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Septic System Installation | Why Do I Need a Soil Morphology Test?


When it comes to septic system installation, the U.S. Environmental Protection Agency (EPA) estimates 20 percent of all homes in the United States are served by septic systems. That number translates to roughly 5.7 million homes in the Midwest. The EPA has found most septic system failures are due to inappropriate design, poor installation or lack of maintenance. An EPA sponsored study found older septic systems, more than 30 years old, septic system installation or those employing outdated and underperforming designs and technologies, report higher failure rates. The same study estimates failure rates are between 10 to 20 percent. In line with the study referenced above, a U.S. Census Bureau survey discovered more than 403,000 septic system failures in a single year. In the event of a system failure, repair costs for private onsite wastewater systems range from a low of $3,000 to more than $7,000 depending on the problem and locality. Given, both initial first cost for a new installation and repair costs on the back end, the best prevention is good design from the outset and impeccable installation from a state licensed septic contractor. Good design starts with the soil. 

Soil Evaluation for Septic System Installation

All septic system design begins with an analysis of the soil. According to the Department of Health 19 CSR 20-3.060 (2) all sites being considered for the construction of a septic system must have a soil evaluation completed. Soils are at the heart of all waste water systems, dispersing wastewater from the septic tank and providing final treatment of effluent. Site specific soil characteristics inform the septic system designer as to the type of septic systems the soil can support. There are numerous basic and advanced septic systems each with different soil requirements. Failure to test, analyze and understand the type and characteristics of site soil results in a doomed system. Evaluation may begin with a preliminary review of the soil surveys archived by the U.S. Department of Agriculture Natural Resources Conservation Service (NRCS). After an initial review of a proximate soil survey, a more detailed evaluation must be conducted. Traditionally, there have been two types of soil evaluations. Over time most jurisdictions have adopted the morphology report over the simple and inaccurate percolation tests. Here is why:

Percolation Tests

Percolation tests have been around for more than a century, and historically, were the standard soil test for septic system design. Studies by the EPA and Department of Health have found many failed systems were installed in soils that simply could not support long term operation. Previously, septic design had been guided by percolation test results. septic system installationToday, the Department of Health, in most counties and the state of Missouri, no longer accept simple percolation tests for septic system design. The reason Authority Having Jurisdictions (AHJs) no longer allow percolation tests are limited soil information, lack of knowledge about restrictive horizons within the soil, and test reliability. A percolation test measures only a single soil characteristic, namely, the capacity of water to move through a soil surface and into the soil. The perc test does not drill soil sample cores to analyze each soil horizon (layer), therefore the test cannot identify shallow bedrock, clay pans, or other deeper restrictive layers that limit soil permeability. Finally, percolation tests are highly susceptible to seasonal fluctuations, specifically water tables. A perc test is a snapshot in time regarding how well water moves through the soil. Obviously when the soil is dry water will move more easily than when the ground is already saturated. A percolation test may pass in the winter but fail in the spring or summer. Additionally, high clay content soil affects water movement especially if the soil is already saturated. Clay soil expands when wet, effectively closing pores in the soil, holding water tight. Septic systems must be designed to ensure longevity and reliability throughout the entire year which brings us to the more comprehensive soil morphology report. 

Soil Morphology Reports

A soil morphology report is a detailed evaluation of the soil performed by a state licensed soil scientist, or registered engineer or geologist with specialized training and field experience in determining soil characteristics. The soil scientist drills one or more 48-inch holes at the proposed septic system location. The soil scientist records the physical properties septic system installation of the soil and analyzes permeability characteristics, noting horizons that will restrict the soils’ ability to effectively dissipate and treat the wastewater. A soil morphology report provides five critical characteristics of the soil as well as the calculated loading rate. Each of the five characteristics play a critical role in the soils ability to treat and covey waste water, while the loading rate determines the size of the septic system absorption field. The first characteristic provides data regarding the soil horizons noting changes by depth. Individual layers of soil process wastewater differently due to physical makeup, and some soils create restrictions that inhibit the soils’ ability to process effluent. The soil scientist looks specifically for clay pans, rock pans, and water tables. The second characteristic is soil color or more specifically the Redoximorphic feature. Redox indicates the drainage conditions of the soil as a whole and are a result of reductions, translocation and oxidation of Iron and Manganese oxides within the soil. Colors within the soil indicate the soil may be saturated much of the year thereby limiting the type of septic systems that can be installed. The third characteristic is soil drainage. The U.S. Department of Agriculture designated septic system installationseven drainage classes based on how well the soil percolates water. If the soil scientist notes issues with soil drainage, certain mitigation strategies may be implemented to help overcome a lack of drainage. One such option is to install a curtain drain in or around the proposed absorption field. The fourth soil characteristic is texture which is determined by the proportion of three soil particles, sand, silt, and clay. Soil texture significantly contributes to the soil’s ability to treat and disperse wastewater. The fifth and final soil characteristic is structure which is determined by the soil texture and unique combination of sand, silt and clay. When the soil particles pack and stick together, the soil forms different shapes and various structures with differing size pores. The pores determine the quantity of air and water that move through the soil. The soil scientist will determine the structural grade of the soil noting potential problems with permeability. Finally, based on the five physical characteristics of the soil, each horizon will be assigned a loading rate. The loading rate is simply the quantity of wastewater that the soil can safely treat and disperse.  septic system installationA licensed septic contractor will use the most conservative suitability rating to determine the type of septic system the soil can support, and the smallest loading rate to calculate the size of the absorption field required for reliable operation. The suitability designation is found in the left column of the morphology report. Each horizon of soil is determined as “S” suitable, “PS” provisional suitable”, or ”U” unsuitable. Suitable soil will support any type of basic septic design which keeps installation cost down. Provisional suitable soil generally will not support a standard septic system but often allows for an open lagoon alternative or more advanced design. Finally, Unsuitable soil almost always requires a high-pressure drip system which is the costliest engineered septic solution available. 

Home Performance Group Septic System Installation in Kansas City

Needing to install a new septic system, or considering renovations or repairs to your existing private onsite wastewater treatment system. Consider hiring a state licensed septic professional with advanced training in proper design, size, and installation of septic systems. A small upfront investment in careful testing and design along with preventative measures can save both the headache of inconvenience and tens of thousands of dollars in future rework.

At Home Performance Group we are both basic and advanced state licensed in septic systems. We continue to invest in technical training so we can correctly design, specify, size and install septic tanks, septic laterals, low pressure systems, lagoons, and high-pressure drip systems. We have performed numerous septic projects for our clients. 

If you are interested in a no-cost in-home consultation, schedule with a Solutions Advisor today



Article by Larry L. Motley Jr., 11 October 2021

Larry is a graduate of both Wentworth Military Academy and Missouri Western State University earning a double bachelor’s degree in Economics and Finance. Additionally, he maintains six professional tradesman licenses in two states and advanced credentialing in green technology, project and program management, and process improvement. Larry is a three-time combat veteran having served in Operation Iraqi Freedom, Operation New Dawn, and Operation Inherent Resolve. He continues to serve through a value-based building science company focused on providing clients the best design, highest quality installation, and most honest repair services in the community.  

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