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Land and site · Study the site: sun, water, wind and soil · site-8

Investigate the soil with test pits and borings

Find out what is under the surface: soil layers, depth to rock and seasonal water table. This sets foundation design, septic options, road build-up and how easy digging will be.

How

  1. Start with the national soil survey (in the US the Web Soil Survey): note the map unit, drainage class, depth to water table, and septic, dwelling and road ratings.
  2. Plan at least three soil pits near the proposed septic dispersal area, placed to capture all soil layers on slopes, plus pits or borings at the house and road locations.
  3. Investigate an area about twice the size of the expected septic field so you have room for setbacks and a reserve.
  4. Have a qualified person log each layer: texture, colour, mottles, structure, thickness and depth, and record depth to rock, hardpan, caliche or signs of seasonal saturation.
  5. Compare findings with your regulator's required separation between the bottom of a dispersal field and groundwater or rock. Examples: Kansas guidance asks for at least 4 ft of suitable soil; Nevada requires 4 ft above the seasonal high water table; Sonoma County, California prohibits less than 2 ft above groundwater.
  6. If a geotechnical engineer is designing foundations or slopes, give them the logs and pit locations.

Safety: Open pits can collapse. Excavators typically dig to about 5 ft for safety; never enter a deep, unsupported trench. Fence or backfill pits the same day.

Do these first

Sources

  1. Site and Soil Evaluation for Onsite Wastewater Systems (MF2645). Kansas State University Research and Extension, accessed 2026-11
  2. Steps for Using a Soil Survey. Missouri Department of Health and Senior Services, accessed 2026-11
  3. RAINSS: Using Web Soil Survey. American Geosciences Institute, accessed 2026-11
  4. WTS-25: Soil Surveys. Nevada Division of Environmental Protection, accessed 2026-11
  5. Onsite Wastewater Treatment Systems Manual, Chapter 5: Treatment System Selection. US Environmental Protection Agency (copy hosted by Iowa State University), accessed 2026-11
  6. OWTS Manual v8.0. Permit Sonoma, Sonoma County, California, accessed 2026-11

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