Repairing leaks in a garden pond

First, measure the loss

Every pond loses water. On a warm, breezy summer day, a pond can shed 2–3cm a week (around an inch) purely through evaporation, and that is entirely normal. What is not normal is a pond that drops 2cm overnight, or that keeps falling through cool, still, damp weather. Before you start hauling rocks about, spend a week measuring rather than guessing.

• Set a straight-edged ruler or a marked cane vertically in the water and note the level each morning and evening.
• Float a bucket of water on the surface alongside it. If pond and bucket fall at the same rate, you are losing water to the sky, not to a hole.
• Switch the pump off overnight. If the level holds steady with the pump off but falls when it runs, the fault is in the pipework or the waterfall, not the liner.

Write the figures down. A pattern emerges quickly, and it saves you from dismantling a rockery for nothing.

Let the water level find the leak for you

This is the oldest trick in the book and it still works better than anything else. Turn off pumps, filters, top-ups and anything else moving water. Top the pond up to its normal level, then simply wait. Let it drain naturally — no topping up — until it stops falling. Where it settles is almost always just below the leak.

If the water comes to rest at the marginal shelf line, the problem is around the shelf or the planting pockets. If it holds at the base of the waterfall, suspect the falls. If it drains all the way down to a few inches, the liner floor or a below-ground pipe is the likely culprit. It is a slow method, taking perhaps a week, but it narrows the search enormously.

Checking the liner

Walk the whole perimeter slowly, on your hands and knees if you must. Folds and wrinkles in the liner are prime suspects: water tracks along the crease and escapes over the edge. Look for punctures from herons, falling stones, sharp roots, dog claws, or a child's toy that spent the winter under the surface.

Pay close attention to the classic weak points:

• Where pipework passes through the liner — a common failure point as the liner ages and slackens.
• Around bottom drains, skimmers and inlets.
• Under heavy stones sitting directly on the liner, especially where the underlay has shifted.
• Areas where the liner is exposed to sunlight, which makes it brittle and prone to cracking.

Feel the underside of the liner where you can reach it. Damp soil, a dripping sound, or an unusually lush patch of grass beside the pond are all useful clues.

Pipework, pumps and filters

Flexible hose hidden behind rockwork is the most commonly overlooked source of leaks. It perishes in sunlight, splits at bends, and works loose from jubilee clips over winter. Check every joint methodically: the pump outlet, the filter inlet and outlet, any UV unit, and the point where the hose meets the waterfall.

Look for mineral crusting or limescale around a joint — that usually means water has been seeping there for months. Run the pump and watch each connection with a torch, even if you must wait until dusk to see the glint of moving water. A split hose can lose more in a day than evaporation takes in a fortnight.

Edges, margins and capillary loss

This one catches out even experienced pond keepers. Water can be drawn out of a pond by capillary action through overhanging grass, moss, marginal plants and even stone edging if the liner edge sits below soil level. The liner then acts as a wick, quietly siphoning the pond into the surrounding border.

Symptoms to watch for: a permanently damp patch of soil at the pond edge, moss that never dries, or loss that only happens after watering the garden. The remedy is simple. Lift the liner edge, fold it so it stands at least 5cm above surrounding soil level, and backfill behind it. Cover the folded edge with stones or turf so it looks intentional, and trim back any vegetation that trails into the water.

Making the repair and refilling

For small holes underwater, clean the surrounding liner with a stiff brush, dry it if you can, and apply a pond-safe sealant designed for the job. For anything larger, use a patch kit matched to your liner material — butyl, EPDM and PVC each need different adhesives.

• Cut the patch at least 5cm wider than the damage on every side, and round off the corners so they cannot peel up.
• Clean and dry both surfaces, apply adhesive to both, and press firmly, working outwards from the centre.
• Allow the full curing time before refilling, however tempting it is to test it early.

If you must drain down, save as much pond water as you can in tubs or a paddling pool, and keep fish in a shaded, aerated container. Never let filter media dry out. Refill slowly, using rainwater where possible, and dose any tap water with a dechlorinator. Test for ammonia and nitrite over the following week, and expect a brief green tinge as the pond rebalances. With patience, the level should hold — and stay held.