Unfortunately, today was the first time that I read this thread but its a good topic so I'll try to answer. Hydrogeologists rarely if ever use the term "spring". Instead we think about aquifers, which can be loosely classified as water table, confined or "perched". The concepts are really simple but of course there is quite a bit of jargon.
Oversimplifying there are basically two things that will determine whether the "spring" will add or subtract water. The first is the vertical and lateral extent of the permeable material (aquifer). If it is an isolated sand lens that is surrounded by clay, there is little potential for water to move in or out of your pond. If it is a thick aquifer that has a regional extent there may be great potential for water flow. Think of this as the diameter of a pipe. The larger the pipe, the more flow. You would need to install borings, test pits or look at water well records to evaluate the capability of the aquifer (transmissivity).
The second major factor is the hydraulic head or gradient. Confined aquifers generally have a hydraulic head that is elevationally higher than the top of the aquifer. This is due to pressure from overlying sediments, or because the aquifer is hydraulically connected to upland areas that have higher water levels. Groundwater moves from areas of high head to low head (downgradient). Note that downgradient can be up! If you cut through the confining layer water flows out because of the higher head, which is sometimes called "artesian" or a "spring". If you were to put a drive point or well in the permeable material that is creating the spring, the water level would eventually stabilize in the well and you could measure it. If the stabilized water level in the well is lower than your expected pond level (it sounds like it would be), then there would be the potential for the pond to leak. The rate of the leak is determined by the size of the hole (diameter of the spring), the difference between the pond level and the hydraulic head of the aquifer (gradient) and the transmissivity of the aquifer. The higher head of the filled pond forces water out through the spring and into the aquifer.
If the water level is near the expected pond level, it would fill your pond, and if enough water was available, it could keep the pond full all year with relatively cool water (groundwater temp is equal to your area's mean annual temperature). Also realize that the water is coming from somewhere and it is possible that wetlands or ponds upgradient from you could go dry when the "plug" is pulled. When the pond level is equal to the head of the aquifer you have reached equilibrium and there is no flow in or out. Of course natural systems never attain equilibrium because of rainfall, evaporation, leaking confining layers, etc., so there is always a small potential in one direction.
From the above discussion, my guess is that you hit an isolated seam, or an aquifer without much head above your 25-foot depth. Adding 25-feet of water will put some stress on the system. The safe thing to do would be to patch it. Sorry about the long post, hope that helped.