Absorption disposal systems must not be used on land where the following conditions exist:
- Slope instability
- Loose sand
- Clay soils that collapse in contact with water
- Soils with a hydraulic conductivity of less than 0.36 mm/hr,
- Rock and shale
- Slopes greater than 15% (steep grades increase the chance of landslip and water seepage from the subgrade to the lower areas of the site reducing the amount of water infiltration)
- Extensively paved areas such as driveways.
As a guide, sand has a hydraulic conductivity of >180 mm/hr, sandy clay 180–36 mm/hr, medium clay 36–3.6 mm/hr, reactive clay 3.6‑0.036 mm/hr.