Tafsiran Imamat 15:28: Or HaChaim, Rabbeinu Bahya, Rav Hirsch & Chizkuni

ואם טהרה מזובה וספרה, But if she be cleansed from her issue, then she shall number, etc. We have to try to understand the use of the past tense for the word וספרה instead of the future tense, i.e. ותספור. Precisely when is the count to commence? If the count is to commence immediately, why does the Torah not write ותספור? If it is to occur only after she has become purified, the words "afterwards she shall be clean" do not make sense. We may have to explain this in light of what Torat Kohanim wrote on the meaning of the words טהרה מזובה. They interpret it as a cessation of the issue. The Torah tells us how we are to know that she may be considered cleansed from her issue, i.e. from the time the flow of blood stops. Her "purity" is then sufficient to permit her to begin the count of seven days (during which the flow must not recur). Rashi concurs with this interpretation in Megillah 8. He writes that she may begin counting without first having to immerse herself in a ritual bath.

ואם טהרה מזובה, “when she becomes purified from her discharge, etc.;” when a woman with such a discharge begins the process of healing and purification she has to count seven days just as has the male who had experienced the discharge from his sexual organ. After the seven days have elapsed she is potentially pure, just as the male is potentially pure after the seven day count. This would have led us to believe that just as the male who had a similar discharge had to immerse himself in מים חיים, water directly from a spring, the woman would have to do the same.

Seeing that the words “and after that she will be purified” are unnecessary, the sages decided that the woman though not requiring מים חיים, spring water to complete her purification rites, does require immersion in the regular ritual bath