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Drain Field

A septic tank is an underground, watertight container installed to receive wastewater from your home. It is designed to allow solids to settle out and separate from the liquid, to allow for limited digestion of organic matter, and to store the solids while the clarified liquid is passed for further treatment and disposal.

Though septic waste can be treated in a number of ways the distribution of effluent wastewater into a subsurface (below ground) absorption area or “drain field” is the most common.

The septic drain field treats and disposes of the wastewater by gradual seepage into the surrounding soils. For effective wastewater treatment, prospective soils should be relatively permeable (allow liquid to drain through them) and should remain unsaturated to several feet below the system depth.

Septic drain fields should be set well above water tables and bedrock and should not be located in steeply sloped area because the effluent will flow too quickly through the soil without allowing sufficient treatment.

For regions with high water tables or shallow bedrock, other systems using more advanced technology may be better options for wastewater treatment.

To avoid contamination of drinking water sources and other problems, septic drain fields must be situated at prescribed distances from wells, surface waters and springs, escarpments, property boundaries and building foundations. These regulations may restrict the feasibility of septic system installation, depending on property size, shape, and proximity to the feature noted.

Drain Fileld Diagram

Conventional septic systems are designed to operate indefinitely if properly maintained. However because most household system are not well maintained, the functioning life of septic systems is typically 20 years or less.

It is common practice to require that a second area of suitable soil be reserved at the site as a “repair area” in the event that the initial drain field septic system fails  to operate properly or to allow for the possibility of a future home addition.

Advantages of Septic Drain Fields:

  • Simplicity, reliability and low cost
  • Low maintenance requirements
  • A properly designed, well-maintained drain field can last for more than 20 years.


  • Siting limitations for septic systems include natural soil type and permeability, bedrock and groundwater elevations, and site topography.
  • Regulations relating to set-backs from water supply, lot lines, drainage lines must be taken into account.
  • Improperly functioning systems can introduce organic matter and bacterial and viral pathogens into the surrounding area and groundwater. 

Drain fields are also referred to as: leach fields; leaching fields; leach beds; leach trenches; leaching pits; seepage pits; dry wells.

Leach beds and trenches are the most common. Trenches are shallow, level excavations usually from one to five feet deep and one to three feet wide. The bottom is filled with at least six inches of washed gravel or crushed rock over which a single line of four inch perforated pipe is placed. Additional rock is placed over and around the pipe. A synthetic building fabric is laid on top of the gravel to prevent backfill (soil) from falling into the gravel trench.

Leach beds are similar to trenches, but are more than three feet wide and may contain multiple lines of distribution piping. While beds are sometime preferred for space savings in more permeable soils, trench designs provide more surface area for soil absorption.

The size of the septic drain field is based on the size of the house and the soil characteristics. Traditionally, soil is evaluated using a “percolation rate”, a measure of the water flow rate through the soil. Acceptable levels of percolation for drain field suitability range between 1 an 60 minutes per inch.

While some states continue to use percolation rate as a criterion for site suitability, many use the more comprehensive measure of long-term acceptance rate (LTAR). The LTAR accounts for texture, structure, color, and consistency of all soil layers beneath the drain field, as well as the local topography, to make a determination of the wastewater loads the area is ale to accept on a long-term basis. 

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