TM 5-814-3/AFM 88-11, Volume III
6-5. Mound systems.
Many Army installations are sited upon low-lying plains, reclaimed swamps, or poorly drained areas.
Ordinarily a septic tank and leach field would be used for small flows, but soil conditions or high clay content,
high water table, shallow depth to bedrock and slow percolation make ordinary soil disposal techniques
unfeasible. (See Boyle and Otis, 1982.) The septic tank-mound system may then have application at Army
installations. Septic tank-mound systems should not be used at Air Force installations.
a. Description. A typical mound system is shown in figure 6-7. A siphon may replace the pump if the
mound is located downslope. The mound itself consists of fill material, an absorption area, a distribution
system, a cap and a covering of topsoil. Effluent is dosed into the absorption area through the distributor
piping. The fill material provides the major zone of purification before the cleansed effluent passes into the
buried topsoil of the original soil line. The cap is of fill, deep enough to protect the piping; it should be sloped
and contain sufficient silt and clay as to encourage runoff of rainfall. The topsoil above is seeded with grasses
to prevent erosion and encourage some evapotranspiration. In pervious soils above shallow bedrock, the
mound must be deep enough to provide absorption of pollutants before they can infiltrate bedrock and enter