Manitoba
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Sustainable Development

Onsite Wastewater Management Systems Program

Soil Evaluation

Soil is the most important part of a septic system as it is an excellent medium for treating wastewater. A detailed assessment of a proposed site and its soil resources is essential for designing an onsite wastewater treatment system that will treat wastewater for decades.

1. Role of Soil in Effluent Renovation
Wastewater delivered to soil from a septic tank contains a number of contaminants (organic matter, soluble nutrients and a variety of microorganisms) that need to be removed before the wastewater joins with surface or groundwater. The role of soil is to remove contaminants by natural physical, chemical and biological processes that are active in healthy soil.

2. Required Soil Conditions
Since many of the natural processes required to treat wastewater depend on adequate aeration, unsaturated soil is essential for wastewater treatment.

Hence the soil must be sufficiently permeable for water to move through and beyond the soil profile. Adequate depth of soil ensures the wastewater is in contact with soil material for a sufficient period of time for treatment to take place. Information on the depth of soil and the ability of the soil to accept and transmit water (permeability) provides the basis for assessing the suitability of soil for wastewater treatment.

Together with information on the amount of wastewater produced, soil depth and permeability are used to design the form and size of the soil adsorption system.

3. Limiting Layers
The depth and nature of the limiting layer is critical to the selection and design of an onsite wastewater management system. Limiting layers are zones in or below the soil where limitations occur.  Many soils do not provide adequate depth above materials that restrict water and air movement, or are seasonally saturated with water. Some soil layers are too permeable for good contact between soil and effluent. Limiting layers and conditions in Manitoba soils include water tables and saturated conditions, hard or dense layers within or below the soil with low permeability, and excessively permeable course sand gravel layers.

4. Soil Depth
In Manitoba a minimum of 1 metre (3.25 feet) of unsaturated soil present beneath the wastewater distribution system is needed to treat wastewater before contact with a limiting layer.

5. Permeability
The ability of soil to transmit water has traditionally been described as its permeability. Permeability is often estimated from other soil properties including texture, structure, consistency and color.

Hydraulic conductivity is a quantitative parameter (a number) that indicates water transmission. Hydraulic conductivity varies with the soil moisture content, but is frequently measured by first saturating the soil.

6. Site Assessment
Soil and site assessment involves the following series of steps: 

  • Studying local soil information from the province’s soil maps 

  • Conducting a visual assessment of the site to identify landscape features 

  • Probing the soil to delineate soil resources on the lot 

  • Excavating soil borings and pits to prepare profile descriptions of major soils on the lot 

  • Matching the soil resource to appropriate treatment technology through a suitability assessment

7. Soil Profile
Physical and chemical properties of the soil materials related to water and air movement and storage, root growth, and biological activity need to be investigated to gain an understanding of the quality of the available soil materials for wastewater treatment. In lieu of acutal measurements of water movement and storage in the soil over an extended period, soil characteristics are used to indicate drainage conditions within the profile.