Cecil, the evaporative cooling would only cool the water falling from the sprinkler and I am not sure how much cooler than 52 degrees it would get. The cool water would mix with the warmer surface water and produce some cooling; it would be interesting to know if it could cool more than the other factors could heat.
My understanding is that using surface aerators warm the water because it mixes as it pulls water into it, but what would happen if you forced it to pull from the very top layer of water. Fountain Aerators I have seen draw vertically, but if they could be rigged to draw horizontally at a couple inches below the surface it should minimize mixing of the water and cool the top layer of water.
Another option that might work if you don’t mind the expense of pumping water from the pond is a bong cooler (check it out at http://en.wikipedia.org/wiki/Bong_cooler).
These are used for water cooled computer systems, but might be able to handle cooling needs of a trout pond. It also could be rigged into a system similar to Cecil’s bucket aeration posted under “Cecil's place”.
I am also interested in the geothermal idea. If anybody has tied this let me know! It should work the trick is finding the right length of Pex tubing to run. If you need to run 400’ of tubing to maintain cooling through the summer it could get pricey and will tear up a lot of the yard. I wonder what would happen if several sets of pex coils were buried beneath the bottom of the pond at construction…
My last idea came from the keg-a-rator I built in college. This would require a lot (maybe 1000’) of 3/8” or ¼” tubing coiled wrapped around kegs (or 5gallon buckets full of water/ice) in a chest freezer. The idea is to pump the pond water through the tubes in the chest freezer and hope the water chills down to the 60s and then pump it out to the pond again.