niddah - What is the punishment spelled out explicitly in the Torah for having relations with a post-menstrual woman who has not gone to the Mikvah?

The accepted halacha is that one who has relations with a woman who is a niddah is punished with kares. But if a woman is no longer menstruating (let's assume several days after it stops), but has not visited a mikvah, does the Torah explicitly state that the couple still receive kares? Is there a lesser punishment written in the text of the Torah for this?

In short, is the concept of mikvah of rabbinic origin or derivation (even by strong implication), or is it spelled out in the Torah itself?