Soils and Septic Systems

What type of soils are good for septic systems?

Septic systems come in many types and sizes but have one thing in common.  They all rely on the soil that they are installed in.

If you hang around septic folks like I do, you often here them repeat the mantra “The soil is the system.”   All septic systems have an absorption field.   If the soil doesn’t  cooperate, the septic system won’t work, no matter how large, fancy or expensive it might be.

Let’s look at some good and bad soil characteristics regarding septic systems:

Desirable Soils:

  • Sandy soil (although, not too coarse)
  • Well drained soils
  • Natural undisturbed soils

Undesirable (and possibly unusable) Soils:

  • High clay content (the more clay the slower the absorption rate)
  • Dense compact till, hardpan and bedrock (all impede water)
  • Shallow seasonal high water table (are not suitable for absorption systems)
  • High organic content soils (i.e. muck)  have very low and unpredictable loading rates
  • Coarse and gravely soils (these drain too fast to properly treat the effluent)
  • Fill and/or compacted soils (soil that has been man-handled and moved around looses it’s natural structure and drainage properties)

Clay soils are made up of very small particles.  Therefore, water drains through this dense soil slowly.  This is why absorption systems in clay are much larger (and expensive) than absorption fields in sandier soils.

In fact, because of it’s capillary action, clay soils like to hold water and not give it up.

Lets now look at soil texture!

All soil is made up of three different sized soil particles called sand, silt and clay.

All soil, and their respective amounts of sand, silt and clay can be found on the textural triangle.

At the top end of the triangle is 100 percent clay.  The right side is 100% silt and the left side is 100% sand.  All soils contains a mixture of sand, silt and clay and fall into a texture class inside the triangle.  When  a soil scientist prepares a soil report, they  include the soil texture they encounter at arious soil depths.

Note: The textures listed in the soil report will be abbreviated (i.e. Sandy Load = SL, Loam = L, Sand Clay Loam = SCL, etc.)

The soil scientist will likely add moderators like “fine”, “coarse” and “very fine” to further describe the texture they find.

How does a soils scientist determine texture?

Soil textures are estimated in the field by a soil professional by texturing a small, moist soil sample between their fingers.  Silty soil has a slippery feel (like wet flour).  Sandy soil is gritty and clay soils are sticky and will smear into a nice shinny ribbon that can be thrown at your brother.

Soil texture is also important.  Think of weak structured soil as being very tight.  The weaker the structure, the less compatible the soil is with drainage and  septic systems (especially in heavy soils).  To further complicate things, some soil has no structure at all.  This structureless soil is described as “massive”.  Water cannot drain though massive soil and therefore it is not suitable for septic systems. In fact, because there is no water in this soil, the roots of plans don’t even bother penetrating it.

Wet soils are a problem for septic systems:

Water is another limiting factor in soil.  As I mentioned earlier, heavy soil likes to hold water like a sponge (actually, even more than a sponge).

Septic absorption fields need to be installed in well drained soil.  If the soil is wet with water, the septic effluent will not be treated properly before it enters the ground water.  To over come the limitation of water in the soil, a perimeter drain might be needed to lower the water table.

Generally speaking… Wet soils are identified by dull and grey colors in the soils called mottles.  Water may be present during some parts of the year and not others.  Grey mottles are produced when iron in the soil is reduced due to lack of air (and oxygen).   Well drained soils are the opposite.  Oxygen and air is plentiful in these soils and therefore iron is oxidized and the soil is brighter in color (rusty colored instead of grey).

How do I drain my wet soil?

Sometimes drains can be installed to intercept the water in the soil and take it away to a hill side, ditch or tile.  These are referred to as Perimeter drains, curtain drains or interceptor drains.

Some wet sites don’t have a suitable outlet for a perimeter drain and therefore are not suitable for septic systems!

Interceptor (or perimeter drains) must be installed well below the depth of your trenches to ensure that they are effective.   Feel free to contact me if you are in need of a drainage system or septic design.

Related Page: Soil Loading Rates

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