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Sodium bentonite is a naturally occurring clay composed primarily of sodium montmorillonite. When it contacts water it absorbs moisture between its mineral platelets and expands up to roughly 15 times its dry volume. Under confinement in compacted soil, that expansion forces clay into the soil's pore spaces and forms a dense, continuous, low-permeability barrier that stops seepage. Because the swelling is a physical property of the mineral, the seal is self-healing: if the liner is later disturbed and rewetted, the clay swells again to close the gap.

The Free Swell Index (FSI) is measured by ASTM D5890 and reports how many milliliters a 2-gram sample of bentonite occupies after hydrating in 100 ml of distilled water. It is the single best laboratory indicator of sealing capacity. USDA NRCS Conservation Practice Standard 520 sets a minimum of 22 ml for pond sealing. PondLock tests at 30 ml, which exceeds the NRCS threshold by 36 percent. Higher swell means a tighter seal and generally allows a lower application rate for the same performance.

Application rates depend on soil classification and are defined per each 6-inch compacted lift under NRCS CPS 520. Silts and clayey silts require 2 to 3 lb/ft2. Silty sands require 3 to 5 lb/ft2. Clean sands and gravelly soils require 5 to 6+ lb/ft2. If the design liner thickness exceeds 6 inches, the rate must be multiplied accordingly rather than assumed cumulative. Laboratory permeability testing can refine these rates for a specific site.

Multiply the surface area to be sealed, in square feet, by the application rate for your soil type in pounds per square foot. For example, a 1-acre pond bottom is 43,560 ft2; at 4 lb/ft2 for silty sand that is 174,240 lb, or roughly 87 tons, which is 88 of the 2,000 lb bulk sacks. Always include the interior side slopes in the sealed area, not just the flat bottom, and add a contingency for detail work around structures.

Yes. PondLock is 100 percent natural sodium bentonite with no chemical additives, binders, or polymers. It is safe for fish, livestock watering, irrigation, and swimming. Bentonite clay is widely used in potable water treatment and in animal feed as an anti-caking agent. Newly applied bentonite will cloud the water temporarily until it settles and hydrates.

Bentonite quality varies dramatically by deposit because the sodium content and mineralogy of the parent ash bed determine swell capacity. Northeastern Wyoming holds the world's highest-quality sodium bentonite deposits. In side-by-side ASTM D5890 testing, PondLock achieved a Free Swell Index of 30 ml versus 12 ml for typical Texas-sourced bentonite. At 12 ml, that material fails the NRCS CPS 520 minimum of 22 ml outright, meaning it cannot be specified for a federally compliant compacted soil liner.

Sodium bentonite has sodium as the dominant exchangeable cation, which allows the clay platelets to separate widely and swell many times their dry volume. Calcium bentonite swells only marginally, typically 2 to 3 times its volume, because calcium ions hold the platelets together more tightly. Only sodium bentonite develops the swelling pressure needed to seal soil pores. Calcium bentonite and blended products are not acceptable substitutes under the PondLock material specification.

API 13A, which is often cited as a bentonite quality benchmark, was written for drilling fluids and limits material coarser than 75 microns, pushing products toward an ultra-fine powder. That is the wrong gradation for open-air earthwork. Powder at 200 mesh has the consistency of baby powder: it blows off the subgrade before it can be mixed, produces inconsistent dosing across the liner footprint, and creates a respirable dust hazard for crews. PondLock is supplied granular at 100 percent passing a No. 8 sieve with a maximum of 12 percent passing No. 200, so it stays where it is placed, spreads uniformly with standard equipment, and hydrates fully under confinement after compaction.

The mixed method blends granular bentonite uniformly into in-place soil and then compacts it to form a monolithic low-permeability liner. It is the preferred approach for engineered ponds where long-term performance and resistance to desiccation and mechanical damage are required. The alternative blanket method places bentonite as a discrete layer covered with soil, and targeted application treats a specific known leak. The mixed method produces the most durable result because the bentonite is integral to the compacted soil matrix rather than a separate layer that can be punctured.

NRCS CPS 520 limits compacted lift thickness to a maximum of 6 inches. Amended soil should be placed in loose lifts not exceeding 6 inches and compacted to at least 95 percent Standard Proctor density unless the engineer specifies otherwise. Use vibratory sheepsfoot rollers on fine-grained soils and smooth-drum rollers on sandy soils, with enough passes to reach a dense firm condition with no visible laminations.

The most common cause of failure is loss of confinement, not material defect. Bentonite reaches low permeability only when it is restrained against volumetric expansion by adequate overburden pressure and proper compaction. Liners fail when they are allowed to dry and crack before hydration, when they are placed on subgrade containing rocks larger than 1 inch that create preferential seepage paths, when mixing is done in a single pass and leaves streaks or dry pockets, when side slopes are steeper than 3H:1V and cannot be properly compacted, or when equipment traffic damages the finished surface.

Yes. Leaks around concrete structures, drain pipes, and spillway penetrations are among the most common pond failures because backfill around a structure is difficult to compact and often develops a preferential flow path. The repair is to excavate a trench along the structure, mix granular bentonite into the backfill at a high rate, and recompact. A verified PondLock customer sealed a four-foot cavity around a concrete drain on a 150-foot-diameter pond and reported no discernible water-level drop afterward.

Yes. USDA NRCS Conservation Practice Standard 520 governs pond sealing and lining by compacted soil treatment and requires a minimum Free Swell Index of 22 ml by ASTM D5890. PondLock tests at 30 ml, exceeding the requirement by 36 percent. It is natural sodium montmorillonite with no additives, supplied granular for field mixing, and sun-dried so delivered moisture content stays at or below 20 percent. A free swell certification is available for project submittals.

PondLock is available in 25 lb bags and 45 lb bags for small repairs, rate testing, and spot treatments, shipped by standard ground. Bulk is available in 2,000 lb sacks at 28 cubic feet per sack and 3,000 lb sacks at 42 cubic feet per sack with four lifting loops and bottom drawstring discharge. Bulk sacks ship by LTL freight or full truckload flatbed direct from Wyoming. Call 877-493-7660 for bulk quoting and delivery logistics.

Kiln drying exposes bentonite to high temperatures that damage the montmorillonite crystal structure and permanently reduce swell capacity. PondLock is sun-dried at low temperature, which preserves the mineral's full expansion potential. This processing choice is a direct contributor to the 30 ml Free Swell Index.