Seven Site Limitations for Septic System Design

Before designing a septic system, the site must be evaluated.  The Arizona state rules call the site review process “Surface Characterization.”  The purpose is to identify site limitations like steep slopes and rock outcrops that will affect the location and design of the septic system.

The reviewer will look for seven limiting factors:

1) Surface Slope.  If the surface slope is greater than 15 percent, the design of the septic system will need to make sure that the water moves downward into the soil and does not surface due to the steep slope.  A 15 percent slope means there is a drop of one and one-half feet in ten feet.

2) Setbacks.  Septic systems are required to be a specific distance from wells, water lines, washes, buildings and other features.  The septic system design will be affected by the space available with the setbacks taken into account.

3) Drainage.  How rain water runs off a site also affects the design of a septic system.  The system should not be located where run off will cause erosion or flood the system or soil so the treated water will reach wet soil and not soak into the ground.

4) Flood Zone.  Similar to drainage, a septic system should not be located near a wash that will flood.  The rules require setbacks based on the local flood control agency, typically 25 to 50 feet.  In addition, washes in Arizona can easily migrate out of their current position.

5) Rock Outcrops.  Rocks and boulders that can not be excavated show that there may be buried rocks or bedrock that could cause the treated water to surface.  The design must account for this possibility by providing advanced treatment or a different location.

6) Soil Fill.  Previous excavation may have imported or moved soil so it is now fill with no natural structure.  The fill area may allow the treated water to soak into the ground fast, but then it will reach the undisturbed soil and move sideways.

7) Fissures.  Large cracks in the ground in many areas of Arizona have been caused by the groundwater table dropping.  These cracks could convey water from a septic system directly to the groundwater causing contamination.  A septic system is required to be 100 feet away from a fissure.   The Arizona Geological Survey has maps of known fissures available on their website.

Once the site limitations at the surface and below the surface have been identified, the septic system design can be completed.

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