I'll jump in...
I'm thinking quarry size and depth...ambiguous, I know. Then again, so is the amount of details in the question. Not that it would help me much. It's been way too long since I studied thermodynamics!
My 10 foot deep 1/4 acre pond, with aerations, has no chance of staying cool. In fact, It gets borderline bath tub temps.
Considering that the 20 to 30 foot deep sub soils tend to stay at a constant 50 to 60° F...you would need a large volume of 50 to 60° water to pull to the top in order to keep the swimming depths near 65° F around the clock or even close.
The pond would have to be that 20 to 30 foot deep just to get to that constant sub soil temp. Then, there is the volume of cool water needed to not be overtaken by the mixing of the upper warmer water. The upper 10 foot would be the swimming volume and I am guessing that the volume of cool water would need to be at least 10:1 (meaning 100 foot deep, maybe 5:1 - that's still stupid deep if you are digging). This guess neglects the heat transfer (or cooling effect) of the deep water to soil interface which might lessen the needed depth. The above mumbo-jumbo assumes that the aeration would turn the pond several times a day and the pond owner wants to maintain the 65° temp pretty much 24/7.
We can cheat a bit. An un-aerated 20 foot deep pond will develop a thermocline (where? - I don't really know - it depends), let's say at 8 foot deep. Set the aerators up near/around the swimming area just below the thermocline and just turn them on before going swimming. This will bring cool water up pretty quick and cool the swimming area down for the time of use. The swimming water probably won't get to 65° in the heat of the summer, but certainly much cooler.
If it's a small pond, I would not want fish in the pond as sporadic aeration would not be easy on them and not having fish would reduce the potential stink of pulling bad water up.
My 2¢ in 1000 words or less.
Last edited by Quarter Acre; 03/05/21 01:18 PM.
