What is this body of water south of Tucson?

I sometimes like to explore random locations on Google maps satellite view in lieu of traveling. This time I noticed something that piqued my curiosity west of Green Valley. If you go to 31.8516124, -111.0455142 on Google maps, there's a fairly circular body of water about a kilometer in diameter on top of some odd, terraced, clearly artificial hill. I'm not from the Tucson area myself, so I was hoping someone local would have an idea what this is.

My best guess is some kind of disposal pond for liquid waste from what appears to be a nearby copper mine. What struck me is how clear blue the water appears. Not what I would've expected from a polluted pond. I also wouldn't have expected it to be so large; this thing is a kilometer wide. Anyone have a better idea? Substantial searching has yielded no concrete answer, so this is my shot in the dark.