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.
They work best on calciumsaturated clay minerals; therefore, soils derived from limestone are the likeliest candidates for the effective use of a dispersant.
Dispersants will work only with aggregated soils (with an open, honeycomb arrangement of soil particles) with more than 50 percent silt and clay-sized particles and at least 15 percent clay.
Because of the technical nature of soil dispersants and clay mineralogy, pond owners should seek expert advice in selecting an appropriate dispersant, if the use of one is indicated.
Common salt is recommended as a dispersant only for soils with a high cation exchange capacity (CEC), typically montmorillonite clays.
Soda ash (sodium carbonate) is also used as a sodiumbased dispersant. For kaolin clays (low CEC), sodium polyphosphates are more effective than plain salt because of the additional reaction of clay with the phosphate.
Three commonly used polyphosphates are polyphosphates pyrophosphate (TSPP), sodium tripolyphosphate (STPP), and hexametaphosphate (SPP). The proprietary compound ESS-13 also functions as a soil dispersant.
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.
Dispersants are applied in the same way as bentonite. Disk the pond bottom and spread the dispersant evenly over the surface. Mix the soil and compact it as outlined in the section on compaction.
Again, a minimum of two, 6-inch-thick compacted layers is recommended. Adequate compaction is an important part of the treatment, as the dispersants work with the force of compaction to change the soil structure.
Typical application rates are:
When ponds treated with dispersants are filled, the water will probably become muddy and may take weeks to clear. (Alum or lime should not be used to treat for this turbidity, as they will counteract the effect of the dispersants.)
Dispersant salts might also leach out into the water and affect its suitability for use as a water source.
Author:
Nathan Stone
