The next day not much got accomplished because some of the workers didn't show up, and they didn't have enough pumps and hoses to drain the water down below the riser pipe in time for the cement truck to show up.
Next morning one of the guys came up with an idea to place a chunk of 36" culvert around the old riser pipe (24"), shove it down deep into the mud and drain the water from around the old riser pipe inside the 36" one. This is a photo they sent me, looking down into the old drain pipe, showing water leaking from the deep drain out in the middle of the pond and passing by through the bottom. I hope this makes sense.
They wasted some concrete trying to fill it, but finally got it. Unfortunately it was one day I couldn't be there so I hope they didn't cut corners.