How to Inspect the Soil Condition of the Septic Drainfield for Evidence of Septic System Failure
     


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This septic drainfield article series discusses the evaluation of drainfield soil conditions among the types of causes of septic system failure in the drain field, leach field, seepage bed, or similar component. We also discuss what can be planted over and near a septic drainfield and what should be avoided.

We list the causes of each type of septic component failure, and list the septic component failure criteria or in other words what conditions are defined as "failure"? How can you distinguish between a blocked pipe, a septic tank that needs pumping, and a clogged drainfield that needs replacement? This is an important question as it distinguishes between relatively low cost maintenance or repair task and a costly septic leach field replacement.

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SOIL CONDITIONS - The Drainfield: Leaching Bed Soil Condition & Liquid level

The absorption system or "drain field" has two jobs

First, it disposes of liquid effluent by permitting it to seep into the soil below. Second, a "bio-mat" of bacteria which forms in the soil below the drainage field processes pathogens in the septic effluent to make the effluent sufficiently sanitary as to avoid contaminating nearby ground water. This distinction between successful "disposal" and successful "treatment" is important to avoid groundwater contamination but has not been addressed by regulation in every municipality.

Municipalities which require a minimum distance between the bottom of the drain field trenches (or equivalent component) and the top of the seasonal high ground water table have recognized the importance of a working bio-mat and the need to provide adequate dry soil for it to function.

Even in a well-designed drainage field, eventually the soil surrounding the drainfield device (perforated pipe in gravel trench or other seepage system) becomes clogged with grease and debris.

Examining an excavated cross-section of a failed drainfield

Examining an excavated cross-section of a failed drainfield will often display a black or gray band of sludge and grease of about 1" thickness at the inside perimeter of the gravel trench. When this layer of soil becomes sufficiently clogged the passage of effluent into the soil below is slowed and eventually blocked, leading to the need for replacement. Keeping a tank pumped so as to reduce the passage of debris and grease into a drain field will extend its life.

This is the most expensive problem to correct. Look for septic effluent seepage to ground surface in area of equipment or downhill from such equipment. Look for (illegal) drain field line extensions to nearby streams, storm drains, or adjoining properties where the temptation to "fix" a failing system by sending the effluent to an improper destination overwhelmed a previous owner or repair company.

In some areas where septic inspections are commonly performed, septic system inspectors use septic loading and dye test. Seepage may be due to overloaded tank, failed absorption system, or blocked/broken piping (may be less costly).

An excavator or septic contractor will often explore one or more drain lines (or similar components) by excavating a portion of it to look for evidence of flooding or soil clogging. We've used a simple probe at the end and along a leach bed to check for flooding of that component. (Be careful not to break or collapse old piping.)

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