PHOTO of new construction placing septic system components in a wet area.The Causes of Septic System Drainfield or Soakaway Bed Failure

  • SEPTIC FIELD FAILURE CAUSES - CONTENTS: Causes of septic system drainfield or leachfield failures, how to recognize & diagnose failing conditions in a septic field. Definitions of septic failure for each component
  • POST a QUESTION or READ FAQs about the causes of septic drainfield or soakaway bed saturation, clogging, or failures

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What causes a septic system failure?

This septic system diagnosis article explains the causes of septic system failure in the drain field, leach field, seepage bed, or similar component. 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"? We also discuss what can be planted over and near a septic drainfield and what should be avoided.

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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Septic Absorption Field Failure Causes: what ruins septic drainfields, leaching beds, soakbeds, or soakpits?

PHOTO of a northern Minnesota field where we have ample room to keep septic components well away from the trees - but notice those tire tracks?Here is a list of the causes of sewage wastewater disposal field failures. These factors explain why soakaway beds, seepage beds, leach fields, disposal fields, drainfields, or other synonymous effluent treatment & disposal systems fail early or at the end of a normal life.

  • Septic Drainfield Age: eventually even a well-maintained SAS will eventually clog and have to be replaced.

    Just how quickly depends on several factors including original construction type, materials, and quality, field size, septic system usage level, soil characteristics, soil water or groundwater control, and of course septic tank pumping frequency to avoid sending solids into the fields.

    But for all systems that depend on disposal and treatment of wastewater in the soil, the biomat that forms around drainage trenches, seepage beds, or soakbeds, cesspools and soakpits eventually clogs.

    In Causes and remedy of failure of septic tank seepage systems. [PDF] Harkin explains simply:
    The reason for [septic absorption ] system failure is the gradual formation underground of an imperable clogged or crusted layer in the soil blow and surrounding the seepage bed. Flow of water through this clogged or crusted zone is severely restricted or even eliminated, although the permeability of the surrounding soil remains essentially unchanged. Consequently, huge voumes of stagnant septic tank effuent accumulates in the seepage bed.
    - Harkin (1975)

    and SEPTIC LIFE EXPECTANCY where we also discuss septic tank life and other septic component life.

Nevertheless a number of additional factors can hasten the demise of a septic soakbed or leachfield.

  • Septic Tank Pumping schedule: don't forget to pump out the septic tank regularly (solids/grease are discharged into the fields, clogging the soil).

    • Soil clogging at the biomat layer which forms below and around the drainfield trenches (or other absorption systems). The biomat is a bacteria layer which forms in soil below and around drainfield trenches where septic effluent or wastewater is discharged.

      This biomat layer is critical in the processing of fine biological solids and pathogens which are in the effluent, and without it the septic system would not be adequately treating the effluent. Inadequately-treated effluent released into the ground risks contamination of nearby ponds, wells, streams, etc.

      The formation, clogging, and measures to protect and extend the life of the biomat
  • Septic Tank Baffle failures: corrosion and rust-off of steel septic tank baffles is a significant cause of disposal field failure in systems that have otherwise been maintained by proper tank pumping schedules. Concrete or plastic tank baffles also fail, break, or fall off; baffles can be repaired or replaced but the drainfield may already have been damaged and its future life reduced.
    See SEPTIC TANK BAFFLES for details.
  • Blocking or sealing the ground surface over a septic drainfield will be a problem over just about any septic effluent absorption field. Don't build anything over a septic mound, no building, no fish pond, no patio, no tennis court, no parking area, no playground (compacts the soil).
    • Don't put a swimming pool on top of a drainfield - yes I've seen people do this including my neighbor! See the photo at the top of this page.
    • Don't put a concrete block pation on top of a septic drainfield
    • Don't pave over the septic drainfield
    • Don't put outdoor carpeting over a septic drainfield
    • Don't install astroturf™ or other synthetic groundcovers over a septic drainfield unless a careful review with the product manufacturer assures that the cover will not interfere with moisture evaporation from the soil below and oxygen entry into the soil below
    • Don't build a building or deck on top of the septic drainfield - you risk damaging the drainfield during construction by equipment, by breaking pipes, by digging into a pipeline, and you prevent future access and repair to the drainfield
    • Don't seal the drainfield or soakaway bed top from contact with air by leaving a plastic liner of any sort - ice rink, swimming pool, tents and tarps in place. Temporary placement of such sealers, depending on weather conditions and temperatures, may not be harmful, such as during solid freezing conditions, but we have not found independent research that confirms or denies the problem. For example, details of the pros, cons, & warnings about building a temporary ice rink over septic fields are found at ICE SKATING OVER SEPTIC
  • Driving vehicles or any heavy equipment over the absorption system, leach field, drainfield. As with any septic absorption system, heavy traffic over the system can compress soil or break pipes, rendering the system inoperative and requiring costly repairs.

    At a recent inspection I found that the septic mound had been placed over a roadway connecting two properties owned by the same family. Family members continued to drive back and forth between homes right over the septic mound. Its future life was rather doubtful.

    Driving vehicles over the septic mound or other drainfield, even for a single project such as construction of a nearby structure or performing other site work, is likely to damage the system and lead to need for costly repairs.

    Kahn et als. also advise keeping grazing animals, horses, cows, etc. off of mound systems.
    In the photo shown here, light snow makes it quite apparent where someone has been driving through this field. Compacting the soil by driving over the drainfield will damage it and may lead to early failure.
  • Septic tank converstion to aerobic or aerator systems without proper design, components, or installation. Simply dropping an aeration system into a conventional septic tank, particularly a single-chamber septic tank will virtually guarantee that suspended solids will be sent into and then destroy the leaching fields. Settlement chambers, septic filters or an equivalent will be needed when converting from a conventional to an aerobic septic system design.
  • Septic tank or field construction mistakes: Improper original construction by bad site selection , especially on rocky, poorly-drained sites (pipes settle, for example) including improper trench grading & routing or extending piping over variations in trench grading without proper bedding (causing pipe settlement).

    Inspecting raised bed and mound systems in New York State I have often seen improperly constructed mounds including efforts by the builder to save on fill cost by using the "mound" as a place to first pile up all of the trees and tree stumps which needed to be cleared from the site, covering this mess with a too-thin layer of fill soil in which the septic absorption trenches are installed.

    The result is a mound system with a short life. If you see trees poking out from the perimeter of your mound further investigation may be in order. Also watch for effluent breakout around the bottom edges of the mound.

    Driving over the septic drainfield trenches during construction, omitting the required gravel under and around drainfield trench piping, and possibly backfilling with heavy wet clay-based soils during construction may also damage the field or reduce its performance.

    See our warnings at (3) Construction of Shallow Septic System Absorption Trenches
  • Shortcuts by the drainfield constructing contractor or excavator such as
    • Failure to provide proper type & quantity of gravel in drainfield trenches - hasty backfill with available soil
    • Failure to install drainfield trenches of depth, length, width, slope specified by the septic system designer & planner
  • Houses clustered around a lake: often will have a marginal system as properties were crowded together, built as part-time summer-camps, were built without code supervision, and often were built using amateur, marginal home-made systems.

    PHOTO of new construction placing septic system components in a wet area.In our photograph (left), new construction placed the septic tank and septic drainfields in the wet area shown in the photo.

    Everything looked questionable: the drainfield is placed near trees, risking root invasion of the piping; the drainfield is placed in wet soils, and the total elevation between the bottom of the drainfield piping and the top of the nearby lake is less than two feet. The drainfield is placed less than 20 ft. from the edge of the lake.

    It is improbable that the septic system belonging to this property will do much more than contaminate the nearby lake.
  • Improper septic absorption field location: one way to have a quick failure of a drain field is to install it in an area of high seasonal water tables (flooding the system) without sufficient fill and elevation, and/or without providing extra site and intercept drainage around the field to keep surface and subsurface water away from the field.
  • Surface & subsurface runoff water control mistakes: Ignoring site runoff and groundwater levels: Improper absorption field siting is found at some properties where the builder fails to consider site runoff or natural groundwater paths. One mound system was constructed using too little fill and placed over what had been a stream bed.

    My septic dye test very quickly produced red-dyed effluent at the low end of the mound where a seasonal stream continued to run under the mound in wet weather. Flooding the absorption system with surface or roof runoff, or rocky, poorly-drained or under-sized sites may simply lack capacity means a short absorption field life.
  • Steep slope septic systems: Placing a conventional septic system drainfield on a steep slope, over a stream bed, over a natural drainage swale (photo of this defect), specifying an under-sized mound or raised bed for the anticipated usage level, and poorly installed piping which does not slope properly or which becomes disconnected, and finally, use of improper fill soil which lacks the proper percolation rate all result in a costly installation with a too-short life.
  • Sloppy pipe layout and connections is a common cause of early failure in new drainfields. Pipes placed in trenches at uneven slope become disconnected; trenches with improper slope and omission of the specified amount and improper placement of gravel (for those systems) are also quite common where there is time or money pressure working against the builder.
  • Placing structures, patios, or paving over the absorption system: Covering an absorption bed, raised bed, or septic mound, such as by installation of paving, a patio, or weed-blocking solid plastic, will damage the mound and prevent proper operation by blocking both oxygen intake into the soil and also evaporation of effluent from the soil top. See "Blocking the ground" earlier in this list.
  • Placing improper substances into the building drains and septic system. Don't use the septic system to dispose of illegal oils, chemicals, fats, greases. One system in New York near the Taconic State Parkway was connected to house in which was operated an illegal drug manufacturing operation. So much contaminant was flushed down house drains that the workers contaminated their own well and poisoned themselves.
  • Discharging excessive salts or other chemicals which destroy the the biomat formation, clogging the soil. This problem and measures to protect and extend the life of the biomat is discussed at "Biomat Formation".
  • Planting trees, deep rooting shrubs, and certain ground covers on the septic absorption field (roots enter pipes) and evaporation can be blocked.
  • Use of septic tank or drain field additives which claim to extend system life can generate so much activity in the tank that solids are held in suspension and forced into the soil absorption system! Do not add any treatments, chemicals, yeast, or other treats to a septic system. In general these treatments don't work, may ruin the system, and are illegal in many localities. There is no magic bullet to repair a bad SAS.
  • Wet weather installations of septic systems & fields: Installing a drainfield in wet weather (which compacts the soil) is likely to mean a shorter field life.


Continue reading at DRIVING or PARKING OVER SEPTIC or select a topic from the More Reading links shown below.


Or see ATU FAILURE RATES, COSTS and also ATU PROBLEMS for aerobic septic system failures

Or see SEPTIC DRAINFIELD LOCATION: how to find the septic drain field or leaching bed and what site features should be kept distant from the drainfield.


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SEPTIC DRAINFIELD FAILURE CAUSES at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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