According to biblical law any collection of water, drawn or otherwise, is suitable for a mikveh as long as it contains enough for a person to immerse himself (Yad, ibid. 4:1).
The rabbis, however, enacted that only water which had not been drawn, i.e., has not been in a vessel or receptacle, may be used; and they further established that the minimum quantity for immersion is that which is contained in a square cubit to the height of three cubits.
A mikveh containing less than this amount (which they estimated to be a volume of 40 seah, between 250 1,000 liters according to various calculations) becomes invalid should three log of water fall into it or be added. However, if the mikveh contains more than this amount it can never become invalid no matter how much drawn water is added to it.

Mikveh
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