On-Site Wastewater Treatment and Disposal Systems - Forced ...

3.3.3.10 Hydraulic Conductivity
Several methods of measuring the hydraulic conductivity of soils have
been developed (l)(ll). The most commonly used test is the percolation
test. When run properly, the test can give an approximate measure of
the soil's saturated hydraulic conductivity. However, the percolation
of wastewater through soil below soil disposal systems usually occurs
through unsaturated soils. Therefore, empirical factors must be used to
estimate unsaturated conductivities. The unsaturated hydraulic conduc-
tivities can vary dramatically from the saturated hydraulic conductivity
with changes in soil characteristics and moisture content (see Appendix
A).
The percolation test is often criticized because of its variability and
failure to measure the hydraulic conductivity accurately. Percolation
tests conducted in the same soils can vary by 90% or more (1)(11)(12)
(13) (14) 0 Reasons for the large variability are attributed to the pro-
cedure used, the soil moisture conditions at the time of the test, and
the individual performing the test. Despite these shortcomings, the
percolation test can be useful if used together with the soil borings
data. The test can be used to rank the relative hydraulic conductivity
of the soil. Estimated percolation rates for various soil textures are
given in Table 3-7.
Soil Texture
TABLE 3-7
ESTIMATED HYDRAULIC CHARACTERISTICS OF SOIL (15)
Sand >6.0 (10
Sandy loams
Porous silt loams
Silty clay loams
Clays, compact
Silt loams
Silty clay loams
Percolation
ml n/in.
0.2-6.0 10-45
39
x0.2 >45