Before determining where to place a mound system, it is probably a good idea to have a picture of the general mound profile. It starts with a layer of clean sand at the undisturbed soil surface which is at least 1 foot thick. The top of the clean sand is level so the media (rock, chambers, etc.) used in the bed can be installed level as well as the pressure distribution laterals. Geotextile fabric is put down if the media is rock and then a sandy loam cap is provided over the top before being finished with topsoil.

Location on the landscape is the first key component. Mounds should be located on a slope whenever possible. If the system is located on at least a 1 percent slope, it is easier to predict how and where the effluent will move through the clean sand layer and then into the original soil. If the site is absolutely level the assumption is made that the effluent will move equally in both directions, making for a symmetrical mound.

Having the mound located on a slope also provides a better opportunity to “landscape” it into the natural slope to avoid the appearance of a 4- to 5-foot-tall hump in the middle of a yard. Humps usually result in unhappy homeowners.

While there is not necessarily a maximum slope limitation, placement is limited by the practicality of bringing enough materials in to finish the mound so that the downslope dike meets the slope. While this can be mitigated somewhat by not needing clean sand over the entire area (only over the width necessary for the effluent to infiltrate the original soil, something we will cover in future discussions), mounds are usually limited to slopes of 12 to 15 percent.

Another important landscape and slope consideration is that the mound needs to be located and constructed so that it does not impede the movement of surface water off the slope. Think in terms of not providing a situation where water runs off from above and then collects behind the upslope dike area, which would interfere with the ability of the mound to infiltrate the effluent being applied and can lead to hydraulic failure. Avoiding this may require building a berm or diversion upslope from the mound to ensure surface runoff is routed around and away from the mound.

As with any other soil treatment unit the mound should be located so it is not subject to additional water impacting the system; so away from runoff areas and waterways, away from areas that may be affected by runoff from impervious surfaces such as roofs, driveways, patios, etc.

About the Author

Jim Anderson is connected with the University of Minnesota onsite wastewater treatment education program, is an emeritus professor in the university’s Department of Soil Water and Climate, and education coordinator for the National Association of Wastewater Technicians.

Send him questions about septic system maintenance and operation by email to kim.peterson@colepublishing.com.

This article is part of a series on mound design:

• Do We Need Design Guidance for Mound Systems?
• Where Should You Put a Mound System?
• The Importance of Sand Specification
• More Mound Design Tips
• How Do You Determine Absorption Width?