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How to Repair a Pond Liner
You are sure you are facing repairing your pond liner. Before you panic, make absolutely sure it is a leaking liner and not another problem that is causing your water level to drop. Check your waterfall, check your hoses, check your spitters. Turn your pump off, fill up your pond. Watch it overnight. Did the level drop? If so, you probably have a hole in your liner.
The first thing you must do is find that leaking liner. If you can see it, skip the next section.
Fixing the leak
If you can see the leak, you have an easy job. Assuming your liner is rubber or EPDM and most are, you can patch it just like a tire. You can buy a patch online and while you are at it, buy a roll of patching material, because whatever caused that liner to leak will most likely do it again.
45 mil rubber liners rarely leak spontaneously. Something causes it. It could be raccoon toenails, doggie toenails, a falling tree branch, or maybe a piece of statuary with a sharp end fell and penetrated the liner. No matter the cause of that liner leak, repairing it, while not always a clean and easy job, it can be done by anyone with a bit of effort.
Make sure the liner surrounding the hole is clean and dry. I scrub the liner with the same scrubber you would use for dishes, just make sure there is no soap in it. After scrubbing it clean, rinse it and let it dry. If you are in a hurry, use a hair dryer, but be careful you are connected to a GFI before you use any electrical appliance around water. When the area is clean and dry, apply the patch according to the directions on the package. Your work is done. Fill the pond back up with water. Don't forget the dechlorinator.
Finding the leak
Finding a liner leak can be one of the most frustrating jobs on earth. The liner is black, usually dirty and wrinkled. The leak should be right at the water line because water can't leak out below that. Or can it? Here in New Orleans, our water table is only inches below the ground, so often there can be a hole in the bottom of the liner, but ground water pushes the bottom of the liner so a leak could well be on the bottom and ground water is seeping in the pond.
Pour some milk in the pond. If the water is leaking fast, the milk will make a trail to the leak. But if the water is leaking that fast, you have a huge tear and you can probably see it.
Pour some fine sand in the water. The sand will follow the water to the leak, but as above, if the tear is that big, you should need no help finding it.
Sometimes you cannot find the leak no matter how hard you search. I have resorted to this method as a last resort. Pump the pond water out. Remove all fish, plants, pots, lights, etc. Using a wet vac, vacuum the bottom to be sure all the fish waste, leaves, and other organic matter is gone. Rinse and wet vac again. Make sure the bottom is dry even if you have to wait several hours or get that hair dryer out again. When you are positive it is dry, stick a garden hose under the liner and turn on the water. Watch the liner carefully. Soon you should be able to see water coming up from underneath. You have found your leak. Dry it again and patch with patching material. Replace all pots, plants, fish, lights and refill the pond with water.
It's a good thing rubber liners rarely leak.
Leaks in concrete ponds
If your concrete pond is leaking, there is only one relatively cheap way to fix it. Because concrete is brittle and our ground is always moving, concrete is freezing and thawing, concrete is one of the hardest materials to repair. Clean out the crack and use Plumber's Epoxy to patch it. If that doesn't work, call a professional. If the professional tells you he can patch your concrete and guarantee it, doubt it. Doubt it a lot.
In all my years of pond building, I have never seen a serious concrete crack patched so it will hold water for more than a few weeks. Get your contractors guarantee in writing, get his home phone, his cell phone, his address and his Landscape Contractor's license number. Getting his insurance certificate can't hurt either.
If your pond is built from recycled swimming pool liner or PVC or visqueen and it has a leak, you must start over again with another liner. Same with pre formed hard liners. Once they crack, they cannot be repaired.
Luckily ponds rarely leak. But if yours does, you are now prepared.