Unless it is a farming type irrigation well, it won't be a heckuva lot of use on a 21 acre lake. I don't remember how many gallons of water there are in an acre foot but it is a bunch. Calculate gals. per acre X 21 X avg depth. Then look at how many gallons per minute your well produces to find out how much effect you will have. It's probably like spitting in the ocean. 21 acres is a bunch of water. I'd worry more about the watershed and the depth of the lake.
