Septic systems and percolation testing

One-line orientation

A septic system treats wastewater onsite where no municipal sewer is available. The tank settles solids, and the leach field disperses liquid into the soil. A percolation test checks whether the soil can accept that liquid. Do not confuse the perc test with a soil boring.

Key points

• It’s the onsite wastewater option where municipal sewer isn’t available (typically rural).
• Flow path: wastewater → septic tank → distribution → leach (drain) field → soil.
  • Septic tank: separates and stores solids (sludge settles, scum floats); clarified liquid flows on.
  • Leach / drain field: distributes the liquid effluent into the soil for natural filtration.
• Percolation (perc) test: evaluates the soil’s absorption rate — its ability to take in water. It decides whether a leach field is feasible and informs its design/size. Poor perc means the soil can’t disperse effluent and a conventional leach field may not work.
• Setbacks: the leach field must be separated from wells and water bodies to protect drinking water and surface water from contamination.

Confusions / comparison

	Percolation (perc) test	Soil boring
Measures	How fast soil absorbs water	Soil profile, bearing capacity, groundwater
Used to design	Septic leach field / drainage	Foundations
Question answered	Can the soil disperse effluent?	Can the soil carry the building?

	Septic (onsite)	Municipal sewer
Where	Rural / unserved sites	Urban / served areas
Treatment	Tank + leach field onsite	Piped to public treatment
Feasibility gate	Passing perc test	Available sewer connection

Related

→ Site utilities (this module): where septic fits in the urban-vs-rural utility decision · Soils (this module): soil type and percolation/drainage characteristics · Excavation dewatering (this module): groundwater considerations near drain fields.