When it comes to a person who needs to purify themselves, they could either purify themselves through rainwater or water from a spring. Depending on the situation, usually a person, or a keli or a utensil, could be purified by dipping in rainwater. But there are unique situations where spring water would be needed, like a zav, somebody who has a certain type of emission. If he needed to purify himself, he would need to use spring water.
Here we'll talk about the mixture of the two. The Mishnah says: When Rabbi Tzadok testified about a mixture of waters, there was spring water—we'll call it here zochalin, which is flowing water—that got mixed together with notfin. Notfin are trickling water. Trickling means it's notef; it falls from the sky; it's rainwater. Shehen kesherim means that this mixture is kosher (fit for ritual use) to be used wherever spring water was needed, even though most of the water here is spring water and some of it is not. Some of it is rainwater. We say, bottom line, that the majority here is spring water. It's okay to be used as spring water, and we ignore the small amount of rainwater.
There was once a story in this place called Birat Pagi. They had this case. There was spring water that got mixed with some rainwater, and the situation came before the rabbis. They said, “It's kosher; the mixture can be used for spring water.”