Halacha

הלכה א
סָפֵק מַיִם שְׁאוּבִין שֶׁטִּהֲרוּ חֲכָמִים כֵּיצַד. מִקְוֶה שֶׁנִּסְתַּפֵּק לוֹ אִם נָפְלוּ לְתוֹכוֹ מַיִם שְׁאוּבִים אוֹ לֹא. וַאֲפִלּוּ יָדַע בְּוַדַּאי שֶׁנָּפְלוּ סָפֵק יֵשׁ בָּהֶן שְׁלֹשֶׁת לוֹגִין סָפֵק אֵין בָּהֶן. וַאֲפִלּוּ יָדַע בְּוַדַּאי שֶׁיֵּשׁ בָּהֶן שְׁלֹשֶׁת לוֹגִין סָפֵק שֶׁהָיָה בַּמִּקְוֶה שֶׁנָּפְלוּ בּוֹ אַרְבָּעִים סְאָה סָפֵק לֹא הָיָה הֲרֵי זֶה כָּשֵׁר:
כסף משנה
1.
What is meant by an unresolved doubt regarding drawn water that our Sages ruled as pure? A mikveh about which one was unsure whether drawn water fell into it. Or even if one was certain that drawn water fell into it, but was in doubt whether there were three lugim or not. Or even when one was certain that there were three lugim of drawn water, but there was a doubt whether the mikveh into which the water fell contained 40 se'ah or not. In all instances, the mikveh is acceptable.

הלכה ב
שְׁנֵי מִקְוָאוֹת אֶחָד יֵשׁ בּוֹ אַרְבָּעִים סְאָה וְאֶחָד אֵין בּוֹ. נָפְלוּ שְׁלֹשֶׁת לוֹגִין מַיִם שְׁאוּבִין לְאֶחָד מֵהֶן וְאֵין יָדוּעַ לְאֵיזֶה מֵהֶן נָפְלוּ סְפֵקוֹ טָהוֹר מִפְּנֵי שֶׁיֵּשׁ לוֹ בְּמָה יִתְלֶה. הָיוּ שְׁנֵיהֶן פְּחוּתִין מֵאַרְבָּעִים סְאָה וְנָפְלוּ לְאֶחָד מֵהֶן וְאֵין יָדוּעַ לְאֵיזֶה מֵהֶן כָּל אֶחָד מִשְּׁנֵיהֶן פָּסוּל שֶׁאֵין לוֹ בְּמָה יִתְלֶה אִם לָזֶה נָפְלוּ נִפְסַל וְאִם לָזֶה נָפְלוּ נִפְסַל:
כסף משנה
2.
The following law applies when there are two mikveot, one containing 40 se'ah and one containing a lesser amount. Three lugim of drawn water fell into one of them, but it is not known into which they fell. Because of the doubt, we rule that the acceptable mikveh is pure, because there is a factor on which one can rely.
If they both contained less than 40 se'ah and three lugim fell into one of them, they are both disqualified, because there is no factor on which one can rely. If the drawn water fell into one, it would be disqualified and if it fell into the other, it would be disqualified.

הלכה ג
מִקְוֶה שֶׁהֱנִיחוֹ רֵיקָן וּבָא וּמְצָאוֹ מָלֵא כָּשֵׁר מִפְּנֵי שֶׁזֶּה סְפֵק מַיִם שְׁאוּבִין לְמִקְוֶה זֶה:
כסף משנה
3.
When one left a mikveh empty, and returned and found it full, it is acceptable, for there is an unresolved doubt whether the water for this mikveh was drawn.

הלכה ד
צִנּוֹר שֶׁמְּקַלֵּחַ לַמִּקְוֶה וְהַמַּכְתֶּשֶׁת נְתוּנָה בְּצִדּוֹ סָפֵק מִן הַצִּנּוֹר לַמִּקְוֶה סָפֵק מִן הַמַּכְתֶּשֶׁת לַמִּקְוֶה הֲרֵי זֶה פָּסוּל מִפְּנֵי שֶׁהַפְּסוּל מוּכָח. וְאִם יֵשׁ בַּמִּקְוֶה רֻבּוֹ מַיִם כְּשֵׁרִים הֲרֵי זֶה כָּשֵׁר שֶׁזֶּה סְפֵק מַיִם שְׁאוּבִים הוּא שֶׁהֲרֵי יֵשׁ שָׁם מִקְוֶה כָּשֵׁר קָבוּעַ:
כסף משנה
4.
When a pipe carried water into a mikveh, but there is a mill next to it, if there is a doubt whether water flowed into the mikveh from the pipe or from the mill, it is unacceptable, because the disqualifying factor is evident. If, however, the mikveh contains a majority of acceptable water, it is acceptable because the doubt involves drawn water and there is an acceptable mikveh whose presence had been established.

הלכה ה
כָּל הַמִּקְוָאוֹת הַנִּמְצָאִים בְּאֶרֶץ הָעַמִּים פְּסוּלִין שֶׁחֶזְקָתָן שְׁאוּבִין. וְכָל הַמִּקְוָאוֹת הַנִּמְצָאִים בְּאֶרֶץ יִשְׂרָאֵל בַּמְּדִינוֹת לְפָנִים מִן הַמַּפְתֵּחַ בְּחֶזְקַת פְּסוּלִין שֶׁאַנְשֵׁי הַמְּדִינָה מְכַבְּסִים בָּהֶן וּמְטִילִין לְתוֹכָן מַיִם שְׁאוּבִים תָּמִיד. וְכָל הַמִּקְוָאוֹת הַנִּמְצְאִים בְּאֶרֶץ יִשְׂרָאֵל חוּץ לַמַּפְתֵּחַ בְּחֶזְקַת טָהֳרָה שֶׁחֶזְקָתָן מִן הַגְּשָׁמִים:
כסף משנה
5.
All of the mikveot found in the lands of the nations are invalid for immersion, for we operate under the presumption that the water is drawn. All of the mikveot found in Eretz Yisrael, in the cities, within the gates are assumed to be invalid, because the inhabitants of the cities wash their garments in them and pour drawn water into them at all times. All of the mikveot found in Eretz Yisrael outside the gates of a city are presumed to be pure. For it is assumed that they came from rainwater.

הלכה ו
הַטָּמֵא שֶׁיָּרַד לִטְבּל סָפֵק טָבַל סָפֵק לֹא טָבַל וַאֲפִלּוּ טָבַל סָפֵק יֵשׁ בּוֹ אַרְבָּעִים סְאָה סָפֵק אֵין בּוֹ. שְׁנֵי מִקְוָאוֹת אֶחָד יֵשׁ בּוֹ אַרְבָּעִים סְאָה וְאֶחָד אֵין בּוֹ וְטָבַל בְּאֶחָד מֵהֶן וְאֵין יָדוּעַ בְּאֵיזֶה מֵהֶן טָבַל סְפֵקוֹ טָמֵא לְפִי שֶׁהַטָּמֵא בְּחֶזְקָתוֹ עַד שֶׁיִּוָּדַע שֶׁטָּבַל כָּרָאוּי. וְכֵן מִקְוֶה שֶׁנִּמְדַּד וְנִמְצָא חָסֵר בֵּין שֶׁהָיָה הַמִּקְוֶה בִּרְשׁוּת הָרַבִּים בֵּין שֶׁהָיָה בִּרְשׁוּת הַיָּחִיד כָּל הַטָּהֳרוֹת שֶׁנַּעֲשׂוּ עַל גַּבָּיו לְמַפְרֵעַ טְמֵאוֹת עַד שֶׁיִּוָּדַע זְמַן שֶׁנִּמְדַּד בּוֹ וְהָיָה שָׁלֵם. בַּמֶּה דְּבָרִים אֲמוּרִים כְּשֶׁהָיְתָה הַטְּבִילָה מִטֻּמְאָה חֲמוּרָה. אֲבָל אִם טָבַל מִטֻּמְאָה קַלָּה כְּגוֹן שֶׁאָכַל אֳכָלִין טְמֵאִין אוֹ שָׁתָה מַשְׁקִין טְמֵאִין אוֹ בָּא רֹאשׁוֹ וְרֻבּוֹ בְּמַיִם שְׁאוּבִין אוֹ שֶׁנָּפְלוּ עַל רֹאשׁוֹ וְעַל רֻבּוֹ שְׁלֹשֶׁת לוֹגִין מַיִם שְׁאוּבִין. הוֹאִיל וְעִקַּר דְּבָרִים אֵלּוּ מִדִּבְרֵיהֶן הֲרֵי סְפֵקוֹ טָהוֹר כְּמוֹ שֶׁבֵּאַרְנוּ. וְאַף עַל פִּי שֶׁנִּסְתַּפֵּק לוֹ אִם טָבַל אוֹ לֹא טָבַל אוֹ שֶׁנִּמְצָא הַמִּקְוֶה חָסֵר לְאַחַר זְמַן וְכַיּוֹצֵא בִּסְפֵקוֹת אֵלּוּ הֲרֵי זֶה טָהוֹר:
כסף משנה
6.
In all the following situations, when an impure person descends to purify himself and:
a) there is a doubt whether he immersed or did not immerse,
b) even if it was known that he immersed, but there is a doubt whether the mikveh contained 40 se'ah or not, or
c) there were two mikveot, one containing 40 se'ah and one that did not contain 40 se'ah, he immersed in one of them and did not know in which one he immersed, he is impure because of the doubt. The rationale is that we presume that the impure person is impure until it is certain that he immersed in the proper manner.
Similarly, if a mikveh was measured and it was discovered that it did not contain the required amount of water, whether the mikveh was located in the public domain or a private domain, all of the pure articles that were immersed in it are considered as impure retroactively until a time when it was measured and it was complete.
When does the above apply? When the immersion was intended to purify the person or the article from severe impurity. When, however, the immersion was because of a light impurity, e.g., one ate impure foods or drank impure beverages, his head and the majority of his body entered drawn water, or three lugim of drawn water fell on his head and on the majority of his body, since the primary aspect of these types of impurity are Rabbinical in origin, he is pure despite the doubt, as we explained. Even though the person is in doubt whether he immersed or not, the mikveh in which he immersed was discovered to be lacking afterwards, or other doubts of this nature arise, he is pure.

הלכה ז
שְׁנֵי מִקְוָאוֹת אֶחָד כָּשֵׁר וְאֶחָד פָּסוּל וְטָבַל בְּאֶחָד מֵהֶן מִטֻּמְאָה חֲמוּרָה וְעָשָׂה טָהֳרוֹת הֲרֵי אֵלּוּ תְּלוּיוֹת. טָבַל בַּשֵּׁנִי וְעָשָׂה טָהֳרוֹת הָרִאשׁוֹנוֹת תְּלוּיוֹת כְּשֶׁהָיוּ וְהַשְּׁנִיּוֹת טְהוֹרוֹת. וְאִם נָגְעוּ אֵלּוּ בְּאֵלּוּ רִאשׁוֹנוֹת תְּלוּיוֹת וּשְׁנִיּוֹת יִשָּׂרְפוּ. וְכֵן אִם נִטְמָא בָּאֶמְצַע טֻמְאָה קַלָּה שֶׁבֵּאַרְנוּ וְטָבַל בַּשֵּׁנִי וְעָשָׂה טָהֳרוֹת. אֲבָל אִם טָבַל בְּאֶחָד מֵהֶן מִטֻּמְאָה קַלָּה וְעָשָׂה טָהֳרוֹת וְנִטְמָא טֻמְאָה חֲמוּרָה וְטָבַל בַּשֵּׁנִי וְעָשָׂה טָהֳרוֹת הָרִאשׁוֹנוֹת טְהוֹרוֹת וְהַשְּׁנִיּוֹת תְּלוּיוֹת. וְאִם נָגְעוּ אֵלּוּ בְּאֵלּוּ רִאשׁוֹנוֹת יִשָּׂרְפוּ וְהַשְּׁנִיּוֹת תְּלוּיוֹת כְּשֶׁהָיוּ. הָיָה בְּאֶחָד מֵהֶן אַרְבָּעִים סְאָה וְאֶחָד כֻּלּוֹ שָׁאוּב וְטָבְלוּ בָּהֶן שְׁנַיִם אֶחָד מִטֻּמְאָה חֲמוּרָה וְאֶחָד מִטֻּמְאָה קַלָּה וְעָשׂוּ טָהֳרוֹת הַטּוֹבֵל מִטֻּמְאָה חֲמוּרָה טָהֳרוֹתָיו תְּלוּיוֹת וְהַטּוֹבֵל מִטֻּמְאָה קַלָּה טָהֳרוֹתָיו טְהוֹרוֹת. הָיָה אֶחָד טָמֵא וְיָרַד לִטְבּל וְהַשֵּׁנִי יֵרֵד לְהָקֵר זֶה שֶׁיָּרַד לִטְבּל בְּאֶחָד מֵהֶן טָהֳרוֹתָיו תְּלוּיוֹת כְּמוֹ שֶׁבֵּאַרְנוּ. וְזֶה שֶׁיָּרַד לְהָקֵר טָהוֹר כְּשֶׁהָיָה. שֶׁזֶּה סְפֵק מַיִם שְׁאוּבִין הוּא שֶׁמָּא בַּשָּׁאוּב טָבַל כְּשֶׁהֵקֵר וְנִטְמָא:
כסף משנה
7.
When there were two mikveot, one kosher and one unacceptable, a person immersed in one to purify himself from a severe type of impurity, and touched pure foods, their status is questionable. If he immersed in the second mikveh as well and touched pure foods, the status of the first remains questionable as before and the second ones are pure. If these two batches of foods touch each other, the status of the first remain questionable and the second should be consigned to fire. This ruling also applies if in the interim he contracted impurity from a light source of impurity, as we explained, immersed in the second mikveh and touched pure foods.
If, however, he immersed in one of the mikveot because of a light impurity and then touched pure foods and then contracted a severe type of impurity, immersed in the second, and then touched pure foods, the first batch are pure and the status of the second is questionable. If these two batches of foods touch each other, the first should be consigned to fire and the status of the second remains the same as before.
The following rules apply if one of two mikveot contained 40 se'ah of acceptable water and the other was filled entirely with drawn water and two people immersed in them without knowing which one immersed in the acceptable mikveh and which immersed in the unacceptable one. One of the two had contracted a severe type of impurity and one had contracted a light type of impurity. They both touched pure foods. The status of the foods touched by the one who immersed because of a severe type of impurity is questionable, but those touched by the one who immersed because of the light type of impurity are pure.
In the situation described in the above clause, if one person was impure and descended to immerse and the other was pure and descended merely to cool off, pure foods touched by the one who descended to immerse in one of them are of questionable status, as we explained. And the one who descended to cool off is pure as he was previously. The rationale is that this is a doubt concerning drawn water, for the question is: perhaps he immersed in the pool of drawn water when he cooled off and thus contracted impurity.

הלכה ח
שְׁנֵי מִקְוָאוֹת שֶׁל עֶשְׂרִים עֶשְׂרִים סְאָה אֶחָד שָׁאוּב וְאֶחָד כָּשֵׁר. הֵקֵר בְּאֶחָד מֵהֶן וְעָשָׂה טָהֳרוֹת הֲרֵי אֵלּוּ טְהוֹרוֹת. הֵקֵר בַּשֵּׁנִי וְעָשָׂה טָהֳרוֹת הֲרֵי אֵלּוּ יִשָּׂרְפוּ. שֶׁהֲרֵי וַדַּאי בָּא רֹאשׁוֹ וְרֻבּוֹ בְּמַיִם שְׁאוּבִין כְּמוֹ שֶׁבֵּאַרְנוּ:
כסף משנה
8.
The following rules apply when there are two mikveot, each containing 20 se'ah, one of drawn water and one of acceptable water. If a person cooled off in one of them and then touched pure foods, they are considered pure. If afterwards, he cooled off in the second and touched pure foods, they should be consigned to fire. For his head and the larger greater of his body certainly entered drawn water, as we explained.

טהרה הלכות מקואות פרק י
Taharah Mikvaos Chapter 10