Baba Basra 66a - Opinion of Rashbam by "She'uvin"

The gemara implies that the p'sul of drawn water in a mikva is only d'rabonon, but the Rashbam is not willing to accept that statement at face value because of the braisa in toras kohanim that learns from a pasuk that a mikvah must be created bi'dei shamayim, rather than being filled by buckets. Tosafos quotes the rashbam to hold that כולו שאוב is d'oraysa. Simply, that would imply that only when the entire 40 se'ah is of drawn water would the mikvah be invalid. But, in actuality the Rashbam has difficulty constructing a case where drawn water will not invalidate the mikvah m'doraysa. The case that the Rashbam describes where it will not be pasul m'doraysa is when the mikvah was already full with 40 se'ah of rain water (not drawn), then one adds drawn water. Clearly, the Rashbam holds that if the initial 40 se'ah is comprised of some drawn water, the mikvah will be pasul m'doraysa.

In my sefer on mikvaos, Mayim Rabim (pg. 40) I pointed out that the proof that Tosafos cites to the Rashbam (and Rabbeinu Tam), actually doesn't fit with the actual opinion of the rashbam. Tosafos proves from the opinion of R' Eliezer that we are more machmir for she'uvin when the mikvah is completely empty (amount is revi'is), than when the mikvah is partially full (amount to pasul is 3 lug). Tosafos assumes that the m'doraysa the mikva would be passul if 3 lug were placed prior to any other water, therefore we are machmir m'drabonon for a revi'is. But once the mikvah is partially full with rain water, it can no longer be ruined m'doraysa, so we are only machmir m'drabonon not to put in 3 lug. This approach assumes that the p'sul d'oraysa will only apply when the she'uvin come before any other water, which is actually not the opinion of the rashbam who holds that until it is full with 40 se'ah the she'uvin have the capacity to passul the mikvah.

Another chiddush of the Rashbam (mayim rabim pg. 43) is that he understands that a 40 se'ah mikvah cannot become passul by adding she'uvin because the she'uvin is batul in the mikvah water. This would imply that if you add more than 40 se'ah of she'uvin, it will indeed invalidate the mikvah (but according to the second answer of rashbam that kama kama batul, perhaps it will remain batul even after the majority is she'uvin). The Beis Yosef points out that all rishonim disagree and maintain as Tosafos writes in the name of Rabbeinu Tam that a full mikvah cannot be ruined even if all the she'uvin in the world is added to it.