Photo of a leak into a septic tank at the sewer line connection (C) Daniel Friedman Septic Tank Leaks
How & Why Septic Tank Leaks Cause Septic System Failures

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Septic tank tank leaks are one of the things to check for during a septic tank inspection. Here we explain where and why septic tanks might leak, why surface water or runoff leaking into a septic tank is bad, and why septic effluent leaking out of a septic tank can also be a problem. We explain why pumping a flooded septic tank does not usually fix anything. Leaks in either direction, into the septic tank or out of the septic tank can be a problem.

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Causes, Effects, & Repair of Leaks Out of or Into the Septic Tank

Our page top photo shows water ponding at the connection of a sewer line to a septic tank. Because this sewer line runs downhill from the house to the septic tank it was particularly good at collecting surface water and aiming it all at the septic tank entry port. Because the sewer line was not sealed at the tank, water entered and flooded the septic tank and drainfield.

See SEWAGE LEVELS in SEPTIC TANKS for details on normal and abnormal levels and what they mean.

Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical reviewers are welcome and are listed at "References." This is a chapter of SEPTIC SYSTEM INSPECTION & MAINTENANCE COURSE an online book on septic systems.

Where do Septic Tank Leaks Occur

A Leaky pipe connection allows ground water to leak into this septic tank (C) Daniel FriedmanA septic tank can develop a leak at just about any location but here are some common ones.

Our photo shows concrete poured around a waste line entering the septic tank. You can see that just as the concrete pooled in this location, the trench dug for the sewer line would, in wet weather, collect and aim a large volume of water into the septic tank.

  • At the sewer line entering the septic tank, or the effluent line leaving the septic tank may leak: if the pipe is not sealed in that location; many older septic tanks provided no sealant unless some home-made system was used. Some installers pour concrete around the waste pipe entering the tank - which can work but it makes future repairs more troublesome.

    Modern septic tanks may include a rubber gasket to help seal at the tank entry and exit openings. But if the waste lines entering or the effluent lines leaving the septic tank are at a sharp angle with respect to the tank, the gasket may not seal properly.
  • Any damaged sewer piping and even effluent piping might permit ground water or surface runoff to flow into the septic tank or into the drainfield.
  • A septic tank cover or cleanout port, especially one that is below ground may permit surface water to enter the septic tank. (Make sure septic tank covers are sound - falling into a septic tank is likely to be fatal).
  • Rust damage to a steel septic tank can let effluent out of the tank and water leak in depending on weather conditions.
  • Cracks in a concrete septic tank also can let effluent leak out or water leak in - though we have not found these occurring as often as rusted out steel septics
  • Damaged fiberglass or plastic septic tanks can also leak at a seam or point of damage - though we heard few reports of this problem.

You can reduce the chances of water leaking into a septic tank by making sure that roof runoff and surface drainage are directed away from the septic tank as well as the drainfield.

Leaks out of the septic tank prevent testing the septic drainfield

Leaks out of the septic tank can occur if the tank has a hole (for example a rusted-out metal septic tank) or if a concrete, fiberglass, or plastic tank is cracked or damaged. A leaky septic tank means that effluent may not be properly treated since it is not reaching the drainfield.

A leaky septic tank also means that a septic loading and dye test to attempt to check on the condition of the drainfield may fail to work. Particularly if the septic system has been unused for some time, and if the leak is near the bottom of the septic tank, the liquid level in the tank will drop very low. The result is that a normal septic dye test volume will simply be filling up the septic tank rather than pushing water out into the drainfield.

In turn this condition means that the septic test could not test the function of the drainfield. The risk is that new owners moving into the property will very quickly discover the bad news that not only has the septic tank got a leak but the drainfield may not really be functional.

A septic tank that is not in use and leaks out may also produce solidified scum and sludge that collect low in the septic tank or on its bottom - making septic tank cleaning extra difficult.

If there is a port to permit safely looking into the septic tank before an inspection or test, be sure to check the sewage level in the tank.

Leaks into a septic tank can flood the tank and drainfield

Leaks into the septic tank can occur if ground water or surface runoff are directed towards the septic tank or pipes that carry sewage into the tank (or effluent out of the tank). Any opening that permits surface runoff to enter the septic tank risks flooding the tank. In rainy weather the result can be a water overload in the septic tank, reducing the level of treatment in the septic tank.

Perhaps more of a problem, the same water running into the tank may also push its way into the drainfield, flooding the septic drainfield. If extra volume of the water entering the septic tank also prevents adequate settling time for sewage entering the tank then an excessive level of suspended solid waste may be forced of the septic tank and into the drainfield, further reducing the life of that component.

Leaks into a septic tank can also occur if the drainfield is so flooded that water is flowing backwards through the drainfield piping and back into the septic tank through its outlet.

Pumping a Flooded Septic Tank - Does that Fix Anything?

Pumping the septic tank won't fix any of these flooded septic tank conditions. A septic tank is normally always "full" to just below the septic tank outlet opening.

But pumping a flooded septic tank might be performed for the following reasons:

  1. Flooded septic tank needs cleaning: If the septic tank was exposed to area flooding it may have become loaded up with mud and silt and needs to be cleaned in order to work at all. In this case, the septic drainfield piping, distribution boxes, and similar components need to be excavated sufficient to permit their inspection as well.
  2. Diagnose a flooded septic tank: If the septic tank appears to be filling from surface runoff or ground water leaking into the tank, pumping the tank permits the owner or septic service company to look for these problems by observing the empty tank for signs of effluent or ground water back-flowing into the tank. See SEPTIC TANK BACK FLOODING for details.
  3. Permit temporary use of a flooded septic tank: if the tank is emptied, and if the building occupants make a maximum effort to minimize unnecessary water usage (showers, laundry, bathing), then the occupants may be able to use the septic system and thus the building and site in a sanitary way, without further contaminating the neighborhood, for a few days to a week, depending on the tank size, number of building occupants, frequency of toilet flushes, etc.

In our OPINION, if the septic tank floods once in 20 years, under exceptional conditions, no design changes or repairs may be needed other than cleaning the septic tank when floodwaters subside. But if this condition happens frequently, the septic system is unsanitary and may be a health risk to the building occupants or its neighbors.

At TANK INSPECTION PROCEDURE we describe how to inspect septic tanks.

See MEASURE SCUM & SLUDGE for a detailed description of how we measure the thickness of septic tank floating scum and bottom sludge levels.

See SEPTIC TANK LEVELS of SEWAGE for a discussion of the normal levels of sewage found in a septic tank.

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