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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.

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NDWRCDP DisclaimerThis 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.

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CIDWT/University DisclaimerThese 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.

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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.

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Restrictive Horizons

"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. 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.

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All solid materials have two measures of strength

Compressive strengthShear 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.

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Compressive Strength

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.

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Shear Strength

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

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Restrictive horizons are cemented

Cement hasHigh compressive strengthLow 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.

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Restrictive Horizons

CompactMassiveSlowly permeable

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.

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Three Types of Restrictive Horizons

Organic PansIron PansFragipans

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.

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Ortstein(Organic Hardpans)

Influence on Onsite Wastewater Treatment

Organic hardpans follow into a category called Ortstein by soil scientists.

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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.

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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.

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Ortstein Formation

Illuvial accumulation of organic matter, Al, with or without Fe, cements soil particles togetherWater saturation prevents downward movement of C, Al and FeChanges in water saturation levels produces multiple ortstein layers

Ortstein forms in acid sandy soils where pines are the dominant vegetation. The acidity puts organic acids, aluminum and sometimes iron into solution. High water tables in the area prevent the downward movement of the dissolved chemicals, which then cement the soil particles together. Changes in water saturation level caused by seasonal or geologic variation may cause multiple restrictive horizons to form in a soil profile.

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Ortstein Characteristics

BrittleDenseIlluvial active amorphous materials composed of organic matter, Al, with or without Fe, that has been cementedHigh water retentionLow permeability

These horizons are brittle and dense. Cementation can be caused by one or all of the listed chemicals. The dense nature of these horizons causes high water retention and low permeability due to massive condition or coarse structural size.

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Ortstein Orientation

Relatively horizontal layer – AquodsRoot restriction

Vertical to irregular columns, tongues, pillars, or bridges

These restrictive horizons occur as relatively horizontal layers owing to the fact that they are created where the water table is near the ground surface. Due to there denseness and acid nature, roots are often restricted above such layers. Occasionally large tap roots from pine trees will penetrate the layers causing large irregular tongues to be observed through the restrictive horizon.

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Impact of Ortstein on Onsite Wastewater Treatment

Accumulation of C, Al, Fe reduces porosityReduce permeabilityReduce aerationCause anaerobic conditions

The accumulation of carbon, aluminum, or iron reduces the porosity of the soil which in turn reduces permeability and aeration. An anaerobic condition is therefore created which inhibits the treatment of sewage. A septic tank system will fail on such sites.

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Disruption, and Reformation of Ortstein

Practitioners have attempted to disrupt these horizons with deep tillage in an effort to improve their permeabilityUnfortunately these horizons recement shortly after disruptionFurther, decomposing organic compounds in effluent may cause recementation since organic acids are responsible for the original cementation of these horizons

Many people have tried to figure out a way to modify these horizons and improve septic tank performance. Deep tillage has been used in the past to disrupt these horizons and improve the soil’s permeability. Unfortunately, research has shown that these horizons recement shortly after disruption. Further, decomposing organic compounds in sewage effluent may add to the organic acids responsible for the original cementation of these horizons.

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Field Identification of Ortstein

Layer >7.5 cm thickBrittle and denseLayer consists of spodic materials

pH of <5.9Organic C of >0.6%Overlying albic horizonSpodic is: 5YR or redder, or

7.5YR & value of <5, & chroma <210YR & value and chroma <210YR 3/1

Our Rules specify that these horizons are unsuitable for a septic tank system when they are greater than 3 inches thick. Research in southeastern North Carolina has shown that when the horizon is thinner it is also discontinuous in a horizontal direction across a site and does not therefore present a true restriction to the vertical movement of water. The layer will be brittle and dense, and is easily shattered by a soil auger due to low shear strength. The layer normally consists of what is called spodic materials which have a pH less than or equal to 5.9, and organic carbon content greater than 0.6%. There will normally be what is called an albic horizon above the restrictive horizon. This is a soil horizon in which all the organic matter and iron has been translocated to the ortstein layer. The albic horizon will be gray or almost white in color. The spodic layer below the albic may have several colors as discussed earlier. Why there is so much color variation is poorly understood.

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Fragipans

Another type of restrictive horizon found in our state is called a fragipan.

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Fragipan

The bleached out horizon starting at 60 cm in this picture depicts what a fragipan looks like in the field. In this case a picture is not worth a thousand words because the picture does not convey the problematic nature of this horizon.

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FRAGIPAN

B horizonRelatively low organic carbonHigh bulk density compared to horizon aboveOften found in a foot slope or toe slope landscape position where dissolved chemicals from upslope soils begin to accumulate in water moving to this landscape position

Fragipans occur in the B horizon of the soil profile indicating that materials have been translocated to this portion of the soil profile from the surface soil horizons. These horizons have low organic carbon and a high bulk density compared to the horizons above them. Fragipans often occur in a foot slope or toe slope landscape position in our state as a result of accumulating chemicals moving with the drainage water from upslope soils.

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Fragipan Formation

Not well understoodSecondary accumulations of Si, Al, Fe, and/or clayPhysical ripening through desiccation of initially wet materials

Fragipan formation is not well understood but may result from the accumulation of silica, aluminum, iron and/or clay. It is clear though that wetting and drying is necessary to physically ripen this cemented layer.

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Fragipan Characteristics

High bulk densityBrittleSlakes in waterSufficiently thick to inhibit root growth and water movement

As with other restrictive horizons fragipans are dense and have a high bulk density. The horizon is brittle as discussed earlier. If an intact piece of the fragipan is placed in water it will slake. This means that it will fall apart similar to a sugar cube in warm water. Fragipans are therefore not as hard when they are sampled in a moist field condition. On the other hand fragipans are extremely hard when sampled in a dry field condition. They are often thick enough to inhibit both root growth and water movement.

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Fragipan Impact on Onsite Wastewater Treatment

Reduced porosity reduces water and air movementReduction in pore continuityMay cause water saturation thereby reducing aeration

Fragipans have reduced porosity which reduces both air and water movement in the soil. Because they are very dense, pores are poorly connected, and internal drainage is slow. The combination of slow internal permeability and reduced soil aeration will cause a septic tank system to fail.

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Disruption, and Reformation of a Fragipan

Dense characteristics reestablished in <11 years after mechanical disturbance

Some people have suggested that fragipans can be broken up by a plow. Research has shown though that the dense characteristics of the horizon will reappear. Disruption of the pan with a plow is not a solution to the problem.

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Field Identification of Fragipan

Layer 15 cm or more thickLayer has very coarse prismatic, blocky structure; separation between peds >10 cm 5-10 cm fragments slake when submerged in waterLayer has firm or firmer rupture-resistance, brittle manner of failure, and virtually no roots

Where fragipans occur they are greater than 15 cm thick and often have very coarse prismatic structure. If a 5 to 10 centimeter fragment of the fragipan is placed in water it will slake like a sugar cube placed in a glass of water. The fragipan will be at least firm and brittle. Virtually no roots will be found in this horizon.

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PlinthiteInfluence on Onsite Wastewater

Treatment

Plinthite is another form of restrictive horizon found in our state. These horizons may be found throughout the state but are most common east of Raleigh.

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Plinthite

The red blotches of color located below 20 cm in this soil profile indicate the presence of plinthite.

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Plinthite Formation

Forms in horizons that are water saturated for some timePlinthite forms as a result of the irreversible hardening of iron nodules The nodules will not degrade even when exposed to repeated wetting and drying

The reason that plinthite is found more often in the Coastal zone of our state is that the soil must be saturated for some time before plinthite will form. Plinthite forms as a result of the irreversible hardening of iron into nodules. The nodules will not degrade even when exposed to repeated cycles of wetting and drying.

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Plinthite Characteristics

Iron-richHumus poorDark red concentrations Platy structureNodules are extremely hard

Plinthite has high iron content and low organic content. Plinthite is dark red in color. Platy structure often is found where plinthite occurs, which greatly reduces vertical permeability of the soil. As stated earlier plinthite nodules are hard.

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Impact of Plinthite on Wastewater Treatment

The formation of plinthite generally is accompanied reduced porosity

Permeability reducedAeration reduced

Water saturation may cause anaerobic conditionsSystem failure will occur

Plinthite layers are normally very dense and have reduced porosity, which gives rise to reduced permeability and aeration. Water saturation often occurs in the layer, which cause anaerobic conditions, that will cause a septic system to fail.

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Field Identification of Plinthite

Red to dark red redox concentrationFirm or very firm when moistHard when dryUpper boundary abruptLower boundary diffuse or gradual

Plinthite is identified by red to dark red concentrations in the soil. These concentrations are firm to very firm when moist and hard when dry. The upper boundary of the horizon is often abrupt, while the lower boundary may by be diffuse or gradual.

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Ironpan or Petroferric Horizon

Influence on Onsite Wastewater Treatment

A variation of a plinthite layer is called an ironpan or petroferric horizon.

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Ironpan Formation

Accumulation of FeParticles cemented by FeFerrous Fe may rise by capillary action from seasonal water saturation

Iron pans occur as a result of iron accumulation in the soil profile. The iron in this case is able to cement soil particles together to form a continuous layer in a horizontal direction instead of nodule formation when plinthite occurs. These layers form above a seasonally saturated zone. Soluble iron rises by capillary action above the saturated zone and cements the soil particles to form a continuous layer.

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Ironpan Location

Tropical and subtropical regions

Although ironpans can be found in our state their extent is not very great. Ironpans are more common in tropical and subtropical regions of the world.

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Ironpan Characteristics

High Fe content: >30% Fe2O3

Roughly horizontalThin but extremely hardLittle, if any, organic matter

Iron pans have an iron content in excess of 30%. They are horizontal to the soil surface and are generally thin but are normally harder than plinthite. Little organic matter is found in an ironpan.

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Impact of Ironpan on Onsite Wastewater Treatment

Accumulation of Fe reduces porosityReduces permeability Reduces aeration

Slows wastewater flowIncreases possibility of saturated flow and anaerobic conditions

Septic system will fail

As with other restrictive horizons, the ironpan has low porosity which leads to reduced permeability and aeration. Slow water movement will cause saturation and anaerobic soil conditions. A septic tank system will fail if exposed to this soil condition.

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Field Identification of Ironpan

Extremely hardRoughly horizontalThickness: few centimeters to few meters

Iron pans are extremely hard and roughly horizontal to the soil surface. You may not be able to penetrate an iron pan with a soil auger because of their extreme hardness. The pans are often just a few centimeters thick in our state but may be a few meters thick in other parts of the world.

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Anthropogenic PansInfluence on Onsite Wastewater

Treatment

Soil scientist recognize another type of pan caused by compaction of the soil as a result of heavy equipment.

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Anthropogenic Pan Formation

Not naturally occurringFormed as result of human activity

Usually when soil was wetPressure from a bacckhoe

Natural structure destroyed – usually massive

Such pans do not occur naturally. They form as a result of human activity when soil is wet and has low compressive strength. Pressure applied by a backhoe constructing a septic tank system can cause formation of such pans. The pressure will destroy the natural structure of the soil causing a massive structureless condition which is unsuitable for a septic tank system because of low soil permeability.

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Anthropogenic Pans

Not classified as restrictive horizonsNot cementedUnsuitable because of massive structureless condition

Anthropogenic pans are not classified as restrictive horizons in our Rules because they lack cementation and are not brittle. Such horizons are still unsuitable for a septic tank system due to their massive structureless condition and low permeability.

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Disruption, and Management of Anthropogenic Pan

Deep tillage Break panNot restore natural structureReduced permeability meansLarger septic system required

Deep tillage has been used to break up anthropogenic pans with some success. Unfortunately tillage cannot restore the natural structure that was originally found in the soil before compaction. The soil will therefore have slower internal drainage after tillage compared to the naturally occurring soil. A larger septic system may need to be installed at such sites after compaction has occurred.

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Field Identification of Anthropogenic Pan

Occur near soil surfaceDense and offer resistance to augerMassive – lack natural pedsHigher bulk density and lower porosity than layer directly above or below the pan

Anthropogenic pans normally occur near the soil surface. They are dense and offer resistance to an auger like other pans. The layer will be massive and lack natural peds. Such layers will have higher bulk density and lower porosity than the layer above or below the pan.

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A final wordA word of caution is necessary before leaving this topic. Many beginners will classify layers of massive clay as a restrictive horizon. While it is true that these layers restrict the flow of water and are unsuitable for a septic tank system, they are not classified as restrictive horizons by our Rules, because they lack cementation and are not brittle, as required by the definition for restrictive horizons.

A word of caution is necessary before leaving this topic. Many beginners will classify layers of massive clay as a restrictive horizon. While it is true that these layers restrict the flow of water and are unsuitable for a septic tank system, they are not classified as restrictive horizons by our Rules, because they lack cementation and are not brittle, as required by the definition for restrictive horizons. The words we use in this profession have very specific meanings. Improper use of terminology can affect your credibility with some clients.