It sounds like you have a holding tank for a spring. My Grandparents had one on the hill above the house, but not nearly as big as yours. Is the water drinkable? I mean, if your in a drought swimming in it seems like a waste. We're in drought here in New Mexico and I'm going to put in a cistern and grey water holding tank, as well as Xeriscape. It would be interesting to know the history of your tank, whatever it is.
All The Best, Ron

If it's potable but not enough water to swim in, you might want to close it against mosquito's and really use it. You can use a cistern-gauge to know the water-level inside.
( www.cisterngauges.com ) Did you do a dry-residu test with the water? At least you can use it as a tank to store water for plants and put a barrel with guppys inside, against mosquito's. Close the barrel with screening to keep the parent-guppys in. The younger ones will eat the mosquito's, when the water-level rises.

Here is a link that might be useful: cistern-gauges water tank gauge

Cistern Supply sells the most inexpensive gages on the market at $40 and up for complete systems that install easy.
cisternsupply.com

Here is a link that might be useful: Cistern Supply Co.