This information can be applied both to new ponds which leak, and established ponds and waterways with damaged and leaking linings.
Water levels that fluctuate with the season and prevailing weather are not necessarily a problem, but artificially lined ponds that hold insufficient water even in the wet season will normally need repair. Artificial linings that are exposed are likely to suffer further damage without the protective covering of water.
Badly leaking puddled clay liners, failed ponds constructed using bentonite powder, and badly cracked concrete ponds will need a rethink. If a pond is still required on the same site, the best solution is probably to reline with a flexible liner. The pond will need clearing out completely of vegetation, debris and other material, in order to start from a clean base, otherwise damage to the liner is almost inevitable. If the space is available, it may be easier to start again on a new site, and turn the leaking pond into a marsh. This will probably be happening anyway, but the process can be made more interesting and attractive by planting and other management, depending on the site and available resources. Rearrangement of clay or other material within the site can create dams or small water features to retain some open water areas. If the area is fed by stream flow or run-off, there is the opportunity to be creative in channelling the water through small pools or ditches. Failed ponds which relied only on mains or pumped water are possibly best abandoned.
Tracing and repairing leaks
The following procedure for tracing leaks is described for puddled clay linings, but the same procedure can be used for other types of linings, and for leaky dams, retaining walls or earth banks.
- If the water level has recently been raised above a level which had been normal for some time previously, it is likely that the clay above the long-standing water line has dried and cracked. Look for leaks between the old low water mark and the new high water mark.
- The lining may have been damaged by disturbance due to recent cleaning, or from trampling by horses or cattle. Check the cleaned area, and the trampled shallows.
- Once the area under suspicion has been identified, trace the leak using fluorescein dye. Fluorescein is available from chemical suppliers, and comes as a magenta or coffee-coloured powder, which turns a bright neon green on contact with water. Localised leaks in small pools may be traced with green ink if fluorescein is not available.
The amount of fluorescein needed depends on the volume of water to be treated, so it is worth limiting this if possible to the area around the suspected leak. For a water line leak, scatter the powder along the water ’s edge, not out away from the bank. For a leaky dam or retaining wall, scatter powder within a few feet of the installation. For a holed bottom, it helps to drain the pond so that only a few inches of water cover the lining before sprinkling the dye. Use your own judgement as to quantity. A tablespoon of powder dyes about a quart of water. - If you suspect that moisture on the outside of a dam or bank is due to a leak through from the other side, wait to see if fluorescein-tinted water penetrates to the damp spot. This may occur within a few hours, but more often takes a day or so. The advantage of waiting is that in this way even an indirect leakage route can be established.
If, after a few days, no sign of green water appears outside the bank, the leak may be slow and extend some distance, so that the dye has been absorbed en route. Alternatively, the damp patch may be not due to leakage. - After allowing the dye to work for a day or two, drain the water from the pond. Rake off the last bit of standing water, using a rake to which a wooden slat has been fixed to form a flat, toothless scraper. Observe the surface of the clay as you rake. Any cracks should show up as green lines in the clay.
- Dig away the clay where it shows green, following the dyed crack as far as you can. Remove this clay and repuddle it carefully, adding fresh clay as needed.
Concrete
Minor cracks in concrete can be repaired as follows. Locate the crack and clean thoroughly. Using a cold chisel, cut out a rough V shaped groove at least 75mm wide along the line of the crack, and at least a third the depth of the concrete. Use a rubber or bitumen based sealant to seal the crack below the groove, and then fill the groove with a mix of 1:2:4 cement/sand/fine gravel with a waterproofing bonding agent. Wet the sides of the groove before mortaring. Leave to dry for a few days, and then paint over with pond sealant.
A concrete pond which has become porous but is otherwise intact may be repairable by using a waterproofing sealant. The concrete surface must be cleaned and primed first, in accordance with the instructions on the sealant.
Flexible liners
Punctures or small tears in butyl and other flexible liners can be repaired with patches and adhesive. Contact the supplier of the liner for details.
Watch out for couch grass, bramble, bamboo and other plants which spread by underground shoots and may puncture a flexible liner. Remove any that are spreading near the edge of the pond.
