Three years ago when they excavated Grasmere Lake in Washington Park and put in a double-layered liner, they said it was because the lake had been leaking. What? Lakes leak. That's part of being a lake.
The real reason was to prevent the Purple Pipe Water (containing Lowry effluent) from leaking into the ground water below the lake. The contents of the Purple Pipe Water is above the allowable standards for leaking into ground water. Just like a tertiary treatment evaporation pond has to be prevented from leaking into ground water, so our "Lakes" must have rubber bottoms if they get the Purple Pipe Water.
Back then I was informed that "in a couple of years" Ferril Lake and Duck Pond in City Park would be outfitted with their rubber bottoms.
Word of warning
If and when they finally drain Duck Pond and start the removal of the sediment and muck, DO NOT HANG AROUND THAT AREA. If it dries out and gets dusty, the dust will be toxic as it will contain at least 6 years of sediment from the Lowry Landfill Superfund Site water.