: two pools of water lying alongside each other, and what each one does to its neighbor (the case of mikvah samuch l'mikvah). Our Mishnah continues along that line. Picture water that has no standing as valid mikvah water sitting immediately beside a kosher mikvah. Does its closeness ruin the kosher water, or is each body of water judged on its own terms? That is the question the Mishnah sets out to answer.
To follow the Mishnah we need to picture the setup. There is a general excavated area which serves as the mikvah. Built into the cement work, sunk into the ground itself, is an ambati, what we would call a bathtub. Because it is constructed into the ground and not a portable vessel, it is not mekabel tumah and it can hold valid mikvah water. But embedded into the floor of that ambati is an avik, a drain, and that drain is a genuine keli, a vessel, which has been set into the depth of the tub.
Now consider how the water travels. Water enters the large mikvah area, flows into the merchatz, the bathhouse pool, and from there down into the avik. Since the drain sits at the lowest point, and since it has the status of a vessel, whatever water reaches it becomes she'uvin, drawn water. And because water always seeks the lowest spot, that drawn water will be sitting in the installation before a single drop of rainwater has settled anywhere else.
The Words of the Mishnah
"Ha'avik sheb'merchatz", the drain found in the bathhouse. This drain, as described, carries the halachic standing of a keli. A plug is kept sitting in it, and in order to empty out the pool one has only to lift that plug away.
"Bizman shehu b'emtza, posel": when the drain is in the middle, it invalidates. The poskim explain that "in the middle" is not merely a matter of geography; it means that this is the first place where water collects when it falls in. Consequently, when you fill the pool, the mikvah is posul, because the water that arrived first gathered in the avik and became drawn water, and that drawn water preceded every drop of kosher water that would afterwards collect in the mikvah.
"Min hatzad, eino posel": if the drain is off to the side, it does not invalidate. Some poskim understand this to mean that the drain is set in the wall, but the essential point is that it is positioned so that it does not collect water before the collection in the pool itself begins. Even if the drain fills at the same time that the mikvah is filling, we treat it as an independent body of water standing alongside the mikvah, a self-contained entity, and it therefore does not spoil the mikvah.
"Mipnei shehu k'mikvah samuch l'mikvah": the drain's standing here is that of one pool of water lying beside another, and a neighbor of that sort does not ruin the pool that is collecting its mei geshomim, its rainwater. The Mishnah signs off on this ruling with "divrei Reb Meir", telling us that what we have heard until now is Reb Meir's view.
The Approach of the Chachamim
"Vachachamim omrim": the Chachamim take a more lenient view, and for them the matter depends on a measurement. "Im mekabelas ha'ambati revi'is ad sheyagia la'avik, kasher": if the ambati, the tub built into the cement work which is capable of holding kosher mikvah water, can gather a revi'is of water before any water reaches the avik, the pit containing the vessel, then the mikvah is valid. A revi'is is the smallest quantity that counts as a shiur of a mikvah from a Torah perspective. Once that amount has settled, we view the mikvah as having begun to fill in its own right, and the drain is then merely a neighbor, a case of mikvah samuch l'mikvah.
"V'im lav, posul": but if the tub cannot hold a revi'is of rainwater before the water reaches the avik, the mikvah is invalid.
Reb Elazar ben Reb Tzaddok
"Reb Elazar ben Tzaddok omer: im mekabel ha'avik kolshehu, posul." Reb Elazar ben Tzaddok holds that if the avik receives even the smallest trickle first, the mikvah is invalid, and it makes no difference whether the ambati, the kosher portion, can hold a revi'is or not. As long as even a little water is gathered in the drain ahead of everything else, all the water that pours in afterwards keeps on becoming she'uvin, and that drawn water in turn renders the mikvah posul.
So this Mishnah leaves us with three positions on a single installation: Reb Meir, who looks only at where the drain sits; the Chachamim, who look at whether a revi'is of valid water was gathered first; and Reb Elazar ben Reb Tzaddok, who disqualifies the moment the vessel captures even a drop before the mikvah begins to fill.