Soil and Site Evaluation for Onsite Wastewater Treatment

Bulletin 905

Using soil observations

The design of onsite wastewater treatment systems requires two important soil interpretations: the soil depth to a limiting condition, and an estimate of the soil’s permeability to water and air. Based on this information, soil adsorption systems can be designed, or appropriate alternative wastewater treatment systems can be selected.

Soil layers that restrict the movement of water and air are a major cause of failure in onsite wastewater treatment systems. The soil evaluator must identify the presence of the following restrictive layers, and measure the depth of soil material available above the limiting layer:

• Zones of seasonal, perched or long-term saturation. Soil color and redoximorphic depletions serve as the most reliable morphological indicators of soil wetness.
• Hard, unfractured bedrock.
• Dense massive glacial till.
• Dense pans or cemented layers (such as fragipans) exhibiting massive structure and high consistence.
• Zones of coarse sand or gravel with few fine soil particles between the coarse fragments, occurring below finer textured soil material. Water will tend to perch above the coarser materials until the finer soil is nearly saturated, and the coarse layer will provide little residence time for untreated wastewater. Fractured bedrock behaves in a similar fashion.

The ability of the soil to transmit water has been called permeability (Table 1). Permeability is often estimated from soil texture, structure and consistence. Hydraulic conductivity is a more quantitative measure of water transmission through soil. Since hydraulic conductivity varies with soil moisture content, it is frequently measured in saturated soil to provide a standardized assessment of the maximum rate of water movement. Measurements of water movement need to be made on individual horizons.

Table 1. Classes of soil permeability.

A percolation or perc test indicates the rate at which water moves into the soil. Local health departments or designers often use this test for site-specific data before sizing the soil absorption system. The perc test is conducted using several test holes placed throughout the area proposed for the soil absorption system. Each hole is presoaked with water for at least 24 hours. The percolation rate is then measured in each hole. The percolation rate is frequently expressed as the number of minutes taken for an inch of water to soak into the soil.

Perc tests do not give reliable information when the soil is dry and cracked, or in soil where a seasonal shallow water table is present. Perc tests by themselves do not provide enough information to site and design an onsite wastewater system. Soil profile information must be gathered to determine the depth to a limiting condition, and to estimate permeability using observed texture, structure, consistence and other morphological indicators. The perc test may provide additional information to confirm the estimated permeability. By assessment at several test holes, perc tests may provide a range of values for use in adjusting a design.