Katafreis Connection

The Gemora (Daf Yomi: Gittin 16a) cites a Mishna in Taharos: A jet of liquid from a jug (nitzok), or liquid flowing down a slope (katafreis), or liquid enough to moisten (mashkeh tofe’ach) does not form a connection for tumah or for taharah. [The explanation that they are not a connection for tumah: Nitzok – if one pours liquid from a tahor vessel into a tamei one and the flow of the liquid is uninterrupted between them, it is not regarded as a connection to make the upper vessel tamei. Katafreis – if a liquid from a pool flows down a board on a slope into a pool of liquid which is tamei, the upper pool remains tahor. Mashkeh tofei’ach – a moist trough has two pools of liquid on opposite sides; one of them tamei and one of them tahor. Although the trough is damp enough that it can moisten anything that touches it, the tamei pool does not contaminate the tahor one. The explanation that they are not a connection for taharah: A mikvah must contain forty se’ah of water in order to be valid. If there are two mikvaos and neither of them contain forty se’ah and they are connected through any one of the three ways mentioned above, they are not considered halachically connected to form one large valid mikvah.]

The Vilna Gaon asks: Even without the connection through katafreis, why don’t we say that each droplet of tamei water should contaminate another drop, and ultimately, the water in the upper pool should be rendered tamei?

He answers that the halacha of katafreis would be necessary in a case where a tevul yom (one who was tamei, but has immersed himself in a mikvah; he is considered a tevul yom until nightfall) touched the liquid, for he cannot contaminate a food item with the capability of contaminating something else (and therefore, one droplet will not cause the other to become tamei). Through the principle of katafreis, it could be regarded that the entire flow is regarded as one body of liquid, and the upper pool could become tamei (if it would be considered connected).

Reb Shimon Shkop asks on his answer from the Rambam, who rules that these halachos would apply by tumas sheretz as well, and a sheretz can contaminate one drop to render the others tamei!?

He answers that one drop cannot render the other drops tamei because of the principle of beis hasetarim (for the drops touching each other are not recognizable).

Rav Elyashiv answers simply that if the liquid would be rendered tamei because of its contact with the first droplet, it would only be Rabbinically tamei, for that which a liquid is ruled to be a rishon l’tumah is only a Rabbinic decree. Hence, kodoshim that contracted tumah in such a manner could not be burned. However, through the principle of katafreis, the entire liquid would be rendered tamei on a Biblical level, and if kodoshim would be involved, it would be required to be burned.