MIL-HDBK-1005/16
6.4.1
Description. A typical mound system is shown in
Figure 15. 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 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 should be deep enough to provide
absorption of pollutants before they can infiltrate bedrock and
enter groundwater.
6.4.2
Site Considerations. Table 9 summarizes soil and site
factors that restrict mound systems. In using Table 9,
percolation tests are usually run at 20 to 24 inches (50 to
61 cm) from the natural surface. As shown for slowly permeable
soil, if the percolation rate is less than 60 minutes per inch
(min/in.)(24 min/cm), the soil is permeable so that the slope of
the site may be cautiously increased to keep effluents in the
upper soil horizons. If the percolation rate is greater than
120 min/in. (48 min/cm), then the soil is so impermeable as to
disallow use of a standard mound system. Soil characteristics,
water table depth, and amount of large fragments dramatically
influence mound design. Figure 19 illustrates a mound system
using two trenches, while Figure 20 shows the bed absorption
system. For further information on design criteria and
installation, see EPA 625/1 80-012.
6.4.3
Depth to Pervious Rock. A minimum of 24 inches (61 cm)
of unsaturated natural soil is required beneath the mount. This
natural soil provides additional purification capacity and serves
as a buffer in protecting the groundwater from contamination. It
also reduces the amount of fill material needed for the mound,
serving as a part of the unsaturated soil needed for
purification.
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