I had the same problem on my pond. Over time more leaks will appear as the pipe weakens. This is what I ended up doing. I got a piece of thin rubber matting (like used in tile applications) and wrapped it around the pipe were the holes were. This stopped the flow of water well enough to allow me to concrete the pipe. After that a had a syphon system installed. This will allow you to raise the water level if you so desire.
Putting the matting on the pipe is the most difficult part. I had to be very carefull not to make more holes. I had to use scuba gear because there were some deeper holes. The water presure held most of it in place and I used large zip ties in the rest. If the holes are near water level is should not be hard. Getting rid of that stand pipe was the best thing I ever did, at least for my peace of mind.
Good luck