Secrets of Septic Tanks with Leach Trenches

What is a Trench Leach Field Septic System?

Septic leach trenches for disposal of septic tank effluent are 1 to 3 feet wide trenches that have up to 4 feet of aggregate rock with a perforated pipe installed down the middle of the trench on the top of the rock.  The pipe is connected to the distribution box and the septic tank outlet pipe.  Typically the aggregate rock filling the trench has a diameter of ¾ to 2-inches and has been washed to remove fines.  The water leaving the septic tank drains to the perforated pipe in the trench where it leaks out the holes.  It then is absorbed in the soil at the bottom and on the sides of the leach trench.  Treatment of the septic tank effluent water also occurs in the soil as microbes grow and use the organics and nutrients.  Physical filtering of the water also occurs in the soil.

Where can they be used?

In Arizona the state rules allow a leach trench system to be used after a septic tank when the site and soil do not have any limiting conditions.  There must be at least four feet of suitable soil below the bottom of the leach trench.

Septic Leach Trench Installed

This photo shows a narrow trench installed with observation pipes and filter fabric over the leach rock.

Soil Testing and Site Investigation

To design a leach trench system, the first step is to have a soil and site investigation completed.  The state rules require that the testing be conducted by a trained and certified inspector.  Some counties require the investigator to be a registered professional engineer, geologist, or sanitarian.   The testing including excavating three holes to a depth of 12 feet or to refusal if there is rock or cemented soil.  The investigator will log the soil profile noting the structure, texture, and consistence of the soil.  In some cases, such as an area with significant rock fragments, a percolation test with water will be conducted.  They will use the data to determine the percolation rate and soil absorption rate or SAR.  The percolation rate or soil characteristics are 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 leach trench.

If the SAR is faster than 1 inch in 1 minute as expected in an area with coarse sand, or gravel and cobbles, then it is too fast for a leach trench.  Additional treatment of the waste water is required to clean the water to prevent it from contaminating groundwater .  If the rate is slower than 1 inch in 60 minutes in clay soil, treatment will also be required.

Design and Sizing

The SAR 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.  A trench is sized based on the effective depth and width.  The effective depth is the depth from the bottom of the leach pipe to the bottom of the trench.  The maximum effective depth allowed by the state rules is 4 feet.  The area per foot is calculated by adding each sidewall plus the bottom.  The design flow from the building is divided by the SAR to determine the area of soil disposal required.  The area is divided by the area per foot of trench to calculate how long the leach trench will need to be for the volume of water.  If the length required is more than 100 feet, two or more leach trenches will be required.

Leach Trench Cross Section Detail


Trenches must meet the normal setbacks from the property line, building, pool, wall, well, or easement.  If there is more than one trench, they must be separated by a distance that is at least two times the effective depth of the trench.

Case Study Example

We recently designed a trench septic system for a new house in Rio Verde, Arizona.  The house will have three bedrooms and a design flow of 450 gallons per day based on the number of plumbing fixtures.  The soil and site evaluation showed sandy loam soil below 14 inches which gives a SAR of 0.6 gallons per day per square foot.  The trench will have a 4 foot effective depth and 2 foot width.  This gives 10 square feet per foot of trench:

Area of Trench = 2 (effective depth of 4 ft) + 2 ft wide = 10 ft per square foot

The area required for 450 gallons is 750 square feet:

Area Required = 450 gallons/0.6 gal/day/sq ft = 750 sq ft

The required length of the trench is 75 feet:

Trench Length Required = 750 sq ft/10 sq ft/ft = 75 ft

We designed the septic system to have two trenches, each 37.5 feet long, to fit the space available.  The overall depth of the trenches will be 7 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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