LIMITING AND RESTRICTING LAYER IDENTIFICATION

There are numerous possibilities for limiting and restricting conditions on any site. Common conditions include:

  • Redoximorphic Features/saturated soils

  • Bedrock (including soils with >50% rock fragments

  • Flood Elevation

  • Restricted Percolation Rates/High Clay Contents

Redoximorphic Feature/Saturated Soils

Soil color changes, including matrix and mottle colors, signal shifts in soil conditions. Redoximorphic features, a specific type of mottling, indicate periods of soil saturation and are key indicators during site evaluations.

Formation

Identification

Limitation

Recognition of Problem Soils

Many times redox features do not exist in the soil, but the soil may still be limiting due to saturated soil conditions.

Red Parental Materials

Soils with Munsell hues of 7.5YR or redder contain iron minerals from glacial deposits that resist change, making redox features hard to observe. These features, if present, are often subtle. Evaluations should consider landscape, landform, and vegetation. MN Rule 7080.1720 Subp. 5 E 1(c) recognizes faint redox features in these soils as indicators of periodic saturation.

Thick Topsoil

Western and southern Minnesota have thick topsoil (over 12 inches) from prairie vegetation, which adds organic matter and masks redoximorphic features. MN Rule 7080.1720 outlines that redox features must be identified below this thick layer or using specific indicators in the upper 12 inches, such as low chroma colors, hydrophytic vegetation, or proximity to water levels, to determine seasonal saturation.

Elluvial Horizons

Elluvial (E) horizons form below the topsoil, especially in forested areas, due to leaching of clay, iron, and organic matter by acids from decomposing leaves. Their light, dull colors can resemble signs of saturation, complicating interpretation. To confirm suitability for an ISTS, soil evaluation must extend three feet below to rule out any restrictions.

Lamellae

Lamellae are finer textured layers in the soil that form due to a slowing of vertical water movement through the soil. As the water slows, anything carried in the water (silts, clays, etc.) is deposited. This creates a feedback mechanism causing more fine textures to accumulate in these areas. By themselves, lamellae indicate nothing about a saturated condition in the soil.

Calcareous

Calcareous soils contain calcium carbonates from glacially crushed limestone, appearing as light-colored masses. These can be misidentified as redox features due to their contrast with the soil matrix. Accurate identification requires spotting adjacent redox depletions and concentrations, especially when both occur in the same horizon.

Saturated Soils

  • Saturated soil conditions are also known as groundwater and the water table.

  • The relationship between soil and water is critical in evaluating the use suitability for a soil.

  • Soil wetness should be characterized by identifying the depth to the uppermost zone of saturation and the approximate duration of that saturation.

Bedrock

  • Bedrock on a site will severely limit the treatment and hydraulic acceptance of septic tank effluent.

  • Locating these areas during a field evaluation is key to an appropriate design.

Restricting Soil Conditions

Flood Elevation

Disturbed Areas

Fill Soils

Cut Areas

  • Flood Elevation

Flood elevations must be obtained, and all designs/constructions must be followed.

  • Disturbed Areas

Areas that have been cut, filled, compacted, or otherwise disturbed often struggle to absorb

  • Fill Soils

Fill soils are mechanically moved soils, creating a man-made layer break (lithological discontinuity)

  • Cut Areas

Cut areas are where land has been lowered by removing soil/materials.

Vertical Separation

Why it is important?

  • For effective treatment, it is essential that on-site sewage systems include, among other things, provisions for an adequate vertical separation. 

  • Vertical separation primarily affects degradation of organic nutrients (i.e. BOD5) and removal of bacteria and viruses.

  • It also plays a role in converting nitrogen to soluble nitrate (NO -) ions which can then readily migrate into the groundwater unless denitrifying conditions are present.