MIL-HDBK-1005/16
6.4.4
Depth to High Water Table. In mound systems, the
absorption area is raised above the natural soil to keep the
bottom of the trenches as least 2 feet (53 cm) above groundwater,
as well as creviced or porous rock or relatively impermeable
soils. High water tables can be determined by direct observation
or by soil mottling. Occurrence of grey and red soil mottling
phenomena can be used to indicate periodic saturation with water.
However, lack of mottling does not always mean that seasonally
perched water does not occur. Looking at mottling is meaningful,
but direct observation is preferable if there is any doubt.
6.4.5
Depth to Impermeable Soil Layer or Rock Strata.
The depth to impermeable soil or rock strata can vary over a
range (see Figures 19 and 20). The optimum distance will vary
for a given site. Provide sufficient area so that the effluent
can move away from the mound. Otherwise, effluent will build up
in the mound and cause seepage out the toe of the mound.
Climatic factors, soil permeability, slope, and system
configuration affect this distance. Slowly permeable soils
require more area to remove the effluent from the mound than do
permeable soils. Frost penetration reduces the effective area
for lateral movement; thus, in warmer climates, depth
requirements are not as great as for colder climates.
Level sites require shallower depths than do sloping
sites, as more area is available for effluent dispersal since the
effluent can move in several directions. Less depth is required
for long narrow mounds than is required for more square systems
because the square system concentrates the liquid into a smaller
area.
6.4.6
Depth to 50 Percent Volume Rock Fragments. Rock
fragments do not assist in purification and disposal of
effluents. They cause the effluent to be concentrated between
the fragments. This may lead to saturated flow and, thus, poorer
purification. If the soil contains 50 percent rock fragments by
volume in the upper 24 inches (61 cm) of soil, then there is only
half the soil available for purification and disposal of the
effluent. Depths greater than 24 inches (61 cm) should be used
if the soil beneath the mound contains more than 50 percent by
volume of rock fragments. This is especially true for permeable
soils over creviced bedrock and in areas where the high water
table may intersect a potable water supply.
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