Can a large outdoor swimming pool be considered a Kosher Mikvah? If not, why not?
There are two types of Kosher Mikvahs, one is a standing pool of water (which is the actual literal translation of “Mikvah”), and the other is a flowing spring. However, when using a Mikvah, it’s waters may not be flowing, rather they must be absolutely stationary, and when using a spring, the majority of the waters in the spring must come from underground springs and not rain water.
Additionally, a Mikvah may not contain water which has been previously accumulated in a vessel, but rather must be gathered from a spring or rain source directly into a pool. This is why, generally speaking, a swimming pool may not be considered a Kosher Mikvah.
An ocean is considered a Mikvah, stationary body of water, since the Torah itself calls it this in Genesis, and may be used as a mikvah (not withstanding modesty concerns). However, a river is actually more problematic, and really depends on the location of the river and the season.
Take care,
Rabbi Aaron Tendler