Reference for Makhshirin 4:9

הַמְמַלֵּא בְקִילוֹן, עַד שְׁלשָׁה יָמִים, טְמֵאִין. רַבִּי עֲקִיבָא אוֹמֵר, אִם נִגְּבוּ, מִיָּד טְהוֹרִים. וְאִם לֹא נִגְּבוּ, אֲפִלּוּ עַד שְׁלשִׁים יוֹם, טְמֵאִים:

[If] one filled [water] through an irrigation channel [the water] is impure [makes food susceptible to impurity] for up to three days [afterwards]. Rabbi Akiva says: If it dries up, it is immediately pure; and if it is not dried it is impure [makes food susceptible to impurity] for up to thirty days.

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