Purdue Residential Onsite Wastewater Disposal:  Septic system information to protect your family and the environment

Soil and Site Characteristics

Conventional septic systems work because the soil beneath the trenches removes bacteria, nitrates, and other pollutants from the sewage. So, the type of soil at the site is important. In general, septic systems should be located in areas with:
  • Good soil drainage and permeability--to allow the sewage to seep into the soil at just the right rate

  • Soil Drainage Classes:  Poorly, Somewhat Poorly, Moderately Well, Well

  • The right amount of slope--too little slope can keep the system from working properly and too much slope can cause sewage to run off into surface water sources rather than permeate into the soil

  • Soil should slope so that surface water does not collect at the bottom of the slope.

  • A low seasonal high water table--to avoid polluting the groundwater

  • Water table well below trench bottoms

  • A far enough distance from the trench bottoms to the limiting layer--this distance determines how much permeable soil the effluent passes through before it reaches the bedrock, gravel, or the water table

  • Low (ideal) limiting layer

  • No soil compaction--do NOT allow access to the proposed septic site since any traffic could compact the soil and make it unfit for septic

  • Soil is too wet if it forms a ribbon in hand.  Soil is OK if it crumbles in hand.

  • No drinking water wells nearby--each county will have specific setback distances such as 50 feet between the septic system and any drinking water wells--including the neighbor's well

  • Do not place septic system within 50 feet of any drinking water well.

Educational Tutorials

If you want to learn more about soil and site characteristics, see these tutorials:

Other Resources