Secrets of Septic Tanks with Seepage Pits

What is a Seepage Pit?

It is a 4, 5, or 6 foot diameter hole drilled straight down 30 to 50 feet.  Typically it is filled with ¾ to 2-inch diameter rock (just like a leach trench) with a perforated pipe installed in the middle of the hole, and continues to the bottom.  The pipe is connected to the outlet pipe from the septic tank with a tee or elbow.  The water leaving the septic tank goes into the perforated pipe to the bottom of the pit and leaks out the holes.  It then is absorbed in the soil at the bottom and on the sides of the seepage pit.

Where can they be used?

In Arizona the state rules allow a seepage pit system to be used after a septic tank, but only in certain areas.  The area must have ground water at least 60 feet below the bottom of the seepage pit.  The soil in the area must be alluvial fill in a valley – this means the area was filled by soil eroded by wind and rain from nearby mountains or carried there by a natural water channel.  The areas of Phoenix and Tucson are typical alluvial fill areas where the valley floors have hundreds to thousands of feet of fill.  The areas near hills and mountains are not suitable because there is not adequate soil to provide filtering and treatment of the septic tank effluent water.  Or there may be too much rock or a slow percolation rate.   Other areas, like along river beds or washes, may have too much loose rock so the water will not be filtered before it reaches groundwater.    Some counties like Gila County and Coconino County do not have any areas suitable for seepage pits.

aerobic treatment system

This photo shows a seepage pit in an area with too much loose rock. An aerobic treatment system was used after the septic tank to treat the water before it drained to the seepage pit.



To design a seepage pit system, the first step is to have a seepage pit test completed.  The state rules require that the seepage pit test results be certified by an Arizona licensed engineer or sanitarian.  The test hole is drilled by a drilling rig and is an 18-inch diameter test hole to a depth of 50 ft.   It is filled with water to presoak the soil and then a seepage pit percolation test is performed.  The water level is measured every ten minutes to determine how fast it drops by soaking into the sides and bottom of the hole.

The seepage pit percolation rate is determined when the rate of drop reaches a steady rate or by graphing the results to determine the rate.   If the rate is faster than 1 inch in 1 minute, then it is too fast.  Additional treatment of the waste water is required or another method (like trenches) is required.  If the rate is slower than 1 inch in 30 minutes, seepage pits are not allowed.  The number of seepage pits would be costly at such slow percolation rates.

The percolation rate is used to determine the Soil Absorption Rate or SAR using a table in the state rules.  This is the volume of water that will soak into the soil each day in one square foot of sidewall of the seepage pit.


Design and Sizing

The percolation rate is used by the designer to determine how much sidewall area is needed to absorb the effluent water that will drain from the new building.  The design flow from the building is divided by the SAR to determine the area of soil required.  The area is divided by the area of the seepage pit to calculate how deep the seepage pit will need to be for volume of water.  If the depth required is more than 45 feet, two or more seepage pits will be required to keep them less than 50 feet deep.



Schematic of  a seepage pit


Seepage pits must meet the normal setbacks from the property line, building, pool, wall, well, etc. just like a leach trench or chamber disposal system.  If there is more than one seepage pit, the pits must be separated by a distance that is at least three times the diameter of each pit.


Case Study Example

We recently designed a seepage pit for a new guesthouse in Buckeye, Arizona.  The site has groundwater more than 200 feet below the surface and is in an area with alluvial fill.  The guest house will have two bedrooms and a design flow of 450 gallons per day based on the number of plumbing fixtures.  The seepage pit test gave a percolation rate of 1 inch in 1.25 minutes which gives a SAR of 1.2 gallons per day per square foot.  The seepage pit will have a 4 foot diameter and an area of 12.56 square feet per foot.  The area required for 450 gallons is 375 square feet:

Area Required = 450 gallons/1.2 gal/day/sq ft = 375 sq ft

The required depth of the seepage pit is 30 feet:

Depth Required = 375 sq ft/12.56 sq ft/ft = 30 ft

The total depth of the drilled seepage pit will be 35 feet to allow for the depth of the plumbing under the house, the slope of the sewer pipe, and drop through the septic tank.

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