Defining the Absorption Area of a Mound Treatment System

Pay attention to several critical siting and construction practices when you incorporate mounds into a septic system plan

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Recently, someone asked us how setback distances were determined for mound systems. In Minnesota, rules state setback distances must be measured from the original soil absorption area. Absorption area terminology is not used in a lot of other states. Although when talking about mounds, it is our opinion that this is the desired approach over other ways of looking at the width and area necessary under the clean sand layer that receives effluent.

The width of original soil receiving effluent under the mound must be capable of absorbing effluent introduced to the infiltration bed (previously called the rock bed layer) of the mound or else surfacing will occur at the toe of the mound berm. Total absorption area is the product of the length of the infiltration bed and the absorption width. The berms on the short ends and the upslope berm are not considered in the total absorption area.

In direct answer to the question then, setbacks would be measured from the edges of the infiltration bed on the upslope side and edges. Setbacks on the downslope side would be determined from the end of the downslope absorption width.


Required soil absorption width depends on the allowable loading rate for the original soil under the clean sand layer. This is determined by soil analysis of the top one-foot layer of soil in contact with the clean sand layer. This analysis can be done either by soil evaluation or by percolation rate.

For a mound installed on a level site — defined as a slope less than 1 percent — the shape of the mound will be symmetrical since the assumption is that effluent will move downward and away from the infiltration bed in both directions. Absorption width is equal to the width of the infiltration bed plus the width of mound berms on either side of the bed. As indicated above, the ends are discounted since, while some will flow out the ends, most of the flow will be to the sides.

When a mound is installed on a sloping site — defined as a site with a slope greater than 1 percent — effluent is assumed to move downslope, and the required width is determined from by the width of the infiltration bed and the downslope berm width.

One of the major reasons we have seen for surfacing at the toe of mound berms is not allowing adequate width for the effluent applied to the infiltration bed to soak in before the end of the berm. This should be accounted for in the design process. However, a check an installer can make on site during installation is to make sure there is adequate width when the sand is placed and the mound is finished.

We will present an example using Minnesota design numbers, but recognize the same analysis can be done using your state’s design numbers for mounds. Research on soils in Minnesota has shown that in the upper foot, the hydraulic conductivity is never less than about 1 centimeter per day. Using this as the bottom design number for heavy textured clay soils, the allowable loading rate (or the expected amount of effluent the soil will absorb) works out to be 0.24 gallons per square foot per day. Many states we have visited use a loading rate number for these types of soils of 0.2 gallons per square foot per day, which makes for easier calculation.


For a clean sand infiltration bed, the loading rate is specified as 1.2 gallons per square foot per day. The original soil under the bed, though, can only accept 0.24 gallons per square foot per day. This means that 5 times the width of the infiltration bed is necessary for the effluent to infiltrate the original soil (1.2 gallons per square foot per day ÷ 0.24 gallons per square foot per day). If the infiltration bed is 10 feet wide and the necessary width is 5 times that (or 50 feet), the width of the mound from the downslope edge of the bed is at least 40 feet to accept the effluent.

When finishing the mound, we often see this downslope width get cut off some during the installation process in the desire to fit the mound within the surrounding landscaping. This results in surfacing at the toe of the berm. We advise installers to watch that absorption width during mound construction. It is a key component of proper mound installation.

A final note on this issue: The reader should recognize that “absorption width” and not “absorption area” is the terminology that best describes how to make sure that there is soil under the mound to allow for infiltration of applied effluent.


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