Shabbos 65a: Shmuel’s father made mikvaos for his daughters in the springtime, but in the fall they immersed in the Euphrates River with mats under them to prevent the mud from sticking to their feet. Why did he not let them use the Euphrates in the springtime as well? Because he held that the springtime swelling of the Euphrates is due to rainfall in the west, and he was afraid that the majority of the river’s water might be rainwater, which cannot be used as a mikveh while it is flowing.
שבת סה ע”א: אבוה דשמואל… עביד להו מקואות ביומי ניסן, ומפצי ביומי תשרי… מסייע ליה לרב, דאמר רב: מטרא במערבא ־ סהדא רבה פרת. ־ סבר: שלא ירבו הנוטפין על הזוחלין.
Over the hot springs in a certain city in England (possibly Bath), a famous spa was built in which the boiling hot groundwater flowed into a pool, where tap water was mixed in to cool it down to bathing temperature. There was always a large amount of hot spring water present – more than 80 se’ah. The spring water continued to flow out of the pool, replenished by the spring, and the tap water likewise flowed out, replaced by new tap water. The question, presented to Dayan Aryeh Leib Grossnass of the London Beis Din, was whether the pool could be used as a women’s mikveh.
Seemingly, this would be the same problem Shmuel’s father had with the Euphrates in the spring. Just as in that case, despite the fact that the river contained a tremendous amount of groundwater, it was still invalid as a mikveh while flowing due to the presence of a majority of rainwater, here too a majority of the spa’s water was tap water.
However, Dayan Grossnass found that this depends on a dispute between the Taz and the Shach on Yoreh Deah 201:2. The Mechaber brings the halacha of Shmuel’s father, and the Taz (3) asks: Why is the problem of flowing water (zochalin) worse than the problem of drawn water (mayim sh’uvim – water that has been contained in a vessel, such as tap water)? In the case of drawn water, we know that once you have a kosher mikveh of 40 s’ah, you can add even 1000 s’ah of drawn water and it will still remain kosher. But here, though a spring can be used as a mikveh even when flowing, rainwater cannot, and once the rainwater becomes the majority of the mikveh, it cannot be flowing!
The Taz answers that actually, a spring can still act as a mikveh while flowing even if a majority of rainwater is added, but only in a place where the spring water itself would have reached. Shmuel’s father feared that the amount of spring water in the Euphrates was so little that, if not for the rainwater, mixed into it, it would have dried up before reaching Babylonia, so the rainwater becomes dominant. According to the Taz, in the English spa there was plenty of hot spring water, so the mikveh would be kosher.
However, the Shach (32) disagrees and says that the rule of Shmuel’s father applies in all parts of the river, even where the spring would have reached alone. Dayan Grossnass followed the Shach and therefore did not permit women to use this spa as a mikveh.
Then he sent the question and his teshuva to the Chazon Ish, who replied that it was permitted for two reasons: 1) When the spring water is there first, you are allowed to add drawn water and it can still be flowing. 2) Flowing is only a problem if the water is visibly moving. But here it was not visibly moving, although it was possible to detect the outflow by measurement. Therefore, said the Chazon Ish, the spa could be used if there was no other mikveh available in that city.
Source: Shailos Uteshuvos Lev Aryeh 37
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