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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

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

A Leaky pipe connection allows ground water to leak into this septic tank (C) Daniel FriedmanLeaks at septic tanks - leaks into and leaks out of the septic tank are problems. Problems of Leaks Out of the Septic Tank & Leaks Into the Septic Tank. Leaks out of the septic tank prevent testing the septic drainfield. Leaks into a septic tank can flood the tank and drainfield. Pumping a Flooded Septic Tank - Does that Fix Anything? Description of test standards for septic tank water tightness.

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 SEPTIC TANK LEVELS of SEWAGE for details on normal and abnormal levels and what they mean.

Where do Septic Tank Leaks Occur

A 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.

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.

Septic Tank Leak Test - Water-tightness Test Standards

The following describe septic tank water-tightness test standards & procedures. While these septic tank leak test procedures focus on the tank's abiltiy to prevent sewage or wastewater leaks out of a septic tank, indirectly they also address in part the risk of groundwater or surface runoff leaking into a septic tank. Keep in mind that these tests do not address:

Septic Tank Water Tightness Testing Procedure & Critera For Pre-cast Concrete Septic Tanks

  Hydrostatic Septic Tank Test Septic Tank Vacuum Test  
Septic tank test standard Septic Tank Test Preparation Septic Tank Pass/Fail Criterion Septic Tank Test Preparation Septic Tank Pass/Fail Criterion Comments
C 1227
ASTM (1993)
Seal tank, fill with water, and let stand for 24 hours. Refill tank. Approved if water level is held for 1 hour Seal tank and apply a vacuum of 2 in. Hg. Approved if 90% of vacuum is held for 2 minutes  
NPCA (1998) Seal tank, fill with water, and let stand for 8 to 10 hours. Refill tank and let stand for another 8 to 10 hours. Approved ifno further measurable water level drop occurs Seal tank and apply a vacuumof 4 in. Hg. Hold vacuum for 5 minutes. Bring vaccum back to 4 in. Hg. Approved if vacuum can be held for 5 minutes without a loss of vacuum.  


Note that these tests do not require absolute water tightness or air tightness.

Adapted from US EPA Septic Design Manual Table 4-14 Watertightness testing procedure/criteria for precast concrete septic tanks


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.


Continue reading at SEPTIC TANK INSPECTION PROCEDURE or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see CAULK & SEALANT CHOICES, products list for possible septic tank lid or pipe connection sealants to keep water out of the tank

Or see MEASURE SCUM & SLUDGE how we measure the thickness of septic tank floating scum and bottom sludge levels.

Or see SEPTIC TANK BACK FLOODING - a common source of flooded septic tanks: failed drainfield / soak bed

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

Or see

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