We have received questions about dispersive soils and the problems they present for onsite sewage treatment systems. Some of these soils are found in western Minnesota. They present problems in siting, design, installation and maintenance — so basically all aspects of a system. What are these soils, how can we identify them, and most important, what are some treatment solutions?

A dispersive soil is structurally unstable. Its soil aggregates — peds — collapse when the soil gets wet because the individual clay particles disperse into solution. This collapse of structure causes the soil to slump, lose porosity and become denser. All these attributes are not good in areas where we want to apply a daily load of wastewater for dispersal and final treatment. The good news — if there is any — is the primary concern with these soils is not treatment but how to get water to infiltrate.

SODIC SOILS

What causes the instability? Soils disperse when they are sodic, which means they contain enough sodium to interfere with the structural stability of the soil. Clay particles have a negative charge on their surface; this charge is balanced by positively charged cations, such as Ca2+, Mg2+, K+ and Na+, distributed around the surface of the clay. 

Cation exchange capacity is a measure of the total number of exchange sites for a given mass of a soil. When the ratio of sodium to other ions at these exchange sites is high, clay particles are less tightly bound to each other, and the soil aggregates easily disperse when the soil becomes wet. (As a separate issue, this is the concern and discussion about water softener or treatment devices impacting this balance causing similar problems in soils not normally considered sodic or dispersive.)

Sodic soils are prone to surface sealing and hard setting (surface crusting) when they dry due to structure breaking down. In the subsoil they have low porosity with dense (massive) structure and high soil strength when dry. Movement of air into these subsoils is poor, resulting in low oxygen availability. Water infiltration is slow resulting in waterlogging or perched water tables. All these conditions are working against what we want to accomplish. From an infiltration standpoint, the surface is more permeable.

How does a site evaluator or installer know to identify a dispersive soil? There are several ways. First, either visit the soil, water and conservation district office or look at the Natural Resources Conservation Service soil map website to see if you are working in an area with dispersive soils. 

FIELD TESTING

If there is a question, the exchangeable sodium percentage can be measured as part of a standard soil test. This measures the proportion of cation exchange sites occupied by sodium. Soils are considered sodic when the ESP is greater than 6 and highly sodic when the ESP is greater than 15. 

There are observations and a field test to evaluate if the soil exhibits dispersive properties. The procedure is as follows and is similar to the test we perform to determine strength of soil structure: 

Collect dry soil peds; place the peds into a clear jar of distilled water, taking care not to mix or agitate the soil or alternatively put the peds in a small amount of water on a tile spade; aggregates will often (but not always) slake (crumble) soon after being placed in the water. However, this is not dispersion; it just indicates how well the structure holds up to being wet.  

Water around the edges of the soil aggregate in dispersive soils will become cloudy and milky looking (water looks dirty) because of the dispersed clay; dispersion will be obvious after about 10-30 minutes in highly dispersive soil. It may take two hours for moderately dispersive soil to be obvious. 

Field indicators of moderate or severely dispersive topsoils are usually obvious: the soil is prone to becoming boggy when wet because of structural instability. Where water ponds on the surface, they are milky colored. Water infiltrates very slowly into the surface. When dry and without vegetation, a surface crust is usually evident. So look at nearby farm fields. When dry cracks are evident and when viewing the soil pits, topsoil is shown to fill these cracks to depth in the profile. These soils are sometimes called self-tilling since the topsoil is turned over naturally.

ADAPT AND OVERCOME

What are the strategies to overcome these soil limitations? The natural soil surface must be used as a part of the system, and this means no excavations into the soil. This not only utilizes the most permeable part of the soil but avoids the perched water tables typically present in these soils due to slow water movement. 

We want to keep the soil consistently moist, so structure is maintained as we apply sewage effluent. To do this our system should spread effluent out as evenly as possible over the soil surface. Further we want to introduce water to the surface slowly over time. A rush or flush of water will cause the structure to disperse. 

Accomplishing this will require a pressure distribution system where our goal is to spread effluent out evenly across the area over time. These are elevated sewage treatment mounds and at-grade systems using multiple doses throughout the day. As a point of sewage history, some of the first mound systems designed and installed were in these kinds of soil. Their form and design parameters have changed since then, but the goal was the same. The other system that can accomplish these requirements is drip application. 

These are difficult soils to work on and they are very unforgiving in terms of their potential for compaction and smearing problems in addition to their natural properties. Proceed with care!