Does Clay Soil Affect the Drainage on a Leach Field?

Septic systems with leach fields collect household waste water in rural areas.
Septic systems with leach fields collect household waste water in rural areas. (Image: Jupiterimages/Photos.com/Getty Images)

When sewage treatment plants aren’t available, septic systems recycle human waste. A typical septic system consists of a septic tank, leach field and soil. Household waste is stored below the soil surface close to the residence. A properly functioning septic system allows waste water to infiltrate into the soil for purification. The type of soil directly influences the type and size of the septic system. Clay soils pose problems for septic systems because of slow drainage.

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Septic System Design

A septic tank is buried in the ground. The number of bedrooms in the house helps determine the size of the septic tank. Household waste water flows into the septic tank. The solid material sinks to the bottom while materials such as fats and oils rise to the top. The liquid in the middle of the tank flows into the septic leach or drain field.

Leach Field Design

A leach field consists of two to five trenches up to 100 feet long. The three-foot-wide trenches are two to three feet deep and approximately nine feet apart. A four-inch perforated pipe runs inside the trenches. The pipe is covered with 12 inches or more of gravel and six inches of topsoil. The waste water passes through the holes in the pipe and into the underlying soil.

Clay Soil

Clay soil particles are very tiny. They measure less than .002 mm in diameter. Water drains very slowly through clay soil. In soils with high clay content, the waste water may not drain properly from the drain field. The clay soil may clog the leach field. In very heavy clay soil, the use of septic systems may not be possible.

Improperly Working Septic Systems

Clay soils may be one cause of an improperly working septic system. Sewage odors may be present near the drain field. Waste water can bubble up to the soil surface and cause ponding on areas of lush green plant growth. The sewage may contaminate groundwater or nearby streams and lakes. Waste water drains slowly from the house or backs up into the house.

Testing for Clay Soil

Each state and county has regulations for septic systems. The county environmental health specialist usually monitors the design and construction of the septic systems. A percolation test measures how fast the soil drains water. The results of the percolation test influence the size of the drain field. Clay soils may require a longer leach field than other types of soils.

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