Bentonite & Pond
Sealing Glossary

Sodium bentonite

Also called: Wyoming bentonite, Sodium montmorillonite

A naturally occurring clay whose dominant mineral is sodium montmorillonite. Sodium as the exchangeable cation allows the clay platelets to separate widely when hydrated, producing expansion of roughly 15 times dry volume. This swelling is what makes the material capable of sealing soil pores, and it is why sodium bentonite — not calcium bentonite — is specified for pond, lake, and dam liners.

Sodium montmorillonite

Also called: Smectite

The specific smectite clay mineral that gives sodium bentonite its swelling behavior. Its crystal structure is a stack of aluminosilicate sheets separated by interlayer spaces occupied by exchangeable cations. When those cations are sodium, water enters the interlayer freely and forces the sheets apart, producing large volumetric expansion.

Calcium bentonite

Bentonite in which calcium is the dominant exchangeable cation. Calcium ions hold the clay platelets together more tightly than sodium, so calcium bentonite typically swells only 2 to 3 times its dry volume — far too little to seal soil pores. It is not an acceptable substitute for sodium bentonite in a compacted soil liner and is excluded by the PondLock material specification.

Free Swell Index (FSI)

Also called: Swell index, ASTM D5890

The volume, in millilitres, occupied by a 2.00 gram sample of dried, ground bentonite after it hydrates undisturbed in reagent water for at least 16 hours, measured by ASTM D5890 and reported as mL per 2 g. FSI is the most useful single laboratory indicator of a bentonite's sealing capacity. NRCS CPS 520 requires a free swell of at least 22 millilitres for pond sealing. PondLock tests at 30 mL, exceeding the requirement by 36 percent.

ASTM D5890

Standard Test Method for Swell Index of Clay Mineral Component of Geosynthetic Clay Liners. The 2.00 gram sample is added to 90 mL of reagent water in a 100 mL graduated cylinder in increments of no more than 0.1 gram at 10 minute intervals, topped to 100 mL, and left undisturbed for a minimum of 16 hours before the settled volume is read to the nearest 0.5 mL. It is the method cited by NRCS CPS 520 for verifying bentonite quality, and its scope states it is not applicable to clays containing polymers.

NRCS CPS 520

Also called: Conservation Practice Standard 520, CPS 520

USDA Natural Resources Conservation Service Conservation Practice Standard 520, Pond Sealing or Lining — Compacted Soil Treatment. It establishes the framework for compacted soil pond liners: sodium bentonite with a free swell of at least 22 millilitres by ASTM D5890, compacted lift thickness limited to 6 inches, minimum bentonite application rates of 0.375 to 0.625 lb/ft2 per 1 inch of lift by soil class, and minimum finished liner thickness of 12 to 24 inches depending on design storage depth. Notably it states no numeric permeability requirement, instead requiring the liner to meet a design specific discharge documented by laboratory testing.

NRCS CPS 521

Also called: Conservation Practice Standard 521, CPS 521

Conservation Practice Standard 521, Pond Sealing or Lining — Geomembrane or Geosynthetic Clay Liner. The companion standard to CPS 520 covering sheet products: minimum thickness by polymer (60 mil HDPE for wastewater, 45 mil EPDM, 40 mil PVC, 36 mil reinforced polyethylene for clear water), a minimum bentonite mass of 0.75 lb/ft2 for GCLs, mandatory installer or manufacturer certification of the installation, and 12 inches of soil cover over PVC liners and GCLs.

Compacted soil liner

Also called: Soil-bentonite liner

A low-permeability barrier formed by amending in-place or imported soil with bentonite and compacting it in controlled lifts. The liner is part of the soil structure rather than a separate sheet, which makes it self-healing, resistant to puncture, and repairable with ordinary earthwork equipment. Also called a soil-bentonite liner.

Mixed method

The installation method in which granular bentonite is spread on a scarified subgrade, mechanically mixed into the full scarified depth with disc harrows or tillers, moisture conditioned, and compacted. It produces a monolithic amended-soil liner and is the preferred approach for engineered ponds requiring long-term durability.

Blanket method

An installation method in which bentonite is placed as a discrete layer on the prepared subgrade and then covered with a protective soil cap rather than blended into the soil. It uses less mixing effort than the mixed method but creates a distinct layer that can be punctured or displaced, so it is generally less durable.

Confinement

The restraint that prevents a hydrating bentonite-soil matrix from expanding freely. Bentonite only achieves low permeability when confined by adequate overburden pressure and proper compaction, because unrestrained swelling produces a soft gel rather than a dense seal. Loss of confinement is the most common cause of liner failure and is not a material defect.

Scarification

Loosening and breaking up the subgrade to a uniform depth, typically 4 to 6 inches, with a disc, ripper, or rotary tiller. Scarification creates the soil volume into which bentonite is mixed and is a prerequisite for uniform blending.

Lift

A single layer of soil placed and compacted as a unit. NRCS CPS 520 limits compacted lift thickness to a maximum of 6 inches, and bentonite application rates are defined per 6-inch lift. A design liner thicker than 6 inches requires multiple lifts and a proportionally larger total bentonite quantity.

Standard Proctor density

Also called: ASTM D698

The maximum dry density of a soil achieved under a defined laboratory compaction energy, per ASTM D698, used as the benchmark for field compaction acceptance. Bentonite-amended soil liners are typically specified at a minimum of 95 percent Standard Proctor density.

Hydraulic conductivity

Also called: Permeability

The rate at which water moves through a porous medium under a hydraulic gradient, usually reported in centimeters per second. It is the property that actually determines whether a liner reduces seepage, and it is a function of the compacted soil-bentonite matrix as built — not of the raw material alone.

Optimum moisture content

The soil moisture content at which a given compaction effort produces maximum dry density. Bentonite-amended soil must be moisture conditioned to near the optimum range before rolling; too dry and the mixture will not densify, too wet and it will pump under the roller.

Mesh size

Also called: Sieve size, Gradation

A sieve designation describing particle size. PondLock is supplied at 100 percent passing a No. 8 sieve (2.36 mm) with a maximum of 12 percent passing a No. 200 sieve (75 microns). Coarser granular material resists wind displacement during open-air placement and generates far less respirable dust than powder-grade bentonite.

Granular bentonite

Bentonite supplied in a coarse, sand-like gradation suitable for mechanical spreading and soil mixing in open-air earthwork. Granular material stays where it is placed, distributes uniformly with standard spreaders, and hydrates fully under confinement after compaction.

API 13A

An American Petroleum Institute specification for drilling-fluid materials. It evaluates bentonite behavior in aqueous slurry — hydration rate, viscosity, gel strength, filtrate loss — and limits material coarser than 75 microns, pushing products toward ultra-fine powder. Those criteria are appropriate for drilling mud and misaligned with compacted soil liner construction, where powder blows away during placement and creates a respirable dust hazard.

Geosynthetic clay liner (GCL)

Also called: Geosynthetic clay liner

A manufactured composite in which a thin layer of bentonite is encapsulated between geotextiles or bonded to a geomembrane. GCLs deliver bentonite in a factory-controlled sheet form and require seam overlap and specialized handling, in contrast to a field-mixed compacted soil liner.

Anti-seep collar

A projecting barrier installed around a pipe passing through an embankment, intended to lengthen the seepage path along the pipe and interrupt preferential flow. Backfill around pipes and collars is hard to compact, which is why pipe penetrations are among the most common pond leak locations and warrant generous bentonite dosing during repair.

Core trench

Also called: Cutoff trench

A trench excavated along the centerline of an earthen dam or embankment and backfilled with low-permeability material, often soil amended with bentonite at roughly 30 percent, to create an impervious core that interrupts seepage through the embankment foundation.

Preferential seepage path

A localized route through a liner or embankment along which water moves far faster than through the surrounding soil. Common causes include rocks larger than 1 inch left in the subgrade, poorly compacted backfill around structures, root channels, animal burrows, and desiccation cracks.

Desiccation cracking

Cracking that develops when a clay or clay-amended soil dries and shrinks. A bentonite liner exposed to drying winds or sun before hydration and confinement can crack through, creating seepage paths, which is why completed liners must be protected and ponds filled promptly.

Self-healing

The property of a sodium bentonite liner by which minor damage closes itself. Because sealing depends on the clay's physical swelling rather than on membrane continuity, a disturbed and rewetted liner swells again to fill the gap — behavior no synthetic sheet membrane exhibits.

Proof-rolling

Passing a loaded roller or truck over a prepared subgrade to reveal soft, unstable, or pumping areas before liner construction. Areas that deflect, rut, or pump must be removed and replaced rather than covered.

Bulk sack (FIBC)

Also called: FIBC, Bulk bag

A flexible intermediate bulk container used to ship granular material in tonnage quantities. PondLock bulk sacks are 2,000 lb at 28 cubic feet and 3,000 lb at 42 cubic feet, with four lifting loops and bottom drawstring discharge for controlled spreading.