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2. Permeameter Tests to Quantify Streambed Hydraulic Conductivity <br />Two primary surface water /groundwater interactions exist at Tamarack: (1) <br />between the South Platte River and alluvial aquifer and (2) between backwater sloughs <br />and the alluvial aquifer. Each interaction is investigated using falling -head permeameter <br />tests to estimate the hydraulic conductivity of the streambed and sloughbed. Several field <br />techniques have been used to estimate the streambed hydraulic conductivity (Landon et <br />al., 2001; Calver, 2001). These techniques include grain -size analyses of streambed <br />samples, slug tests, in situ permeameter tests, and seepage flux measurements with <br />seepage meters. Landon et al. (2001) compare these techniques for estimating the <br />streambed hydraulic conductivity in sandy streambeds in the Platte River in Nebraska. <br />They conclude that field permeameter tests are most advantageous for determining the <br />streambed conductivity in the upper 0.25 in of the streambed. They also note that spatial <br />variability between stream transects is greater than variability in measured streambed <br />conductivity between different techniques. <br />Permeameter tests generally involve the use of a pipe that is pushed partially into the <br />streambed. Water is added to this pipe to induce a hydraulic head on the sediments inside <br />the pipe. In a constant head permeameter test, a target displacement above the water <br />level in the stream is reached. This displacement is maintained over a given time period <br />by adding a known volume of water. In a falling head permeameter test (Figure 2), the <br />water level inside the pipe. is allowed to fall while the displacement is measured. Vertical <br />streambed hydraulic conductivity can be calculated for the falling -head permeameter tests <br />using an application of Darcy's equation or the Hvorslev (195 1) equation. Application of <br />Darcy's equation for a falling -head permeameter yields <br />Ksb = L In Ho (1) <br />t, — to (HI) <br />where Kb is the vertical, streambed hydraulic conductivity, L is the sediment interval <br />being tested, tl -to is the elapsed time, Ho is the initial displacement, and Hl is the <br />displacement at a later time. The Hvorslev (195 1) solution is similar to the Darcy's <br />equation in that it assumes uniform sediment within and below the permeameter, but also <br />accounts for anisotropic conditions: <br />- D +L <br />Ksb _ 11 m In Ho (2) <br />t, —to (HI <br />where D is the diameter of the permeameter and in is the isotropic transformation ratio, <br />Kb , where Kh is the horizontal, streambed hydraulic conductivity. <br />/sb <br />3 <br />