My Pond is Leaking. How Do I Find Water Leak? Beware Natural Siphons
Q: My pond leaks, but only sometimes! If I leave it alone, the water may only drop less than an inch in a week, but sometimes, particularly just after I have topped it up, it will drop six inches overnight. It is partly raised out of the ground at one end. It has a butyl liner, but I cant check it for leaks because it has stone-wall facing built on the inside down to the level of the marginal shelf.
A: The pool is siphoning empty. When you top it up right to the top your are effectively filling up a siphon tube that once it is full and overflowing out of the pool just sucks it dry. It is same principle as a mens urinal automatic flush, except instead of bit of pipe and plumbers fittings providing the siphon, one has accidentally been set up underneath your stone work. I have seen this a couple of times in formal style pools that usually have a block work or concrete framework. It generally occurs where there is a flap of liner folded flat against the side of the pool and then the liner is over the framework and down the outside. The siphon tube is effectively the flap of liner, but it can occur just between a layer of concrete or cement mortar and the liner. This is more incipient as it does not act as a siphon straight away because the mortar will have originally stuck to the liner. Eventually it comes away just enough to enable a steady draw of water up the side of the pool by a capillary action effect. This can then find it way over the side.
An effective cure can be achieved by dismantling the walling where you think the siphon is. The whereabouts are sometimes revealed by tell-tale damp patches on the outside wall or at the base. I have also managed to cure the problem by enabling air to get into the tube and thus breaking the siphon effect by drilling small holes in the pointing of the coping or capping stones at the critical point.