How Much Bentonite to Seal a Pond?

How Much Bentonite to Seal a Pond?

Bentonite is a popular choice for sealing ponds – and if you own a pond that is leaking, you may be wondering how much bentonite you need to effectively seal it. The compound is made up of clay particles that swell and fill in cracks and crevices when mixed with water, providing an effective seal. 

But sealing a pond with bentonite is not a one-size-fits-all job, as the amount of bentonite needed to effectively seal your pond depends on several factors. These include the size and depth of the pond, its shape, the type of soil present in the pond, how much water is being lost due to seepage, and more.

From the team here at Living Water Aeration, let’s break down what you need to know about sealing a pond with bentonite to help you determine the amount of product needed for your project.

What is Bentonite?

Bentonite is a type of absorbent swelling clay that is composed mostly of montmorillonite. It gets its name from where it was first discovered along the Benton Shale near Rock River, Wyoming.

Bentonite clay comprises tiny platelets, each only 1 nanometer thick, with a strong negative charge. When exposed to water, these negatively charged plates attract the positive ions from the water and draw them in. Through this absorption process, bentonite swells up to 15 times its dry size when hydrated.

The ability of bentonite to aggressively absorb water gives it a long history of use as a sealant, binding agent, and containment barrier. Its water-absorbing, gap-filling properties lend well to sealing ponds by expanding into the surrounding soil and bedrock to cut off seepage routes for water.

How Bentonite Clay Works to Seal Ponds

Bentonite clay contains a very fine, sponge-like network of spaces between its particles. When hydrated with water, bentonite absorbs the moisture aggressively and swells up to many times its dry size. The bentonite essentially forms a gel-like water barrier that fills in gaps, cracks, and voids.

This swelling action is what allows bentonite to seal ponds so effectively. As bentonite hydrates at the bottom and sides of the pond, it presses into the surrounding soil and expands to create an impermeable liner. The bentonite will even continue swelling to seal small cracks, if any form, over time.

A major advantage of bentonite over other sealant options is that it is self-sealing in this manner. Its ability to absorb moisture and contort to fit available spaces makes it versatile for filling gaps in almost any type of native soil or rock around a pond. Proper application methods are key, but bentonite clay provides an efficient, adaptable waterproofing solution.

Calculating How Much Bentonite You Need

When taking on a bentonite pond seal project, determining the required amount of product is key. The bentonite must have adequate coverage across the entire pond surface area and depth to expand into all the surrounding soil pores. Some key notes on calculations:

Pond Size Matters

  • Surface area and depth are critical factors. Larger and deeper ponds require more bentonite to penetrate deeper into the walls and bottom. Most recommendations are given per square foot.

Typical Rate Examples

  • 2 pounds per sq ft for a 3-4 foot deep pond
  • 2.5 pounds per sq ft for a 5-7 foot deep pond

Amount Guidelines by Pond Area:

Pond Area

Bentonite Needed

1/4 acre

18 - 36 tons

1/2 acre

35 - 70 tons

1 acre

63 - 125 tons

2 acres

125 - 250 tons


These are rough estimates only. On-site testing by drilling small test holes and adding bentonite to assess water loss rates can help dial in the most cost-effective application rate. 


To learn more about the possible options for our pond, make sure to reach out to our team to chat about our range of products and pond-care methodology.


Applying Bentonite Sealant to Your Pond

Once you have determined the required amount of bentonite clay, the next step is application. There are two main methods - the blanket approach or mixing bentonite with native soil.

The Blanket Approach

The blanket approach involves spreading bentonite clay granules evenly across the pond's bottom and sides without mixing them into the soil. This simpler method works best for newer ponds that have little to no water, drained ponds, or very shallow water levels.


  1. To start, prepare the surface by removing rocks, sticks, and debris that could potentially puncture the bentonite layer. 
  2. Then, calculate the amount needed based on the square footage measurements. Spread the product systematically using a spreader machine for larger areas or by hand using a dust mask and gloves for safety.
  3. The key is forming an even, unbroken layer of at least 1/2 inch over all surfaces. Avoid pile ups and bare spots. Gentle raking can help regulate. 
  4. Reapply to any areas that crack once water is reintroduced.


The Mixed Method

For existing ponds with established water levels, the mixed method combining bentonite and native soil may work better. 


  1. Using an excavator, scrape away and mix in bentonite thoroughly with the top 6 inches of sediment,
  2. Mix at a ratio of 30% bentonite to 70% soil for proper moisture absorption. This blended layer then compacts down into an impermeable liner sealed to the harder underlying layers.
  3. Repeat the process up the slope to where moisture appears. Lay a fabric barrier and replace any decorative rocks or cobblestones after application.


Either method can provide a reliable and permanent fix for those pesky leaks. Plus, the natural clay-like particles of bentonite create a protective barrier and cushion around your pond that prevents any further water loss.


Frequently Asked Questions

How much bentonite do I need for a 1-acre pond?

For a 1-acre pond, you will need an estimated 63 - 125 tons of bentonite to achieve full coverage for sealing. The exact amount depends on the depth of your pond and the type of native soil. We recommend starting on the higher end of that range when calculating how much bentonite clay to purchase.

Will bentonite seal a pond liner?

Bentonite can help reinforce and seal tears or gaps in synthetic pond liner materials. Prepare the liner surface thoroughly and use powdered bentonite to fill small cracks and holes best. Mix with native soil if fully relining previously unlined ponds.

How long does bentonite clay take to seal a pond?

Most swelling and waterproof sealing happens within 2-3 hours after hydrating bentonite with water. However, maximum expansion may take 24-48 hours. Track water loss for 1-2 weeks and reapply more bentonite to any leaking areas beyond that initial sealing timeframe.

What is the ratio of bentonite to soil?

A typical bentonite-to-soil mix ratio is 30% bentonite clay granules combined with 70% native pond soil. This allows enough bentonite to fully saturate and expand while considering cost-effectiveness. Adjust the mix upwards for heavier leaks or highly porous soils.


Learn More About Bentonite Sealing for Ponds with Living Water Aeration

Using bentonite clay to seal a leaking pond is a versatile, environmentally safe, and easy-to-install solution. While budget-friendly compared to rigid liners, calculating the correct amount of bentonite and proper application remains critical. 


For further assistance determining the ideal bentonite amounts and application methods for your pond sealing project, contact the professionals at Living Water Aeration to discuss your unique pond needs. 


Our team has hands-on experience efficiently solving stubborn pond leakage with bentonite solutions. Let us put together a custom plan for your needs! 

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