I know you are all dying to know what ponds have to do with propane, so I researched it for you:
Once operational, as NGLs are pumped into the cavern for storage, brine is displaced to the surface and deposited in large brine ponds. To remove NGLs from storage for delivery to customers, brine from the surface ponds is allowed back into the cavern, displacing NGLs to the surface.
Because NGLs, such as propane, butane and ethane are lighter than brine, they will float on top of the brine within the caverns. To inject products into the cavern, the products are pumped down the outer annulus of the pipe casings and the brine is displaced up and out of the inner brine pipe. The reverse operation brings product out of the cavern by pumping in the brine and retrieving the product.
https://www.keyera.com/titanweb/keyera/w...25768500707E23?OpenDocument#.VK7cvZ34CUk