Perhaps nothing is more frustrating that finally constructing the pond or lake of a lifetime only to find that it does not hold water. Just as frustrating are repairs and repair theories that do not hold water.
We will focus our leak location and repair strategies in 2 main areas – 1) SYNTHETIC LINED BASINS from small garden ponds to multiple acre ponds and lakes. 2) NATURALLY LINED BASINS – can be excavated ponds or lakes with no dam embankment or a pond or lake constructed with a dam.
First things first – how do you locate the leak? This can be exceptionally simple or painfully depending on the situation. Let's state the obvious and move on from there – look for the wet spots around the pond. A soil probe will allow you to find moisture below the surface. If you do not have access to a soil probe, a ditch shovel will work just fine. Probe or Dig as deep as you can in the suspect area and examine the soil for moisture content. The best time to look for moist areas around the pond is after an extended dry period – preferably a week or more. If you find significant moisture, you are likely in the vicinity of the leak and can narrow your search. Pay attention to vegetation around the pond. Lush green vegetation during extended dry periods can help you narrow the search as well.
Once you've narrowed the search or if you just can not narrow the search, allow the pond water surface to fall without replenishing the water until the water level stabilizes. Mark this level and explore the pond around this elevation. For a synthetic liner pond, you have found the elevation of the leak, careful inspection of the liner at this elevation should allow you to find the leak.
If you have an EPDM rubber liner (Firestone Pondgard or similar) the repair is quite simple once you've found the leak. Purchase an inexpensive EPDM Repair Kit and apply per instructions. If this is the only leak in your pond, you may refill and enjoy!
If the pond is a natural bottom or unlined pond, the search becomes a little more complex. As with the lined pond, it is worth examining the pond at the elevation where the water level stabilized. There are 2 reasons that the water level may have stabilized at this level. One, this is the elevation of the leak or two, this is the natural groundwater elevation (water table). A simple pump test will determine with reasonable certainty whether or not the pond bottom has intercepted and is connected to the water table.
To accomplish the pump test, select a pump that will allow you to draw down approximately 6 inches of water from the pond surface in less than 3 hours. Mark the starting water surface elevation and ending water surface elevation. Over the next 6-8 hours monitor the water surface carefully. If the water surface rebounds toward the starting point, the pond has most likely intercepted the water table. If the water surface stays at the same elevation, the initial water surface elevation is the elevation of the leak.
Now that the leak has been located or at least the location has been narrowed down, it's time to review the options for leak repair.
First and foremost let's dispell a common misconception. Sprinkling a modest, or even substantial amount of bentonite in any of it's various commercial forms into standing water is VERY unintentionally to stop or slow a leak. Bentonite is an expanding clay found in Wyoming which does have a role in repairing leaks and sealing water bodies. To be certain it is a great product when used properly.
To properly apply bentonite to seal a pond, the pond must be drained and the bentonite then incorporated into the soil of the pond bottom and sides. The application rate for bentonite ranges from 2 pounds to more than 5 pounds per square foot depending on the soil characteristics. For clayey and silty soils, 2 pounds per square foot MAY be sufficient. For sandy soils 4 pounds MAY be sufficient and for soil containing gravel and rock, in excess of 5 pounds will be required to obtain a seal. Once the bentonite is applied, the soil should be disk harrowed to mix the soil and bentonite and then compacted.
For a leaking embankment, if the location of the leak has been identified, it may be possible to construct a core trench in the embankment to slow the leak. To install a core trench, excavate a minimum 12 inch wide trench to 3 feet below the pond bottom elevation. The trench should be located as far back from the pond edge as possible while still remaining on the flat top of the embankment. The goal is to create an impervious core in the embankment which will not allow water to pass through. This impervious core can be created in a number of ways. Bentonite can be mixed with the excavated soil at a rate of 30% bentonite and 70% native soil. The material should be placed back in the trench and compacted with a trench roller, whacker, or excavator bucket in 6 inch lifts. Alternatively, EPDM liner can be buried in the embankment by draping a solid sheet of EPDM liner along the trench wall closest to the pond and carefully backfilling the trench. Again, the soil should be compacted as the trench is backfilled. Applying a bentonite mix at the bottom of the liner curtain and at the ends of the liner curtain will help to prevent water from seeing around the curtain.
Another common location for leaks is along pipes which penetrate the pond embankment. All pipes that are installed through the pond embankment should be fitted with an antiseep collar to prevent water from flowing along the pipe. If a pipe is installed through the embankment without an antiseep collar, the pipe can be excavated and a bentonite plug installed around the pipe. Ideally, the bentonite plug should extend at least 3x the pipe diameter around the pipe.