Tap water that comes from natural springs cannot serve as mikvah due to water pipe and pump system. I am interested then, how does the rainwater enter the bor? If it can not go trough pipes or filters, how is it done?
Going through pipes is not the issue. The problem is if somewhere along the length of the pipe there is a slight dip or depression, elbow etc. That place causes the water to be gathered, and it then becomes "drawn" and is no longer rain or spring water. A mikveh is constructed in such a fashion that the rain water is directed with very carefully placed pipes that do not have a place water can collect.
An excellent resource is "Understanding Mikvah" by Rabbi Schneur Zalman Lesches. PDF is available here. See page 62.