Hi. I hope someone can help. I have just moved house and have inherited a pond. An upper and lower pond joined by a channel, all concrete constructed and completely lined with one sheet of pond liner. The channel is approx. 43cm wide x 56cm deep. it is bridged by a sandstone slab over which the water flows from the upper to the lower pond, forming a waterfall. Behind the sandstone slab on the upper pond side there is a concrete barrier the same width and depth as the channel and approx 5cm thick. This is set against the sandstone block and on top of the liner to prevent the top pond flooding the lower pond. this seems to work, however there seems to be a leek in one vertical corner of the pond liner where the concrete meets the liner and is allowing the water to go under the pond liner and find it's way down to a drain hole at the lowest point in the concrete floor of the lower pond, so very slowly draining the pond. hence I was hoping I could seal this off. Is there any kind of sealant, something on the lines of bath and shower silicon sealant that can be used. Sorry this is so lengthy. Thanks in advance for any help.