Short answer: To fix a leaking dam, first work out whether the loss is evaporation or seepage, then match the repair to your soil. The four proven methods are compacting a clay blanket, laying a bentonite blanket, applying gypsum to dispersive clay, and installing an HDPE or polypropylene liner. Cost rises in that order, and so does reliability.

A leaking dam is the most common call we take. Most owners assume the wall has failed; in practice the leak is almost always in the floor or a poorly compacted contact zone, and the repair you choose has to match the soil you actually have. Guess wrong and you can increase the leak. Lindsey Hughson has built and repaired farm dams across Queensland and northern New South Wales since the early 2000s, and the diagnostic order below is the one we use on every site before quoting any work.

First, work out whether your dam is actually leaking

Evaporation across an Australian summer is brutal — a typical Queensland farm dam can lose 1.5 to 2.5 metres of depth in a hot summer through evaporation alone, with no leak whatsoever. Before you spend money on repairs, separate the two losses. Drive a star picket into the floor of the dam in the shallows and mark the waterline. Run a second mark on a length of PVC pipe floating on a styrofoam block, sealed at both ends so it cannot fill. The floating mark records evaporation; the fixed mark records evaporation plus seepage. The gap between them, over a week, is your actual leak rate.

Seepage is the technical term for water moving through the dam floor, wall, or contact zone. A 1 metre thick compacted clay blanket should reduce seepage to no more than 2 millimetres per day, according to the WA Department of Primary Industries and Regional Development. If your gap-between-marks is larger than that, the floor is the prime suspect.

How can I tell if my dam is leaking or just evaporating?

Run the two-stake test described above for at least seven consecutive dry days. If both marks drop at the same rate, you are watching evaporation, not a leak. If the in-water mark drops faster than the floating mark, the difference is your seepage rate in millimetres per day. A reading above 5 millimetres per day usually justifies repair work.

What does a leaking dam wall actually look like?

Walk the downstream batter of the wall the morning after a calm night. A leaking wall shows damp soil, brighter green grass, or a slow trickle exiting at or below the original ground line. On heavy clay soils you may see no surface flow at all — the water is moving through the embankment but emerging diffusely. Tunnels, sinkholes on the upstream face, or boils on the downstream face are warning signs of a piping failure and should be assessed before the dam is refilled.

Why farm dams leak: the three real causes

Most leaks come from one of three sources. Knowing which one matters because the fix changes completely.

Wrong soil for the site. Sandy soils, gravelly soils, and red loamy soils with broken rock are inherently leaky. DPIRD WA specifically flags red loamy soils as more likely to leak.
Poor compaction at construction. If soil was placed too dry, in layers thicker than 200 millimetres, or compacted with the wrong machine, air voids remain in the embankment and water finds them.
Dispersive clay that has tunnelled. Dispersive clays break down when wet and form internal tunnels. A 2017 summer storm in WA caused tunnelling in dispersive clay linings that had previously held water for years.

The four proven ways to seal a leaking farm dam

Australian government extension services have road-tested four methods. Each one suits a different soil and failure pattern. The table below summarises the trade-offs; the sections beneath it explain when to use each.

Success rates and cost ratios above are drawn directly from DPIRD WA’s leaky dams guidance and reflect what we see in the field on Queensland and NSW jobs.

When does blending and compacting work?

Blending and compacting suits a dam where the existing floor has enough clay content but was never compacted properly. The dam must be drained. Soil is ripped, brought to optimum moisture content, placed in maximum 200 millimetre lifts, and rolled with a self-propelled padfoot roller. Optimum moisture content sits between 24% and 12% for most clays and silty clays. This is the cheapest fix at one-third to two-thirds of replacement cost, but its 50% success rate means it is best on dams that were marginal rather than catastrophic.

When is a local clay blanket the right repair?

A clay blanket is the most reliable on-site repair when you have access to suitable clay nearby. The target is a 1 metre thick blanket compacted to at least 92% of maximum dry density. Suitable dam-building clay contains 20% to 40% clay, has a well-graded particle size, shows only slight to moderate dispersion, and is dominated by kaolinite. On a property near Goondiwindi in 2024, we rebuilt a 4 ML stock dam with a 1 metre kaolinitic clay blanket sourced from a borrow pit 600 metres from the site; the dam now loses less than 2 millimetres per day to seepage and holds through summer.

How much bentonite do I actually need?

Bentonite is a swelling clay that expands roughly 10 to 15 times its dry volume when hydrated, forming a low-permeability seal. Application rates depend on the type. For high-grade sodium bentonite, DPIRD recommends approximately 15 kilograms per square metre. For Watheroo calcium bentonite, the rate is about 60 kilograms per square metre. The NSW DPI farm dams guide sets a mixed-blanket rate at around 7 kilograms per square metre and a pure-blanket rate at 10 kilograms per square metre on a dry, drained dam floor.

Whichever rate you use, the bentonite must be covered with at least 100 millimetres of site soil to stop it cracking as it dries, and 450 millimetres if stock will access the dam. Broadcasting bentonite onto a full dam from the surface is the last resort; NSW DPI describes the method as “hit and miss” and we have only ever seen it work on small leaks in already-tight clay.

What about gypsum, STPP, and other chemical sealers?

Gypsum and sodium tripolyphosphate (STPP) are dispersion modifiers, not waterproofing materials. Gypsum reduces dispersion in dispersive clays and is mixed into the top 200 millimetres of soil at 4 kilograms per square metre, or 1% by weight, then compacted. Success rate is around 80%. STPP does the opposite — it disperses stable but porous clay so it can be compacted, and is applied at around 0.5 kilograms per square metre after laboratory soil testing. The catch: in 20% of STPP-treated dams the application has increased seepage through induced piping failure. Never apply STPP without a soil dispersion test in hand.

When does a synthetic liner make sense?

A high-density polyethylene (HDPE) or polypropylene liner is the most reliable repair, with a success rate above 90% on properly installed jobs. A 1.5 millimetre HDPE liner has a manufacturer-rated life of 20 years exposed; 1 millimetre polypropylene is rated for at least 17 years and costs roughly 25% more for supply and installation. Liners require an even subgrade, no protrusions, and a compacted contact face. They are the right choice for critical water supply — household, dairy, intensive horticulture — where a 50% success rate is not acceptable.

What we do on a Big Ditch leaking dam site visit

Every leaking-dam quote starts with a site inspection and soil assessment. The sequence is the same on every job.

Measure the actual seepage rate with the two-stake test or recent records.
Auger or backhoe-trench the floor to confirm what soil sits below the waterline.
Take soil samples to depth and send them for dispersion and Atterberg testing where the visual inspection is inconclusive.
Walk the downstream batter for emergence points, tunnels, and stock-induced damage.
Match the repair method to the soil, the leak rate, and the budget — not the other way around.
Quote a fixed price for the works, including drainage, treatment, compaction, and refill rate.

Refilling matters. DPIRD WA recommends filling at no more than 30 centimetres per day so the lining material wets evenly and the embankment loads gradually. We hold clients to that rate; rushing it has cracked more than one well-built blanket.

When to call a professional

Three situations need a builder on site rather than a DIY attempt. First, if the leak is in the embankment rather than the floor, the dam wall has likely lost compaction and any treatment that does not address the wall will fail. Second, if you see tunnels, sinkholes, or boils, the dam may be one rainfall event from breaching and requires immediate engineering assessment. Third, if the dam serves household or stock-critical supply, the 50% success rates on cheaper treatments are too low — a properly installed liner or compacted clay blanket pays for itself the first dry summer.

If you would like a no-obligation assessment of your dam, contact Big Ditch directly — Lindsey or one of the team will look at your soil, your leak rate, and your water needs before any work is quoted.

Frequently Asked Questions

How much does it cost to fix a leaking dam in Australia?

Costs vary by method. Blending and compacting existing soil runs at one-third to two-thirds of the cost of building a new dam. A 1 metre compacted clay blanket costs 1.5 to 3 times a new dam. A bentonite blanket sits around 2 to 3 times. A full HDPE or polypropylene liner is the highest cost but carries the highest success rate. Each job is quoted on soil, area, access, and water depth.

Can I fix a leaking dam without draining it?

Rarely with confidence. Broadcasting bentonite or applying gypsum to dam water can reduce leakage in some cases — gypsum at 1 kilogram per 10 cubic metres of water has roughly a 50% success rate in dispersive-clay dams — but the treatment usually needs repeating every 2 to 3 years. For a permanent repair, the dam must be drained so the floor and walls can be inspected, soil-tested, and properly compacted.

Will bentonite seal any leaking dam?

No. Bentonite works best on moderately leaky floors with existing clay content. On pure sand it will eventually wash away; on calcium-carbonate-heavy soils it can fail to swell. Sodium bentonite swells more than calcium bentonite and is more effective per kilogram, but is mostly imported from the eastern states or overseas. Always run a soil suitability check before ordering bentonite by the tonne.

Watching your dam drop faster than the rain can refill it? Book a site inspection with Big Ditch and we will diagnose the leak before you spend a dollar on the wrong fix.

Lindsey Angus Hughson

Angus Hughson is the founder of Big Ditch Aquatecture, specialising in dam design, construction, repair, and sealing across NSW, Victoria & Queensland. With over 30 years of hands-on experience in permaculture based earthworks, water security and water infrastructure, Angus and his team deliver practical solutions for landowners, councils, and developers.

Method	Best for	Typical cost vs new dam	Success rate
Blend and compact existing soil	Slightly leaky clay floor	One-third to two-thirds	About 50%
Local clay blanket (1 metre compacted)	Sandy or gravelly floor with clay available on-site	1.5 to 3 times	About 80%
Bentonite blanket (mixed or pure)	Moderately leaky floor, no local clay	2 to 3 times	About 50%
HDPE or polypropylene liner	Severely leaky dam, critical water supply	Highest	Greater than 90%

How to Fix a Leaking Dam: 4 Methods That Actually Work