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so SOIL MOISTURE AND GROUND WATER ' ' S1 system of units hydraulic conductivity is in meters per day A derived unit of gallons per day per square foot was used in the past It can be converted to feet per day by dividing by 7 48 43.3 PERMEABILITY OF SEDIMENTS Unconsohdated coarse-grained sediments represent some of the most prolific producers of ground water Likewise, clays are often used for engineer- ing purposes, such as lining solid-waste disposal sites, because of their ex- tremely low intrinsic permeability There is obviously a wide-ranging continuum of permeability values for unconsolidated sediments (Table 4.5) The intrinsic permeability is a function of the size of the pore opening The smaller the size of the sediment grains, the Iarger the surface area the water contacts (Figure 4 13) This increases the frictional resistance to flow, which reduces the intrinsic permeability For well-sorted sediments, the intrinsic permeability is proportional to the grain size of the sediment (25) For sand-sized alluvial deposits, several factors relating intrinsic perme- ability to grain size have been noted (25) These observations would hold true for all sedimentary deposits, regardless of origin of deposition 1. As the median grain size increases, so does permeability This is dile to larger pore openings 2. Permeability will decrease for a given median diameter as the stan- dard deviation of particle size increases The increase in standard deviation indicates a more poorly sorted sample, so that the finer material can fill the voids between larger fragments. i 3. Coarser samples show a greater decrease in permeability with an increase in standard deviation than do fine samples 4. Ummodal (one dominant size) samples have a greater permeability TABLE 45 Ranges of intrinsic permeabihties and hydraulic conducnvf- ties for unconsolidated sediments Intrinsic Hydraulic Permeability Conductivity 0 Materia! (darcys) (cm/sec) i Clay 10-6-10-3 10-'-1o`6 Silt, sandy slits, clayey sands, till l o-3-1 o-1 10-s-10 4 Silty sands, fine sands 10-1-1 10'5-10'"3 Well-sorted sands, ' glacial outwash 1-1 oz 10-1-10-1 Well-sorted gravel 10_103 10-2--1 I I