Septic Systems: Site Evaluation and Testing - Evaluating Soil Morphological Characteristics

Soil characteristics are critical in determining the suitability of a soil for treating wastewater. In North Carolina, the four soil characteristics evaluated are texture, structure, clay mineralogy, and organic soils. These characteristics are evaluated by using soil borings and soil pits.

Soil borings and soil pits for soil evaluation.

Four of the evaluation factors, wetness, depth, soil morphological characteristics, and restrictive horizons, require soil borings or digging a soil pit for proper evaluation. Soil borings are less expensive than soil pits and are sufficient to determine the suitability of the four soil factors for many sites. On sites that require a more detailed evaluation, a soil pit should be dug to determine site suitability. Pits provide a much better means to view and evaluate the soil than soil borings and should be used when a detailed soil description is necessary for site suitability determination.

  • Soil borings are 2-4 inch diameter holes in the soil used to view the soil at a site. The borings or pit should be at least 48 inches deep or go down to a depth where an uncorrectable soil factor is encountered.

  • Soil pits are holes large enough for a person to enter and view the soil closely. Backhoe-dug pits are an excellent diagnostic tool for soil depth, wetness, and restrictive horizons. Because a pit allows the evaluation of a larger cross-section than does the soil auger, the soil may be found to have different characteristics than those identified through auger sampling alone. Since the use of a backhoe-dug pit allows a more thorough soil evaluation, it may be possible to reclassify a site as PROVISIONALLY SUITABLE that would have been considered UNSUIT­ABLE from auger borings alone prior to the pit evaluation.

    • For example, a restrictive horizon at a site may be discontinuous. A soil pit analysis would probably reveal this discontinuity whereas a soil-auger test might not. By using soil pits, the site might be reclassified to reflect the restrictive layer discontinuity.

    • Another situation in which pits are useful is for soils with stony or gravelly layers found in the Piedmont or Mountains. For these soils, a soil boring may lead the evaluator to think that a stony layer is impervious bedrock. A soil pit, on the other hand, may reveal that the layer is a stony or gravelly horizon that will not impede water flow.

  • A soil pit, in conjunction with soil borings, must always be used when evaluating saprolite.

Soil texture.

Soil texture is defined as the relative proportions of the various soil separates in a soil. Texture is a soil morphological property that affects a site's suitability for treating and safely disposing of wastewater. Texture influences the hydraulic conductivity, the porosity, and the structure of a soil. Soils with poor drainage due to heavy texture, such as clay soils, may not allow wastewater to move rapidly enough through the soil to dispose of the needed volume of wastewater. Soils that are too wet or that become too wet do not provide sufficient air for the beneficial microorganisms, or bacteria, that are "treating" the wastewater.

  • Texture in each soil horizon can be determined by hand. Table 4.5.2 presents the criteria used to determine soil textural class by hand as described in Soil Taxonomy (USDA-SCS, 1975).

  • If, however, laboratory analyses are used, the examination should follow the American Society for Testing and Materials D-422 procedures for soil textural testing but use the USDA particle size system for classifying textural categories. Additionally, fine loamy and clayey soils (Groups III and IV) should be soaked in a dispersing agent for 12 hours prior to the hydrometer analyses.

  • The soil texture determinations of the fine earth fractions ( < 2mm diameter) usually divide soil into 12 textural classes. In North Carolina, for the purpose of on-site wastewater evaluation, the 12 classes are combined into four textural groupings:

    • Group I - Sandy Texture Soils

    • Group II - Coarse Loamy Texture Soils

    • Group III - Fine Loamy Texture Soils

    • Group IV - Oayey Texture Soils

  • 1. Soils in Group I and II are SUITABLE for on-site systems.

  • 2. Group III and IV soils are PROVISIONALLY SUITABLE for on-site systems.

Criteria for Soil Textural Class Determination

Criteria for Soil Textural Class Determination

Soil Texture Groupings for Septic Systems North Carolina

Soil structure.

Soil structure is a way to describe how individual soil particles are arranged into larger groupings of particles called aggregates. Structure affects the rate of water movement through the soil, the amount of air that can get into the soil, and thus the soil's ability to treat wastewater. Table 4.5.4 describes soil structure categories, as designated in the rules, and assigns suitability classes to the different soil structures.

Five soil structures are recognized for site evaluation purposes.

1. Crumb and granular

2. Block-like

3. Platy

4. Prismatic

5. Absence of structure: a) single grain and b) massive

The presence of block-like structure is particularly important in some Piedmont and Mountain soils, since water flow around these block/peds allows these soils to be used for on-site waste management (refer back to the section on soil structure in the Basic Soil Concepts chapter for more details).

For information on soil structure refer to Soil Taxonomy, Appendix I (USDA-SCS, 1975).

Clay mineralogy.

The type of clay mineralogy and the amount of clay in a soil influence water movement.

There are different types of clays. The two major types of clay are 2:1 and 1:1 clays. 2:1 clays expand when wet, whereas 1:1 clays expand only slightly when wet. These concepts are discussed below.

Clays with a 2:1 mineralogy, such as montmorillonite, shrink when dry and swell upon wetting. When a soil swells, the soil particles expand into the structural voids, reducing the size of the openings, and reducing total porosity. The hydraulic conductivity of the soil is therefore reduced, which limits the movement of wastewater through the soil. Soils with 2:1 clay mineralogy are generally not suitable for on-site systems because the soil swells and restricts the flow of water.

  • Soils with 1:1 clays, such as kaolinite, have less shrink/swell potential. Soils with 1: 1 clay mineralogy are suitable for on-site systems because these soils do not swell and restrict the water flow.

  • If the clay fraction of the soil has between 10 and 50% 2: 1 clays, the soil has mixed mineralogy. Some soils with mixed clay mineralogy can be used for on-site systems; some cannot. Soils with mixed mineralogy are not suitable for on-site systems iftbe consistence is very firm, extremely firm, very sticky, or very plastic. Each soil in this class must be evaluated to determine if it is suitable for on-site wastewater disposal.

  • Soils with predominately 1:1 clays and less than 10% 2:1 clays are usable for on-site systems.

  • In lieu of a laboratory analysis to determine clay mineralogy, the American Society for Testing and Materials (ASTM) procedures must be used to determine liquid limit, plastic limit, and the plasticity index of the soils.

Soil consistence.

Clay mineralogy can be determined in the field by evaluating soil consistence. Soil consistence is a measure of how well the soil forms shapes and how well it sticks to other objects. Consistence can be determined when the soil is dry, moist, or wet. In North Carolina, the best test for soil consistence is when the soil is moist or wet.

  • In a moist soil, consistence is determined by looseness, friability, and firmness.

  • In a wet soil, two consistency factors, soil stickiness and plasticity, should be determined. Stickiness, how well the soil sticks to other objects, is determined by pressing the soil between the fingers and thumb. Plasticity, how well the soil forms shapes, is determined by rolling the soil between the thumb and forefinger to determine whether a thin rod or wire of soil can be formed. See Table 4.4.9 in the section on Basic Soil Concepts for more details on how to evaluate soil consistence. Additional information on soil consistence can be found in the publication Soil Taxonomy (USDA-SCS,1975).

  • If the soil is UNSUITABLE because of structure or clay mineralogy, the classification may be changed to PROVISIONALLY SUIT ABLE if an investi­gation determines that a modified or alternative septic system would function appropriately on this site. See 15A NCAC lSA.1956 or .1957 for the rules governing installation of modified or alternative septic systems.

Site Suitability

The suitability of a soil for on-site system installation, based on consistence measured at either wet or moist soil conditions, is shown in Table 4.5.5.

Soil Consistency Criteria for Siting Septic Systems

Soil Consistency Criteria for Siting Septic Systems

Organic Matter

Organic soils, soils with 20% or more organic matter by weight to a depth of 18 inches or greater, are always UNSUITABLE as locations for on-site systems. These soils remain wet throughout most of the year because they drain too slowly. Organic soils may also bum or subside causing the on-site system to be destroyed.

From the North Carolina Onsite Guidance Manual