Pond Leaks Part Two, Advanced Investigation

Figure One. A relatively simple leak test of the outlet tubing is to use an extra piece of outlet tubing. These photos also show the temporary tubing on top of the ground. It is easy to move the end of the tubing to various points in the waterfall or stream to narrow down finding a possible leak in the stream.
Figure One. A relatively simple leak test of the outlet tubing is to use an extra piece of outlet tubing. These photos also show the temporary tubing on top of the ground. It is easy to move the end of the tubing to various points in the waterfall or stream to narrow down finding a possible leak in the stream.

Investigating pond leaks is a simple step-by-step procedure if approached systematically. Most leaks are found in the initial investigations. In Part I of this article [click here] we discussed the easy to find leaks that take very little time to investigate, and we covered a large number of potential leak locations. Time is money, as they say, and we want to be as efficient as possible in finding leaks for our clients. Now, we are going to cover more advanced investigations of leaks in flexible lined ponds. Most of these less obvious sources of leaks take more time to find than the easy ones, the majority of which you will have already eliminated by now.

Waterfall Leaks

Some of the more difficult to find leaks in ponds occur in the waterfall/stream of the pond. An easy way to determine whether this is the case is to shut down the waterfall/stream for 24 hours. If after doing this, the water level of the pond stays constant, you know it has to be in the waterfall/stream or the outlet tubing of the water pump that supplies water to the top of the stream. This is an efficient place to start to narrow down the most likely location of the leak.

Always provide extra circulation within the pond during the waterfall/stream shutdown period, especially if the pond has fish. The pond ecosystem is extremely dependent upon this circulation to keep things well oxygenated. So add extra circulation with aeration and/or water pumps within the pond.

 On one occasion I shut off the waterfall/stream in an attempt to find a leak in a simple oval small 900-gal. pond. With the flow stopped the pond level continued to drop. So… “Ah Ha” – we “knew” the leak was in the pond itself. The pond was emptied of all water and stone. On my hands and knees I went over the entire liner looking for a leak – without success. I was frustrated but I did notice that there was some groundwater welling up under the liner. It felt like a waterbed under foot. I was initially grateful for this groundwater because, if there was a hole in the liner that low in the pond it should be leaking back into the pond through it. It did not do this so I was still stumped.

Another “Ah Ha” moment came and I finally figured it out. It was actually the stream that was leaking. How can this be even though it was off? When the stream was running it was providing a constant source of water seeping under the pond liner and running beneath the pond itself. The ground was saturated and the water table was high. The water did not have anywhere to go but to create a bubble under the liner even though the pond was filled with water. In our situation, the water under the liner was still able to soak away very slowly. During the test of shutting off the stream for 24 hours, the groundwater would soak away and the bubble that was created by the groundwater decreased in size. This dropped the level in the pond and gave the impression that the leak was in the pond. I believe that this is really not all that unusual, so beware.

If, after shutting off the waterfall the pond level does not go down, you can now be certain that it is in the waterfall or outlet tubing. An easy check to determine whether the outlet tubing has a leak is to inspect the water level in the filter falls’ box after a 24 hour shutdown. As long as the check valve is holding water and keeps it from back-flowing in the outlet tubing into the skimmer box/pond, this is a good test. You know there is not much of a leak if the water level stays at the top of the filter falls’ box overflow. Because of the higher water pressure when the pump is running, an outlet tubing leak is much faster when the pump is running. So, give this static test more time to determine if the leak is in the outlet tubing.

Isolating Leaks in the Outlet Tubing or Filter Falls’ Box

Outlet tubing can be compromised in several ways. Good quality tubing is a must, especially if it is to be buried. Once hidden this creates much more work in looking for leaks so we recommend starting off with good stuff. Rodents and burrowing critters are notorious in chewing through outlet tubing. These critters include moles, voles, and mice. For those of us that live where the ground freezes down several feet we have the added variable that the tubing may freeze with water in it, thereby cracking the tubing. Tubing buried in these colder zones must be allowed to drain or be blown out before freezing weather. Of course, if the main water pump is run all winter then this is not a problem.

A test to help find leaks in the stream or outlet tubing is to temporarily replace the tubing with a “test” piece (Figure One). Bring along a piece of tubing of the same size and length that is presently used on the outlet of the waterfall pump. Disconnect the old tubing at the pump and connect the temporary tubing. Then lay this new tubing on top of the ground up to the top of the waterfall or into the filter falls’ box, depending on what you have for the start of the waterfall. Start the pump and run it for 24 hours. If the pond level does not drop then the leak is in the outlet tubing or filter falls’ box itself.

The bulkhead fitting at the bottom of the filter falls’ box is a prime candidate for a leak. I like to use a 24 to 36˝ long soil test probe (Figure Two) for an inspection of areas like this. As long as the stream has been leaking for awhile and it has not been raining much lately this is a very effective tool. Poke the probe down beneath the bulkhead and pull out a soil sample from that depth. If the dirt is muddy somewhere along the length of the soil profile you know you have likely found it.

Locating leaks around the filter falls’ box lip will require the removal of some of the rocks and dirt next to the lip. Attempt to feel with your hand behind the liner under the lip. This can be difficult in some situations but the soil probe may work here, also. If the soil is wet, then the liner attachment to the lip will need to be resealed.

If there were no leaks in those places then the leak must be in the buried outlet tubing. To pinpoint the leak there, follow the outlet tubing with the soil probe looking along its entire length. Of course, we have to be careful to not poke the probe through the tubing. In areas where the ground freezes the leak most likely is at the lowest point of the tubing. This is where the tubing would have had the most ice. Sometimes we get lucky and find a wet spot on top of the ground – most times we are not this lucky.

Isolating a Leak in the Stream

We have ruled out most of the problems with outlet tubing and filter falls’ boxes. If the leak is still occurring with the test tubing lying on top of the ground and the pump running, move the outlet of the new tubing halfway down the stream. Run another 24 hour test. If the pond level continues to drop then move the new tubing to where it is dumping water only a couple of feet up the stream. You get the idea – systematically move the tubing down the stream, waiting 24 hours and assessing the change in pond level each time, repeating until the pond level remains unchanged. This is a good test to localize the leak that is occurring in the stream.

Stream leaks can be caused by several factors. I have already mentioned in Part I, the stream overflowing the sides of a stream, and we have ruled this one out by cruising the edge. Burrowing critters are another very likely cause of a stream leak as well as a leak in the pond itself. Look for evidence of animal tunnels or chewed liner along the edge of the stream and pond. Moles, mice, muskrats and Meadow Voles can cause huge damage to liners by chewing through them. Generally speaking they do not chew a lot in one area because they get wet, but it does not take much chewing to cause big problems. You also do not normally see significant damage much below a foot beneath the soil surface. So, burrowing critters under streams happen quite often. If a client sees much burrowing critter evidence, they need to go on the offensive and take care of the animals before they do any damage.

Finally stream leaks can also be caused by liners that have insufficiently overlapped areas without lap seaming tape. Knowing how the stream was built can save some effort. If available, refer to the original construction notes describing the liner seams.

There is almost always an overlap of liner at the juncture of the stream and the pond where two separate pieces of liner are used. Some installers do not use seam and cover tape in this overlap. Just by the process of capillary action water can flow uphill in-between this joint if not overlapped properly. It is amazing how much water can be lost due to this action. Ninety degree vertical drops need a minimum of a 6˝ lap above the water level of the pond without seaming the joint. I must repeat, this lap must be above the water level of the pond itself. The less vertical the lap, the more lap overlay there should be. Of course, properly joined laps with seam and cover tape are always the best choice. This insures that capillary action will not occur.

If a liner lap leak is suspected, then employ the soil probe mentioned earlier. Attempt to get a soil sample under this lap. However, if the test outlet tubing was performed on the stream, simple elimination will tell you the leak has to be in a certain section of stream. This just may be the best place to tear the stream apart to find it.

Finding Leaks in the Pond Itself

When everything points to a leak in the pond then the investigation turns to waiting for the pond level to stop dropping. The stream and waterfall may be left running during this period because you have already determined the leak is not there. Of course, auto fill valves are shut off during this period. This test period can take from a few days to a couple of weeks depending on how fast and where the leak is. When the level does quit dropping then the hole in the liner can be investigated at that level. Rocks and plants will need to be removed to get to the liner so that it can be felt and observed for a hole or tear.

Leaks in the pond itself are more unusual provided a good underlayment has been installed. The underlayment can help prevent punctures by sharp objects but they still can happen, of course. Deer hooves can tear a hole. The same burrowing critters mentioned earlier (moles, voles and mice) can also chew holes in the liner. Sharp rocks can poke holes, especially when walked on. Abrasion of the liner is a very common occurrence, especially when there are very rough objects against the liner. Concrete is like sandpaper working on the liner. The protective layer of an underlayment is critical in areas like this. It is recommended to also place an extra piece of scrap liner on top of the pond liner to take the brunt of any rubbing of a heavy abrasive object like a big rock or block.

Other Causes of Leaks

Capillary action or wicking action can occur around a pond’s edge. This can occur with the underlayment and may cause water to show up in a different spot from where it is actually leaking, which can confuse the issue. Roots or mud may be overhanging a pond or stream’s edge and suck up a lot of water. It does not take much to provide a route for the water to be wicked away.

Skimmer box openings are very common leak points. The soil can settle in this area and the liner can pull away from the skimmer faceplate (Figure Three). If this is leaking the water level will drop to the bottom of the skimmer box opening and stay. Move the rock from around the front of the skimmer so that you can feel with your hand under the opening. If it is wet the faceplate will need to be taken off and resealed.

Certain plant rhizomes or growing tips can easily puncture a liner from within the pond as well as from outside. Plants like Gold Rush Reed (Phragmites australis), or Bamboo have very sharp growing tips, so either avoid planting this type of plant around the pond or make provisions to keep those sharp rhizomes from touching the liner. Placing them in pots is one way but even this technique is dangerous unless you are watching for this. The rhizomes are so vigorous that they can climb over or break the pot (Figure Four).

Locating punctures under rock is almost impossible unless the rock is removed. The soil probe can work but not much more than a couple feet below grade. Once the rock is removed the puncture or tear can be detected by feel. You may be able to see it but the feel method has worked for me on several occasions. It is unusual to find a leak in the bottom of a pond so this is the last place I look. Of course, this requires a total pond cleanout.

Operating the stream/waterfalls through the winter is done with some ponds to help keep high oxygen levels in the pond. Leaks can develop in the winter as well and if a client is not monitoring things closely then disasters can happen. Ice dams in the stream can occur, thereby backing up water and overflowing the sides of the stream. If high flows are maintained ice dams are less common but can still happen. Power outages can create a problem if the power is out for a long period of time when it is very cold. The stream can freeze, as well as the exposed parts of the outlet tubing. Then when the power is restored major overflows of the stream can occur, as well as other problems.

Those who remove their waterfall pump will need to go through a spring start-up process when everything is returned to normal. This is always an interesting time for those who live where it really freezes hard. Returning the main water pump to service needs to be done cautiously. When the pump is started, always remain around the water feature for several hours to determine if a leak has developed over the winter in the hardware that is located outside the main body of water. I have known clients whose ponds emptied over a few hours after start-up while they left to do some errands.

This brings up a very important principle. Locating the water pump’s intake towards the bottom of the pond can potentially empty your pond if the outlet tubing develops a leak. When the pump is located in a skimmer box, then the problem shifts to a different one. The pond water level will only go down to the bottom of the opening of the skimmer box. The problem then turns to burning up your water pump without any water to pump. Spring water pump start-ups are a dangerous time so beware and be ready.

Almost all possible causes of leaks and how to look for them have been discussed in this two-part article. Considering the expense of water, clients will want that leak fixed – even the small ones. Most are easily found and fixed, whereas only a few really are a challenge. The step-by-step investigation presented here should give you the tools necessary to find or prevent any leak.

4 Responses to Pond Leaks Part Two, Advanced Investigation

  1. Mary Buzza August 31, 2017 at 2:23 PM #

    I built a pond which included one large poly for ponds. The entire pond was included in the large piece of poly, the skimmer and the waterfalls. It has been great for about 8 years, but now I have determined there is a leak on the left side of the waterfall. I bought black expanding foam and spray every where in the left of the pond. (I was not very scientific when I did this. Please give me some advice. or if you live Tampa Fl come by. I was 72 when I rented a bobcat and dug my large hole. Now I will be 80 next month and I don’t mind getting to it and fixing it, but I don’t know what to do. I am losing 3-5 inches every day. My pond is 13ft by 11ft, The deepest end is 3 feet

  2. Lora Lee Gelles September 5, 2017 at 10:28 PM #

    From Jamie Beyer:
    It is good that you figured out that the leak is on the left side of the waterfall. This is a huge step. However, expanding foam does not necessarily stop leaks. It can possibly slow them down but do not rely on the stuff to waterproof. Pulling up the liner along the stream requires rock work but is a better choice. Good luck.

    Have fun water gardening. Jamie

  3. Jeff Krenner September 24, 2017 at 9:14 PM #

    Pond leaks can be a difficult to find, but when you use the right approach and take your time to really gather information about the leak you will be surprised on easily it can be narrowed down. Check out this link if you are still stuck on finding your leak.

    Paradise Ponds and Waterfalls – Pond Contractor
    Buford, GA


  4. Jeff Krenner September 24, 2017 at 9:16 PM #

    Hello Lora Lee,

    The correct link for my comment above is


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