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Sanitation · Approvals, site and separation distances · sanitation-5Have the site and soil evaluated
Soil decides whether a drainfield can work. A soil and site evaluation (often including a percolation test or soil profile) shows how fast water soaks in and how deep the seasonal water table and bedrock are.
How
- Use the site survey from the site module to pick candidate areas: away from wells, watercourses, steep slopes and places where water collects.
- Book the soil evaluation or percolation test required by your authority. It is often done by a licensed evaluator or the authority itself.
- Ask for the evaluation in the wettest season if possible, so the seasonal high water table is seen.
- US EPA notes that a conventional drainfield relies on roughly 2 to 4 feet of unsaturated soil under it to treat effluent. Shallow soil or high groundwater points to mound, sand filter or other advanced options.
- Legal minimums differ. Example, Washington State (USA): at least 36 inches (0.9 m) between the bottom of the drainfield and the seasonal high water table or a restrictive layer, reducible to 24 inches only with advanced pre-treatment and extra studies.
- Identify a second suitable area as a reserve for a future replacement drainfield, as US EPA recommends.
- Mark and fence off the chosen areas so no vehicles compact them during construction.
Safety: Soil test pits can collapse. Never enter a deep pit that is not shored or benched.
Do these first
Sources
- Decentralized Systems Technology Fact Sheet: Septic Tank Soil Absorption Systems (EPA 932-F-99-075). US Environmental Protection Agency, accessed 2026-11
- Types of Septic Systems. US Environmental Protection Agency, accessed 2026-11
- WAC 246-272B-06100 Vertical separation. Washington State Legislature, accessed 2026-11