Sodium bentonite clay is a naturally occurring material widely used to create a low-permeability liner for sealing leaking ponds. It has a unique capacity to absorb significant amounts of water. When hydrated, sodium bentonite clay can swell up to 15 to 30 times its dry volume, filling soil voids and creating a dense, watertight barrier against seepage. Determining the correct quantity of this material is the primary challenge for any sealing project, as the required amount varies drastically depending on the pond’s unique characteristics.
Assessing Soil and Leak Conditions
The amount of bentonite required is directly proportional to the porosity of the native soil in the pond bottom. Highly porous, coarse-grained soils like sand, gravel, and silty sand allow water to pass through easily, necessitating a much higher dosage of bentonite. Conversely, soils that already contain a substantial amount of fine particles, such as native clay or heavy silt, require less bentonite to achieve a sufficient seal.
Soil analysis is a prerequisite to accurate calculation, often involving a simple field test or laboratory analysis to classify the soil type. This analysis determines the soil’s composition, which dictates how much bentonite must be blended into the existing earth to form an effective barrier. For most effective application, the pond should be drained and allowed to dry so the native soil can be prepared and treated directly. Treating the entire pond bottom is the most secure method, but localized leaks can sometimes be addressed if the precise point of seepage is known.
Highly permeable soils, such as clean sand or gravel, require high application rates to ensure the bentonite fills all the large voids. Ignoring the native soil type risks applying too little bentonite, which will result in continued water loss.
Calculating the Required Amount of Bentonite
The application rate, measured in pounds of bentonite per square foot of area to be sealed, is heavily influenced by the soil type identified during the assessment phase. For example, a heavy clay soil might only require an application rate of 1.0 to 1.5 pounds per square foot to achieve a seal.
In contrast, a sandy silt soil typically demands 2.0 to 2.5 pounds per square foot, while clean sand or gravelly areas often require 4.0 to 6.0 pounds per square foot. The general calculation involves multiplying the total square footage of the area to be sealed by the determined application rate. For a pond of a significant size, such as a quarter-acre (10,890 square feet), the total bentonite needed could range from approximately 18 tons for a relatively clay-rich soil to 36 tons for a highly porous soil.
The total weight must then be converted into volume for purchasing and logistical planning. Deeper ponds, those exceeding eight feet, may require an increase of one pound per square foot for every additional eight feet of depth to account for the increased hydrostatic pressure. Calculating the amount precisely based on the soil type ensures the final seal is effective without the unnecessary cost of over-application.
Techniques for Applying Bentonite
Dry Application Methods
The most effective and common method is the mix method, which requires the pond to be completely drained and dried. The calculated amount of bentonite is spread evenly over the dry pond bottom. The clay is then thoroughly mixed into the top four to six inches of the native soil using a tiller, disc, or harrow to create a homogeneous, low-permeability layer.
A related approach, the blanket method, involves applying a layer of bentonite directly onto the cleaned pond bottom, followed by covering it with a four to six-inch layer of native soil. Both dry-application techniques allow for compaction of the treated layer, which enhances the final seal’s density and effectiveness.
Wet Application Methods
For ponds that cannot be drained, the broadcast or sprinkling method may be used, though it is generally less effective and requires a higher application rate. This involves scattering a granular or pelletized form of bentonite directly over the water surface, ideally targeting the area above the suspected leak. The heavier granular particles sink through the water column and settle into the seepage points below, where they hydrate and swell to form a localized seal.
Finally, the slurry method is used for highly localized leaks or specific cracks, where bentonite is mixed with water to form a thick mud and pumped directly into the leak area.
Hydration and Curing the Seal
After the bentonite has been spread and mixed into the native soil, the treated layer must be compacted. Compaction is generally achieved using heavy equipment, such as a sheepsfoot or pad-foot roller, to press the mixed soil and bentonite firmly together. If the soil is too dry for optimal compaction, water must be added by sprinkling to achieve the correct moisture content before rolling.
Hydration occurs when the bentonite absorbs water and swells to form the impermeable barrier. Ponds should be refilled slowly and gradually to prevent erosion of the newly formed seal, especially along the side slopes. The flow rate of the incoming water must be gentle enough to avoid washing away the fine bentonite particles before they can fully hydrate and bond.
While bentonite begins to swell immediately upon contact with water, it takes a curing period of several days for the seal to achieve its full effectiveness. During this initial period, stable water levels must be maintained to allow the bentonite to fully swell and create a durable, self-healing barrier against future seepage. Once cured, the bentonite seal can provide a long-lasting, low-maintenance solution for water retention.