Before you panic: does your pond really have a leak?

Water levels in a British garden pond are never static. In a warm, breezy week, a shallow pond in full sun can easily lose two to five centimetres to evaporation alone, and that figure climbs if a fountain is throwing spray about or if marginals are transpiring hard in July. Before you start hauling out the liner, work out whether you are watching normal seasonal behaviour or a genuine hole in your habitat.

What separates the two is the pattern. Evaporation slows in cool, still weather and stops overnight. A leak keeps going regardless. The simplest check is to fill a bucket with pond water and stand it on the edge or float it at the same level, then mark both water lines with a pencil or a stone. Compare them after 48 hours. If the pond has dropped noticeably further than the bucket, you have a leak to find.

Tracking down the source

Most leaks announce themselves by where they stop. Fill the pond to the brim, mark the level, and let it fall. When the water settles and stops dropping overnight, the leak is almost certainly sitting at that waterline — a puncture, a fold, or a crack at exactly that height.

If the level keeps falling to the very bottom, the hole is somewhere on the base, or the problem is not in the liner at all but in the pipework. Turn off the pump and filter for 24 hours and watch again. If the water holds steady with everything off, your liner is fine and the fault lies in the plumbing, a waterfall, or a filter that is quietly overflowing down the back.

Two further checks are worth doing before you start digging:

• Look for wicking. Long grass, moss, or a soil edge that has crept over the liner lip can siphon water out continuously. Trim it back and re-check.
• Use a dye test. A few drops of milk or food colouring near a suspect spot will be drawn towards a hole with surprising speed, even a small one.

Repairing a flexible liner

Butyl rubber and EPDM are forgiving materials and patch beautifully. Hose down the area, clean it with fresh water and a soft brush, then let it dry completely — adhesive will not bond to a damp, algae-slicked surface. Cut a patch of the same material at least five centimetres larger than the damage on every side, and round off the corners so they cannot peel up. Coat both surfaces with a liner adhesive, wait until it turns tacky, press the patch down firmly, and work any trapped air outwards with your thumb. Weight it with a smooth stone for a few hours before refilling.

PVC liners are trickier because they stiffen with age, and HDPE usually needs heat welding. If your liner is more than fifteen years old and brittle, patching is a stopgap — budget for a replacement in the next year or two.

Repairs in a live pond need care. Drain only as far as you must, keep any newts, frogs, and invertebrates you find in a bucket of pond water in the shade, and let aquatic-safe silicone sealant cure fully — usually 24 to 48 hours — before refilling. Refill with rainwater where you can, or with tap water treated to remove chlorine.

Preformed ponds and rigid liners

Rigid ponds rarely leak through the shell itself. The usual culprits are hairline cracks from frost heave or clumsy footfall, a sunken edge letting water escape over the rim, and failed seals around a bottom drain or a planting pocket. Small cracks can be cleaned, abraded, and sealed with an aquatic-safe two-part epoxy or silicone. Anything structural is usually easier to replace than to chase.

Do check the ground around a preformed pond. If the surrounding soil has settled, the shell can tilt, and water simply runs out over the lowest point of the lip. Lifting the edges back to level with a bed of sand or a firm mortar mix often cures the problem outright.

Pipework, pumps, and waterfalls

Flexible hose is the most common leak in any pumped pond, and the least often suspected. Check every joint, clip, and hose tail first. Look for kinks, splits at the pump outlet, and connections that have worked loose over winter. Jubilee clips should be stainless steel and snug, not overtightened onto soft plastic.

Waterfalls and streams lose surprising volumes through splash and capillary seepage along the sides. Line the channel properly, edge it with overlapping stones, and watch the flow for a full hour before assuming the job is done. A filter that sits slightly out of level will spill down its back and drain the pond quietly overnight.

Protecting your wildlife while you work

Ponds are habitat first and water feature second. Time repairs for a mild morning in spring or autumn rather than the height of froglet season or a heatwave. Keep a large container of pond water shaded and oxygenated for any creatures you lift out, return them the moment the water is safe, and leave a ramp of stones or a piece of wood in the refilled pond so hedgehogs and amphibians can climb out if they fall in. A well-repaired pond, topped up with rainwater and edged with plenty of cover, will recover its wildlife within a season — often sooner than you expect.