Niddah and washing

Question

I'm looking for the source of the mitzvah of going to the mikvah after being in niddah. There is a part in vayikra 15 which speak about niddah, but there it says a woman should bring a korban when she is finished being in niddah. What I'm looking for is where the mitzvah of going to the mikvah comes from. Looking at verse 19 and on, I can find a lot of verses teaching that anything or anyone one who comes in contact with a woman being in niddah becomes ‘unclean’, but none of those talk about the woman herself having to bathe in living waters, only about the counting of days after which she is considered to be clean (verse 28).

The only verse I could think of was 15:13 if we take it as a general rule it could be read as: When one (male or female) with a discharge becomes clean of the discharge, one shall count off seven days for ones cleansing, wash ones clothes, and bathe ones body in fresh water; then one shall be clean.

Any other thoughts or sources derived from the verses mentioned in Leviticus 15 which could show us a reason for taking a mikvah?

[P.s. would it be allowed to take a normal bath if there isn’t such a command to be found?]

Answer 1

From yoatzot.org:

The requirement for a niddah to immerse is a Torah level, D'oraita, law. However, it is not explicitly stated in any verse in Torah. This is an instance where the Rabbinic tradition is essential for a proper understanding of the Torah text.

There are a few different Torah sources given for the requirement that a woman in niddah immerse. These include VaYikra 15:18, which is understood by the Rambam as a 'binyan av', or archetype, for anyone who is tamei (ritually impure) needing to immerse. An alternative scriptural basis is VaYikra 15:21, which requires one who touches a niddah's bedding to immerse. Kal vachomer (a fortiori), a niddah herself must immerse. Another source is Bamidbar 31:23, in which a mikveh is referred to as "the waters of niddah." (For further reference, see Bet Yosef, Yoreh Deah 197:1.)

On another note, a zavah brings a sacrifice when she exits that status (VaYikra 15:25-30), but a niddah does not (VaYikra 15:19-24). Again, the Rabbinic tradition is important in understanding the verses.

It seems none of the verses given describe one should literally take a mikvah, yet it can be derived out of the context.