Farm Dam Leaks: Causes to Check Before Buying Bentonite

For bulk bag prices, coverage per bag and indicative delivered costs to regional towns, see the sodium bentonite delivered price guide.

A falling waterline is a reason to investigate, not a diagnosis on its own. The NSW Department of Primary Industries advises checking other losses, including evaporation and an accidentally open outlet valve, before treating a dam as leaking (Leaking Farm Dams).

For landholders in NSW, Victoria and Queensland, the useful starting question is: where is the water going? We recommend separating water use, soil seepage and a possible embankment defect before discussing a bentonite order.

What causes farm dam leaks?

Seepage is water moving through soil or other material rather than leaving through the intended outlet. The distinction matters because NSW DPI identifies leakage pathways through unsuitable embankment material, beneath the wall and through exposed rock, sand or gravel in the basin (NSW DPI guidance).

An embankment is the constructed earth bank that holds back the stored water. It needs investigation as a structure, not just as a surface that can be coated: Agriculture Victoria recommends understanding the cause of failure and available repair options before work begins (Why do dam repairs often fail?).

Possible cause	What to investigate	Why diagnosis comes first
Unsuitable soil or exposed porous material	Sand, gravel, rock and the soil used in the wall	A surface product does not establish whether the foundation and wall are sound.
Poor compaction	Construction history, soil moisture and how the fill was placed	The repair may require rebuilding material rather than adding a coating.
Seepage beneath the embankment	Foundation conditions and the original cut-off trench	The visible wet area may be separate from the entry point.
Cracks, roots or animal damage	Changes in the bank, vegetation and burrows	The condition of the earth structure must be assessed.
Water use or outlet losses	Pumps, valves and the water-use record	These losses must be separated from a suspected soil leak.

The table is an inspection guide, not a way to diagnose a particular dam remotely. Its cause categories draw on NSW DPI’s leaking-dam guide and Agriculture Victoria’s maintenance guidance.

Start with water use and a consistent record

We recommend keeping a simple record before choosing a repair: photograph the waterline from the same safe position and note rain, pumping, stock water use and outlet-valve settings. Take another photograph after a change rather than relying on a recollection of where the water used to sit.

Do not assign every centimetre of water loss to leakage. NSW DPI specifically identifies evaporation and open outlet valves as possible explanations for apparent leaks, while Agriculture Victoria recommends monitoring and photographing changes in water levels and visible defects (NSW DPI; Agriculture Victoria).

We would also ask when the problem began: after construction, after a long dry period, after cleaning out the dam, or after heavy rain? Treat the answer as a lead for assessment, not proof of a particular cause.

Unsuitable soil and poor dam compaction

Permeability is a material’s ability to transmit water. Exposed rock, sand or gravel below the stored water level can provide a leakage pathway, and not every clay is suitable for an embankment (NSW DPI).

Compaction is the mechanical densification of soil. The NSW guide explains that fill placed too dry or too wet can compact poorly, and that embankment material should be placed and compacted in layers rather than treated as one deep mass (soil compaction guidance).

A firm-looking surface is not enough information for us to recommend a material order. Ask the assessor what is known about the soil below it, the construction method and whether the proposed repair needs new material, reworking or another approach.

Agriculture Victoria recommends testing soil from the failure site, surrounding areas and potential borrow pits, which are places where repair material may be excavated (dam repair guidance). That is a better basis for selecting a treatment than assuming that any clay-coloured soil will hold water.

Seepage beneath the wall, cracks and root damage

A cut-off trench is an excavated and compacted barrier beneath an embankment intended to reduce water escaping through the foundation. Failure to provide an effective cut-off is one of the construction problems identified by NSW DPI.

Roots can disturb compacted soil and provide seepage paths, while cracks may indicate settlement or possible slumping rather than a harmless surface blemish (NSW DPI; Agriculture Victoria). Do not turn that observation into a do-it-yourself instruction to pull out a mature tree or excavate a loaded dam wall.

Wet spots at the wall or its toe, the base of the embankment, indicate water movement through the bank according to Agriculture Victoria. Record their position and whether they change, then obtain advice on the cause and significance.

Tunnelling is the development of internal passages as soil is removed along a water pathway. Agriculture Victoria says tunnels indicate poor bank integrity and warrant professional advice (dam maintenance guidance).

If you see growing cracks, slumping, a tunnel or an obvious change in seepage, keep people and machinery away from suspect ground and seek qualified help. This guide does not authorise excavation, tree removal or rapid drawdown as an emergency repair.

Where does bentonite dam repair fit?

Sodium bentonite is a clay that swells to several times its original volume when wet, as described in the Big Ditch Bentonite specification sheet. That property is not a diagnosis of your dam and does not establish that a bentonite treatment will correct a structural defect.

Bentonite Pro uses 30 kg/m² as its company application and preliminary ordering standard, subject to assessment of soil suitability, the leakage cause and installation method (Bentonite Pro method guidance). This is not a rate specified by the material sheet or an endorsement by NSW DPI.

The current retail pack contains 1,200 kg of sodium bentonite fines in a 1.2 tonne FIBC bulk bag, where FIBC means flexible intermediate bulk container (1.2 tonne bentonite bulk bag). Use the mass of material required, not the dam’s water volume, when calculating an order for the treated surface.

At the company standard, 500 m² of treated floor and sloping-wall area requires 15,000 kg before an allowance: 500 × 30 = 15,000 (company quantity guidance). Dividing by 1,200 kg gives 12.5 bags, which rounds up to 13 whole bags before an allowance.

One 1.2 tonne bag provides material for 40 m² at that standard before any allowance, calculated as 1,200 ÷ 30 (company quantity guidance). Calculated coverage is not a guarantee of sealing, and a rough water-surface measurement should not replace measurement of the area to be treated.

These drained-dam methods require emptying and drying the dam, clearing loose rocks and vegetation, and preparing the treatment surface after assessment (Bentonite Pro preparation guidance). Do not interpret the short method comparison below as a substitute for the site-specific work plan.

For suitable light or loam soils, the mixed-blanket method incorporates bentonite into the upper 150 to 200 mm of prepared soil before roller compaction; a pure blanket on suitable heavier soils is instead covered with at least 100 mm of site soil before the covered treatment is compacted (NSW DPI method guidance). The same guide increases pure-blanket protective cover to at least 450 mm where stock have access (protective-cover guidance).

These are distinct methods, not interchangeable steps. Review the full preparation and installation guidance with the person assessing the dam before selecting one, and keep our company ordering standard separate from the government’s generic method advice.

What should you send for an initial assessment?

We recommend collecting the following without entering deep mud, unstable banks or the water. A useful enquiry gives the assessor a record of the problem rather than asking them to infer the cause from one distant photograph.

1. Photograph the basin, wall, spillway and any wet area from safe ground.
2. Record when the loss started and what changed before it.
3. Note pumping, outlets, rainfall and the observed water-level change.
4. Describe known soil conditions, previous work and any available construction records.
5. Explain whether draining is practical and how stock and machinery access are managed.

You can send those details through the Bentonite Pro contact page. Ask for the assessment and access requirements to be clarified before treating a quantity estimate as a repair specification.

Frequently asked questions

How do you stop a dam from leaking?

Identify the loss mechanism before selecting a repair. NSW DPI lists clay lining, bentonite and commercial liners among the available sealing approaches, but suitability depends on the problem; Agriculture Victoria advises understanding the failure cause before carrying out repairs (NSW DPI; Agriculture Victoria).

How to fix a leaky farm pond?

Begin with an assessment rather than a product dose. If the pond cannot be emptied, do not assume broadcasting bentonite over the water is an assured repair: NSW DPI does not recommend that technique because success cannot be assured (broadcasting limitations).

How to control seepage in earthen dam?

The relevant measures depend on the leakage pathway: suitable compacted soil, a functioning cut-off and treatment of exposed porous material are among the measures described by NSW DPI. A wet spot alone is not enough to select the design, so obtain a site-specific assessment before opening the bank or covering the basin.

Investigating farm dam leaks before committing to treatment? Contact Bentonite Pro to discuss the site information needed before ordering.