How to Troubleshoot Mound Systems

Site, design, construction and homeowner misuse can all be contributing factors to system malfunction
How to Troubleshoot Mound Systems
Investigating a mound system leaking out the toe.

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It has been estimated that mound systems should perform hydraulically for 25 to 35-plus years. However, some systems do not perform as they should for that long. Soil treatment system problems are often traced to improper design and construction practices, but improper operation and maintenance of the system also contribute to problems with systems. In a 2004 study in Wisconsin, Blasing and Converse investigated mound toe seepage. Their results indicated that 80.8 percent of the samples had fecal coliform levels of less than 200 col/100 mL, which is the standard for swimmable and fishable waters. This data raises the question of whether mounds with minimal and seasonal toe seepage are an environmental health problem.

Reasons for system malfunctions
The materials with which systems are constructed can lead to malfunction. Systems’ siting, design, construction and homeowner misuse commonly contribute to malfunction. Some common problem situations are described below, including where the seeping problem occurs (the toe or the side/top) in the system.

Malfunction due to poor-quality materials
A common cause for seeping mounds is poor-quality materials including:

  • Sand with too many fines: There should not be over 5 percent silts and clays (side/top)
  • Sand with too much fine sand: This is a problem when fines total more than 5 percent (side/top)
  • Soil treatment system rock with too many fines: There should be less than 1 percent fines (side/top)

Malfunction due to poor siting or design
Poor siting and design are another common cause for seeping mounds including:

  • Mounds placed on soils that do not have at least 1 foot of soil above the periodically saturated soil or bedrock (toe)
  • Insufficient treatment sand to treat the wastewater contaminants prior to the native soil (toe)
  • Systems placed on disturbed or compacted soils (top/toe)
  • Soil treatment systems not placed on the contour (top/toe)
  • Soil treatment systems placed in swales/upland drainage ways (top/toe)
  • Misestimation of the soil texture and structure or percolation rate (top/toe)
  • Miscalculation of slope (a system designed for a flat site instead of a sloping site) (toe)
  • Miscalculation of the bottom area/absorption area — the area where sewage enters original soil (top/toe). This issue can arise in heavy clay soils where a large absorption area is often needed along with a low contour loading rate.
  • Stacking several mounds on a slope without sufficient separation between the two
  • Upslope drainage not designed; soil treatment systems receive upslope runoff (top/ toe)

Malfunction due to construction errors
Installation-related construction errors include:

  • Sewage absorption area compacted during construction, reducing infiltration (toe)
  • Sewage absorption area smeared during excavation or scarification because the soil moisture was over the plastic limit (toe)
  • Excess vegetation was not removed, reducing infiltration from vegetative mat (toe)
  • Cracked pipe or pipe that became disconnected (toe or side)
  • Non-watertight joints in tanks (excess infiltration) (toe)
  • Incorrect pump selection or float adjustment (toe or side/top)
  • Water stands in sagging pipe and freezes in winter (side or problems at pump tank)

Malfunction due to system misuse by homeowners

  • Excessive water use or leaky fixtures (toe)
  • Clean-water source hookups to the septic system, such as the sump pump (toe)
  • Lack of pumping solids from septic tank or other cause of high-strength waste (side/top)
  • Improper landscaping causing compaction of area around system (toe)
  • Rooftops or impervious areas draining to the tank or system areas (toe)
  • An upset septic tank due to excessive use of cleaners, medicines and disposal of chemicals in system (side/top)

About the author
Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association (MOWA) and the National Onsite Wastewater Recycling Association (NOWRA), and serves on the NSF International Committee on Wastewater Treatment Systems. Send her questions about septic system maintenance and operation by email to


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