A Study in Soil

When called on to diagnose a septic system problem during a pump-out, a strong knowledge of soil structure will make you a better drainfield detective.
A Study in Soil

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QUESTION: Why is understanding soils important?

ANSWER: The first question I received when I started writing this column referred to fine, sandy soils and how they react when septic tank effluent is applied.

Since then I have had the opportunity to visit several areas of the country and one comment I consistently hear from regulators and system designers is, "Correctly identifying and dealing with soils questions is one of our biggest problems with having good systems." From installers and pumpers I have consistently heard, "Why do I have to know about soils?" Then later in the conversation they usually ask a question relating to specific problematic soil or site conditions.

So I will present a case for why soils are important, and in the coming months we'll look at specifec soil and site questions I have been involved with in the last few months.


Why are soils so important? Almost all the systems we work with rely on soil to accept wastewater and provide final treatment. This is what I call the "big balancing act." The homeowner only considers one side of the balancing act: Does all the water I use disappear into the soil? As professionals we are worried about the other side of the act as well: Is the wastewater treated before it ends up someplace else?

That someplace else is back into the groundwater, and in a lot of cases, into nearby surface waters. In the realm of soil science, this is called the hydrologic cycle. This means we bring drinkable water into the house and we discharge the wastewater into the soil and, from there, to ground or surface water.

A number of specific soil characteristics and site factors impact both sides of the balancing act. Soil factors include texture, structure and consistence. Site factors involve land slope, landscape position and the presence of limiting conditions, such as depth to bedrock and seasonally saturated conditions. Other factors affect proper balance, most of which can be influenced by the practicing professional.

Examples are how the soil and site were treated during installation: Was the site protected and were the soils kept in their natural condition? Or was the soil compacted or smeared during installation? Was the area driven over repeatedly before, during or after installation? Is the soil treatment unit installed in the proper landscape position and on contour?

Other factors relate to design choices, such as: Was the daily flow estimated properly? Is the wastewater strength from an organic-loading standpoint consistent with the choice of pretreatment and at a level – following biomat development – that will allow for acceptance of the wastewater.


An installer asks: "Since all I need to do is follow the plan laid out by the site evaluator/designer, why do I need to know about soils?" My response: While you don't need to be a soil scientist, having a working knowledge of soils and landscapes will keep you from working the soil when it is too wet, allow you to recognize when the design does not match with soil or site conditions, assist in planning the installation, and properly install the type of soil treatment unit required to maintain the proper balance.

For the pumper's question: "Why do I need to know about soils?'' My answer is that you are the person usually called upon to troubleshoot

and give advice about solutions when the system is not working. This means you need to have the knowledge to identify a problem with water use in the house or sewage tanks and know whether the problem exists in the soil treatment unit. This could mean identifying if the system was undersized during the design, if there is not proper separation to a limiting soil or site condition, or if there was a problem during installation.

To both of these profes-sionals I ask: Who is the first person called if the homeowner has a problem? Who is expected to fix the problem? My answer? The last professional who has been on the site. So if the problems occur within months or a couple of years, the system installer gets the call. If it is shortly after the septic tank has been pumped and the problem has supposedly been fixed, the pumper's phone will ring. These are the calls we all want to avoid and a working knowledge of soils can prevent them.


One of the most basic soil properties we need to recognize is texture. It is one of the properties used to identify the Long Term Acceptance Rate (LTAR) for designing onsite wastewater treatment systems. Soil texture refers specifically to the percentage of sand-, silt- and clay-sized mineral particles present in a sample. Organic material is excluded from the determination as well as rock fragments larger than 2 millimeters. Soil texture refers only to particles smaller than 2 millimeters. Sand is defined as 0.05 to 2 millimeters, silt as 0.002 to 0.5 millimeters and clay as less than 0.002 millimeters.

A couple important items to note: These size criteria are from the United States Department of Agriculture (USDA) classification system used with the national soil survey program. The classification system relates best to the relationship with the movement of water through soils and is used in research done on soils that defined the relationships after biomat formation and the LTAR.

While other engineering classification systems for highways and structures rely on particle size, they do not directly correlate. So any identification and reporting of soil texture should be done using the USDA system with either the determination of soil texture by feel for field evaluation or laboratory determinations. The table provided here shows different classes of materials for the three major classification systems. The USDA soil textural triangle shows the 12 soil texture classes.

In coming issues we will explore some of the soil and site questions I have encountered most often.


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