Date: Tammuz 5583
From: Rabbi Moshe Sofer
To: Rabbi Elyakim Getz Konigsberg
When drawn water was poured into an adjacent spring-fed well, causing the combined water to flow through a connecting pipe into an empty mikvah, the Chasam Sofer rules the mikvah is entirely valid. Since the drawn water was absorbed into the spring water — which alone contained sufficient volume — the water that flowed into the mikvah is considered valid spring water, not disqualifying drawn water.
Source: Shu"t Chasam Sofer YD §212