Restrictive Horizons

Restrictive Horizons Some soils in North Carolina have a soil horizon which may restrict the vertical movement of water and detrimentally affect the the performance of a septic tank system. Identification of restrictive horizons is therefore an important component of a site evaluation for septic tank systems. This presentation will introduce you to information regarding restrictive horizons. A thorough understanding of restrictive horizons is not possible though without considerable field experience. 1 NDWRCDP Disclaimer This work was supported by the National Decentralized Water Resources Capacity Development Project (NDWRCDP) with funding provided by the U.S. Environmental Protection Agency through a Cooperative Agreement (EPA No. CR827881-01-0) with Washington University in St. Louis. These materials have not been reviewed by the U.S. Environmental Protection Agency. These materials have been reviewed by representatives of the NDWRCDP. The contents of these materials do not necessarily reflect the views and policies of the NDWRCDP, Washington University, or the U.S. Environmental Protection Agency, nor does the mention of trade names or commercial products constitute their endorsement or recommendation for use. 2 CIDWT/University Disclaimer These materials are the collective effort of individuals from academic, regulatory, and private sectors of the onsite/decentralized wastewater industry. These materials have been peer-reviewed and represent the current state of knowledge/science in this field. They were developed through a series of writing and review meetings with the goal of formulating a consensus on the materials presented. These materials do not necessarily reflect the views and policies of North Carolina State University, and/or the Consortium of Institutes for Decentralized Wastewater Treatment (CIDWT). The mention of trade names or commercial products does not constitute an endorsement or recommendation for use from these individuals or entities, nor does it constitute criticism for similar ones not mentioned. 3 Citation -Mokma, D. L., D.L. Lindbo, M. H. Stolt, and R. Miles. 2005. 6. Restrictive Horizons – Power Point Presentation. in (D.L. Lindbo and N. E. Deal eds.) Model Decentralized Wastewater Practitioner Curriculum. National Decentralized Water Resources Capacity Development Project. North Carolina State University, Raleigh, NC. 4 Restrictive Horizons ¾ "Restrictive horizon" means a soil horizon that is capable of perching ground water or sewage effluent and that is brittle and strongly compacted or strongly cemented with iron, aluminum, silica, organic matter, or other compounds. ¾ Restrictive horizons may occur as fragipans, iron pans or organic pans, and are recognized by their resistance in excavation or in using a soil auger. A restrictive horizon is defined in our Rules as a soil horizon that is capable of perching ground water or sewage effluent and that is brittle and strongly compacted or strongly cemented with iron, aluminum, silica, organic matter, or other compounds. Because the horizon can prevent sewage from flowing through the soil, a septic tank system will not perform properly if the trench bottom is located near this horizon. Restrictive horizons may occur as fragipans, iron pans or organic pans. Further, they are recognized during a site evaluation by their resistance to excavation and the twisting of a soil auger. Because some of us are stronger than others the perceived degree of resistance to excavation will vary depending on the person doing the site evaluation. This is the reason that much field experience is needed by each person before they can properly identify these horizons. 5 All solid materials have two measures of strength ¾ Compressive strength ¾ Shear strength In order to properly identify restrictive horizons in the field it is important to understand the components of material strength. All solid materials have two measures of strength: Compressive strength, and Sheer strength. 6 Compressive Strength ¾ The ability of a material to resist deformation when a force is applied perpendicular to the surface of a material. Compressive strength is a measure of a material’s ability to resist deformation when a force is applied perpendicular to the surface of the material. A couple of examples may help you to understand this property. Wet soil has low compressive strength, which is why our feet easily sink into the ground after a rain. Concrete on the other hand has high compressive strength, which is why this material is useful for a sidewalks across the wet soil. Our feet do not sink into the concrete. 7 Shear Strength ¾ Is the ability of a material to resist deformation from a force applied horizontal to the surface of a material. Sheer strength is the ability of a material to resist deformation from a force applied horizontal to the surface of a material. A couple of examples may help you to understand this property. Ice for instance has fairly high compressive strength which allows it to hold our weight when we ice skate across a pond. On the other hand the ice can be easily chipped away with a glancing blow because it has low shear strength. Steel has both high compressive and sheer strength. For this reason it is often used to reinforce buildings. Steel can both support the weight (compressive force) of a tall building and at the same time resist the sheer force that might be caused by high winds 8 Restrictive horizons are cemented ¾ Cement has z High compressive strength z Low shear strength We saw in the definition of restrictive horizons that these are soil horizons which may be cemented by iron, organic matter, aluminum, and other substances. Cement has odd properties. It is a material which has high compressive strength but low shear strength. For this reason it may be difficult to recognize such a horizon with a soil auger. We twist an auger to advance the teeth into the soil. This twisting motion applies shear forces to the soil. Since the cemented horizon is likely to have low shear strength, the horizon may not offer much resistance to excavation by the auger. This may cause a problem since resistance to excavation by an auger is how these horizons are identified in the field. If on the other hand you press down hard on the auger at the same time you twist, it there will be greater resistance felt by the auger because of the high compressive strength of the cemented horizon. For this reason, if you suspect you are on a site where restrictive horizons may be encountered, you should bear down with your weight on the auger at the same time as you twist it to feel the true resistance to excavation. 9 Restrictive Horizons ¾ Compact z Massive z Slowly permeable ¾ Brittle In addition to being cemented, restrictive horizons are also compact. These horizons are very dense as a result of compaction and cementation. Soil structure in these horizons is often absent. That is to say they are often massive, or if structure is present, it is very large. This condition leads to their very slow permeability and ability to perch water and sewage. These horizons are also brittle. What does this mean and why is it important. Materials which are brittle are strong in place but weak out of place. An example may give you a better understanding of this concept. Glass which is in a window frame (that is in place) is pretty strong and able to resist the forces applied when opening and closing the window. If one takes that same piece of glass out of the window frame on the other hand, it is easily shattered into thousands of tiny pieces. This is because glass is brittle. In like manner a restrictive horizon in place within the ground at a site will have high strength within the frame of the soil profile. Once this horizon is excavated with an auger, it shatters into individual soil particles because the horizon is brittle like the glass. For this reason it is often hard for an individual to understand why a restrictive horizon may be a problem. After all, what you pull out of the auger boring will be seen as nothing but individual soil particles by the property owner who is watching you perform the test, instead of the compact, dense and cemented horizon which really occurs within the soil profile. The odd properties of these horizons make them very difficult to deal with and explain to others. 10 Three Types of Restrictive Horizons ¾ Organic Pans ¾ Iron Pans ¾ Fragipans Our definition of of restrictive horizons recognizes three types of restrictive horizons. Other types of restrictive horizons are found elsewhere in the United States but are not discussed here. So called iron pans may occur as continuous thin layers or discrete chunks of what is called plinthite. In both cases iron is the agent which is cementing soil particles together. An explanation of each type of restrictive horizon follows. 11 Ortstein (Organic Hardpans) Influence on Onsite Wastewater Treatment Organic hardpans follow into a category called Ortstein by soil scientists. 12 Ortstein The restrictive horizon often has a dark color as pictured above due to the presence of organic acids. This type of restrictive horizon is commonly found from the city of New Bern to the South Carolina Border and east of I-95 highway in our state. 13 Ortstein Black color is not always found in the restrictive horizon. In the picture a second restrictive horizon at deeper depth has a reddish color. In fact these horizons have a great deal of color variation. Color therefore is not a good indicator of a restrictive horizon. 14 Ortstein Formation ¾ Illuvial accumulation of organic matter, Al, with or without Fe, cements soil particles together ¾ Water saturation prevents downward movement of C, Al and Fe ¾ Changes in water saturation levels produces multiple ortstein layers Ortstein forms in acid sandy soils where pines are the dominant vegetation.

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