2013-07-03_REVISION - M1985112 (3)

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OCCURRENCE AND MOVEMENT OF GROUNDWATER 75 hydraulic gradient. If the hydraulic gradient (head loss per unit length of travel) is doubled, the rate of flow in a given sand is also doubled. Conversely, doubling of the flow rate requires doubling of the hydraulic gradient. These ratios apply only to laminar flow, however. If turbulent flow is present, the flow rate does not change in direct proportion with the hydraulic gradient; doubling of the hydraulic gradient may increase the flow rate by only 1.5 times. The information in this paragraph is vital to understanding water -well hydraulics, which is presented in Chapter 9. The slope of the water table or potentiometric surface is the hydraulic gradient under which groundwater movement takes place. The total flow through any vertical section of an aquifer can be calculated if we know the thickness of the aquifer, its width, its - average hydraulic conductivity, and the hydraulic gradient. The flow, q, through each foot of aquifer width is: q = Kb1 (5.12) where K is the hydraulic conductivity averaged over the height of the aquifer, b is the aquifer thickness in feet, and I is the hydraulic gradient. Hydraulic conductivity (gpd /ff� 109 106 10' 100 10' 10 1 10-' 10-' 10-3 to-. 10 -5 10-6 10-1 10 -, Fine to coarse gravel Fuze to coarse sand sit, loess Glacial to Unweathered mane clay Shale Unfractured igneous and metamorptuc rocks Sandstone. wel cemented. unlonted Limestone, unffointed crystalline Tuff Sandstone, fnable Fractured gneous AM matamrrNur mrkc Vesicular basaff Karst limestone 106 10' 100 109 10 1 10-' 10-2 10-3 10-' 10 -5 10-6 10) 10 6 10 -9 10 10 Hydraulic conductivity (m /day) Figure 5.14. Typical K values for consolidated and unconsolidated aquifers. (After Davis, 1969; Dunn and Leopold, 1978; Freeze and Cherry, 1979).