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Stanislaus County
Environmental Resources
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  • What Does the Evaluation of a Septic Tank System Tell Us?


People often ask this agency to describe what should be done when the on-site sewage disposal system (septic system) is evaluated for a home sale or refinancing loan.

In Stanislaus County, these evaluations are performed by private septic tank service companies. To learn what the local septic pumper industry is doing in this area, we asked representatives to tell us what they do when evaluating a septic system. From what we hear, they are using very much the same procedures and criteria that were standard protocol for the departments’ inspectors in the past. One important aspect of any evaluation, is that it follow a comprehensive, systematic inspection procedure. Equally important is that the report to the client provide an overview of what the inspection entailed and the basis of the assessment.

It is important to understand from the start, that an evaluation of the septic system provides a picture of how the system is operating at the time of inspection. To obtain some indication of how the system has operated in the past, some detective work is necessary. To identify clues to the past performance, look at the ground surface and deep into the septic tank and disposal field itself.

Surface check

Walk around the building, looking for indicators that perhaps the drain field is not working properly. When the homeowner has disconnected the washing machine from the house plumbing and waste water (gray water) is discharging onto the ground, this is usually an indicator that the absorption field is saturated or near its capacity to take water. Walking around the yard will often uncover surface seepage of sewage. Checking for any lush vegetation growth is a clue. Sticking a probe into the soil where lush growth is present often allows the sewage to bubble up to the surface through the probe hole. The telltale odor of sewage also may be present.

Checking the septic tank

After removing the soil over the septic tank, the lids are removed to give the inspector a view of the contents. If there is a thick mat of grease, detergent scum, and floating organic material, it is very likely that the septic tank has not been pumped in the last two or three years, perhaps longer. Another sign of lack of periodic pumping of the septic tank is the buildup of a thick layer of sludge in the tank bottom. A thickness of a foot or more will significantly reduce the tank=s liquid capacity and thus reduce the time the liquid stays in the septic tank. For a tank to operate properly, a holding time of one to two days is needed to allow the suspended organic matter in the sewage to settle out or float to the surface. The clear liquid (effluent) in the middle layer of the tank flows out into the drain field and percolates through the soil where additional treatment of the sewage occurs. Floating scum should be pushed aside to get a view of the liquid level, particularly around the outlet sanitary tee or elbow. A water level just above the bottom of the outlet pipe is a good indicator of a leach field that is satisfactorily accepting liquid. Whereas water that completely fills the outlet pipe or the tank=s interior is a common indicator of soil saturation. Both compartments of the tank should be pumped out as part of the evaluation procedure. An empty tank is needed to get a better view of the condition of the tank’s sides, the interior baffle, and the sanitary tees. The integrity of the sides, bottom and top of the septic tank should be sound. The tank’s internal baffle or septum should be in good condition, as should be the inlet and outlet sanitary tees. Older septic tanks often used clay or cement pipes for sanitary tees or elbows and they are often damaged or missing. Missing or damaged sanitary units must be replaced since they are important to the tank’s operation. The outlet tee is especially important since the bottom of the tee is below the floating scum layer, deep into the clear liquid in the center. Clear water is drawn out and flows into the disposal field, while grease and other materials that might clog the soil pores, are left behind.

The Leach field Area

The condition of the leach field is more difficult to evaluate when there is not an obvious lush vegetation growth in the disposal field area or the tank is completely full of liquid. A common technique used to determine if past periods of saturated leach fields have occurred, is to uncover the distribution box immediately downstream from the septic tank’s outlet tee. By removing the lid and checking for high water rings above the outlet pipes, you can get some idea of the system’s past performance. One of the best indicators of a saturated leach field, is when water runs back from the disposal field into the septic tank as they pump the tank out. This liquid represents sewage effluent that collects in the gravel filled disposal field because the surrounding soils cannot accept the water flowing out of the septic tank. The longer the flow occurs, the more extensive the leach field saturation.

On occasion, especially if there is not a record of the original installation and sizing of the system on file with this Department, challenging the field with a hose and running water placed deep into the drain pipe can gauge the condition of the leach field. Water is usually run for 20 to 30 minutes. If the water does not run back from the drain pipe, the field is passed as accepting water.

If you have any questions, feel free to call the Division of Environmental Health at 525-6700, weekdays 7:30 a.m. to 5:00 p.m. or e-mail me at bbadal@envres.org .


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