Date: Between Rosh Hashanah and Yom Kippur
From: Rabbi Moshe Sofer
To: Rabbi David
The Chasam Sofer endorses the Noda BiYehudah's ruling that boards and nails lining a mikvah pit to prevent water loss do not constitute mahaveh bidvar hamekabel tumah, since the boards are not themselves any form of mikvah water. He distinguishes this from Rabbeinu Tam's sukkah case, where a sheet is itself a type of s'chach and thus retains independent identity as pasul s'chach, whereas the boards are categorically different from mikvah water and merely support the existing valid volume.
Source: Shu"t Chasam Sofer YD §204