The short answer is that a spring is where an underground aquifer runs out above the ground. So you are building a pond with a connection to another body of water below the ground. Water is heavy. The pressure of your ponds water column might not allow the spring to flow but make it suck. Now wouldnt that suck. Your pond would have a drain.
Or it might work fine at times of the year, and suck water from your pond at dry times. Or others might have wells that tap into the aquifer, and your pond would provide a supply to them.
Too many potential problems.
