Site Evaluation, Soils, and Drainfield/Absorption Area
What this domain tests
Site and soil questions ask whether a lot can support an absorption area, how the field is sized, and what limits separation from groundwater or bedrock.
Core rules and concepts
- Soil texture and structure control permeability. Sandy soils accept water quickly but treat less; clay soils accept water slowly and are prone to clogging.
- A percolation test or soil loading rate determines field size. Design flow is usually based on bedrooms, not occupants.
- A limiting zone is any layer that restricts treatment or water movement: seasonal high water table, bedrock, fragipan, or compacted till. Required separation below the infiltrative surface is commonly 2 to 4 feet, depending on local code.
- Slope affects layout. Steep sites may need a mound, pressure distribution, or a different location.
- Setbacks protect wells, surface water, property lines, and foundations. Stormwater must be diverted away from the field.
- The infiltrative surface must remain unsaturated and must not be smeared or compacted during construction.
Worked example
A proposed trench bottom is 30 inches above a seasonal high water table, and the local rule requires 36 inches of separation. The site fails as designed. Adding more trench length does not fix it because the problem is vertical separation, not field area. The designer might raise the field with a mound or choose another area.
Traps that cost marks
Do not accept a percolation test performed in disturbed or filled soil. Do not ignore a seasonal high water table because the test pit is dry in summer. Do not size a field by house square footage. Do not assume surface slope tells you the depth to a limiting layer.