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80 SOIL MOISTURE AND GROUND WATER - <br /> SI system of units hydraulic conductivityis in meters e <br /> 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 /> 43 3 PERMEABILITY 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 /> ing purposes, such as lining solid-waste disposal sites, because of their ex- <br /> tremely low mtnnsic permeability There is obviously a wide-ranging continuum <br /> of permeability values for unconsolidated sediments (Table 4 5) <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 Bow, which <br /> reduces the intrinsic permeability For well-sorted sediments, the mtrtnsic <br /> permeability is proportional to the gram 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 /> 1. As the median gram size increases, so does permeability Timis 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 mn 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 /> 4. Unmmodal (one dominant size) samples have a greater permeability <br /> TABLE 4S Ranges of intrinsic permeabilities and hydraulic conductivi- <br /> ties for unconsolidated sediments <br /> Intrinsic Hydraulic <br /> Permeability Conductivrty <br /> Material (darcys) (cm/sec) <br /> clay 10-6-10-3 10_9-10_., <br /> Silt, sandy silts, <br /> clayey sands, till 10-3-10'' 10-6_10-4 <br /> Silty sands, fine sands 10-2--1 10-5-10_3 <br /> Well-sorted sands, ° <br /> glacial outwash 1 -102 10-1-10-1 <br /> Well-sorted gravel 10--103 10-2_1 <br /> I <br />