What is "Makor Niftach l'Veis David"?
Radak, from Shekalim 17a: This is like it says "ba'Yom ha'Hu Yetze'u Mayim Chayim mi'Yrushalayim" (14:8). Do not say that it mixes with other springs - Makor Niftach (one spring)! Once the water reaches the opening of Beis David, it becomes a great river. Malbim ? the water will go "Chetzyam El ha'Yam ha'Kadmoni v'Chetzyam El ha'Yam ha'Acharon" (ibid.) after the last war of Gog. Here discusses after the first war. Then, it is only for Beis David and residents of Yerushalayim, for Chatas, Lehischatei 1 , and for Mei Nidah ? to put Efer Parah Adumah on it. It will remove Chatas (the sin of) foreign mindsets, and to purify from the Tum'ah of folly and errant Emunos and any trace of idolatry.
To become Tahor via immersing in it, so he can engage in Chatas or other Taharos. (PF)
What is "l'Chatas ul'Nidah"?
Rashi, Targum Yonasan: For sprinkling. (Water sanctified with ashes of Parah Adumah is called Mei Chatas and Mei Nidah - Bamidbar 19:9). Radak ? until that day, water did not come out (flow) in Yerushalayim 1 . Then, water will be in the city, and they will not need to go outside the city for Mei Chatas, which requires Mayim Chayim (a spring). Also a Nidah who is a Zavah requires Mayim Chayim 2 .
Yuma 31a: Ein Eitam was higher than the Azarah; from it came the channel that brought water to where Kohanim immersed (Rashi 31a)! An Amah of water flowed through the Mikdash (Yuma 44b)! Malbim said that there was never a spring in Yerushalayim. Yaskil Avdi (6 EH 91:3) suggests that perhaps it is not considered a spring, for by its source it mixes with ponds of rain-water. However, Radak says that water never came out! This requires investigation. (PF)
Also Abarvenel says so. Malbim ? this is wrong. A Zav needs Mayim Chayim, but a Zavah does not (Tosefta Megilah 1:11)! (Also, nowadays we are stringent to consider every Nidah a Safek Zavah, but we do not require Mayim Chayim! Perhaps there is a printing mistake. They wrote that "ul'Nidah" hints to a Zav ? the Torah put Nidah Samuch to Zav (Vayikra 15:32-33), and their Tum'ah is the same. Also a Zav (like Mei Chatas) needs Mayim Chayim. The verse says Nidah, for Mei Chatas is called Mei Nidah. - PF)
