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�, 83 SOIL MOISTURE AND GROUND WATER <br /> Sl system of units hydraulic conductivity is in meters per day. A derived unit of <br /> gallons per day per square foot was used in the past. It can be converted to feet <br /> Per day by dividing by 7.48. <br /> 4.33 PERME6,BiL1TY OF SEDIMENTS <br /> Unconsolidated coarse-grained sediments represent some of the most <br /> Prolific producers of ground water. Likewise, clays are often used for engineer. ' <br /> mg purposes, such as lining solid-waste disposal sites, because of their <br /> ex-tremely low intrinsic permeability. There is obviously a wide-ranging continuum <br /> Of permeability values for unconsolidated sediments (Table 4.5). j <br /> The intrinsic permeability is a function of the size of the pore opening. <br /> ' The smaller the size of the sediment grains, the larger the surface area the water <br /> contacts (Figure 4.13). This increases• the frictional resistance to flow, which <br /> reduces the intrinsic permeability. For well-sorted sediments, the intrinsic <br /> permeability is proportional to the grain size of the sediment (25). <br /> ' For sand-sized alluvial deposits, several factors relating intrinsic perme- <br /> ability to grain size have been noted (26). These observations would hold true <br /> for all sedimentary deposits, regardless of origin of deposition. <br /> I. As the median grain size increases, so does permeability. This is due <br /> to larger pore openings. <br /> ' 2. Permeability will decrease for a given median diameter as the stan- <br /> dard deviation of particle size increases. The increase in standard <br /> deviation indicates a more poorly sorted sample, so that the finer <br /> material can fill the voids between larger fragments. <br /> 3• Coarser samples show a greater decrease in permeability with an <br /> increase in standard deviation than do fine samples. <br /> a• Unimodai (one dominant size) samples have a greater permeability <br /> TABLE 4.5 Ranges of intrinsic permeabilities and hydraulic conductivi- <br /> ties for unconsolidated sediments <br /> Intrinsic Hydraulic <br /> Permeability Conductivity <br /> Material (darcys) (cm/sec) <br /> '• Clay -6- --3 --9 -6 <br /> 10 10 10 -10 <br /> Silt, sandy silts, <br /> clayey sands, till 10-3-10-' 10-6-3 o-4 <br /> ' Silty sands, fine sands 70-2-1 10-5--10-3 <br /> Well-sorted sands, <br /> glacial outwash 1 -102 1 o-3-1 o-' <br /> ' Wel i-sorted gravel 10-103 10-2-1 <br />