Hey Crazyj, Nicely written post, I'm surprised you haven't gotten any feedback as your dilemma is interesting. I love your pond setting - you've got in-ground water sprinklers on the dam, I'm jealous...
My guess is that your old system was made from steel? If so, it is now rusty steel and probably has some rusted/rotted holes in it allowing water to seep into the now extended drain pipe. This rusted/rotted piping may be where you iron is coming from. Or it could be merely iron in the dam soil surrounding the old drain system. Did you take your pond water sample from the bottom of the pond near the old drain pipe or just off the surface? Iron levels might be different at the bottom, I'm guessing. Can you determine any water level changes. I'm no expert on the topic as my newly renovated pond is trying to fill up. Hopefully I don't have to worry about leaks, but you never know. I think there are only three possibilities here. One is that the old system is allowing water to seep from the pond out the old drain pipe (with extension). Another possibility is that natural ground water (water table or spring) is seeping into the old drain system. Or a combination of the two. I'd bet on the first. I like your approach using test kits to determine where the water is coming from and think other methods may be necessary as well to pinpoint the actual problem (like monitoring water drop in pond and comparing it to the output at the extension drain pipe, other water chemistry tests, ect.) As far a s fixing it...not sure at this point. Plugging the abandoned drain pipe might not totally stop the leak, but the leak may eventually slow to a stop as the leaking water carries dirt and debris and fills the crevasses kind of like "stop leak" for radiators. How was the old system plugged? Could it be seeping around that?
Hopefully my thoughts help you out and maybe this bump will get more to respond.
