Use of Soil Dispersants in Clay Liners

Animal waste pond and lagoon clay liners impede contaminated runoff water from entering ground water aquifers and surface water sources such as streams, lakes, and rivers.  The National Design Construction and Soil Mechanics Center (NDCSMC) Laboratories have performed thousands of Flexible Wall Permeability Tests (ASTM D5084) on clay based soils for use as low permeability compacted soil liners for Animal Waste Ponds and Lagoons.

There is a common misconception that soils with adequate amounts of clay will not leak.  In fact, not all clays are alike, and the arrangement of clay particles in a soil greatly influences permeability. Dispersants can be added to soil to reduce aggregation and permeability.  As the name implies, dispersants aid in changing the soil structure from the open, aggregated form to a dispersed form that is less permeable to water.  Dispersants work best on calcium rich saturated clay minerals.  Therefore soils derived from limestone bedrock are the best candidates for the effective use of a dispersant.

A soil dispersant can be any chemical with a free sodium ion (Na+) that can attach to the clay mineral replacing the Ca++ for example.  The NDCSMC laboratories have conducted tests verifying the effectiveness and recommend the following commonly used chemical dispersants. Laboratory tests show that soda ash (sodium carbonate) works as a dispersant best for clay soils with a high cation exchange capacity (CEC) typically montmorillonite clays.  For kaolin clays (low CEC), sodium polyphosphates are more effective than soda ash because of the addition of phosphate. Three commonly used polyphosphates are tetrasodium polyphosphate (TSPP), sodium tripolyphosphate (STPP) and hexametaphosphate (SPP). NDCSMC laboratory experience has shown that usually about twice as much soda ash is required to effectively treat a given clay than the polyphosphates.  However, because soda ash is commonly less than half as expensive, it may be the most economical choice in many applications.

A field test for the effectiveness of a polyphosphate dispersant is to add a pinch of the dispersant to a handful of moist soil.  If the soil becomes noticeably wetter as it is rolled between the fingers, the dispersant should be effective. However a permeably test utilizing any dispersant should be performed to obtain the amount of chemical needed.

For information on the use of soil dispersants or related issues related to clay liners or permeability testing you can contact the NDCSMC Laboratories.
Learn more, including who to contact for more information, at the NRCS National Design, Construction, and Soil Mechanics Center (NDCSMC) Website.