From: Rabbi Moshe Sofer
To: Rabbi Yitzchak
The Chasam Sofer rules that water gradually absorbed through the gaps of paving stones in a mikvah does not constitute zechilah (flowing water) and the mikvah remains valid. He accepts the general principle from the Rashba but questions extending it to a mikvah with an actual piercing, and offers a novel resolution to the dispute regarding the interaction of notfim and zochlin based on the directional mechanics of hashakah and zeri'ah.
Source: Shu"t Chasam Sofer YD §211