Comparison of instream methods for measuring hydraulic conductivity in sandy streambeds

Streambed hydraulic conductivity (K) values were determined at seven stream transects in the Platte River Basin in Nebraska using different instream m easurement techniques. Values were compared to determine the most appropria te technique(s) for use in sandy streambeds. Values of K determined from fi eld falling- and constant-head permeameter tests analyzed using the Darcy e quation decreased as permeameter diameter increased. Seepage meters coupled with hydraulic gradient measurements failed to yield K values in 40% of th e trials. Consequently, Darcy permeameter and seepage meter tests were not preferred approaches. In the upper 0.25 m of the streambed, field falling- and constant-head permeameter tests analyzed with the Hvorslev solution gen erally had similar K values that were significantly greater than those dete rmined using the Hazen grain-size, Bouwer and Rice slug test for anisotropi c and isotropic conditions, and Alyamani and Sen grain-size methods; median differences between these tests and the Hvorslev falling-head 60 cm diamet er permeameter were about 8, 9, 17, and 35 m/day, respectively. The Hvorsle v falling-head permeameter test is considered the most robust method for me asuring K of the upper 0.25 m of the streambed because of the inherent limi tations of the empirical grain-size methods and less sediment disturbance f or permeameter than slug tests. However, lateral variability in K along tra nsects on the Platte, North Platte, and Wood Rivers was greater than variab ility in K between valid permeameter, grain-size, or slug tests, indicating that the method used may matter less than making enough measurements to ch aracterize spatial variability adequately. At the Platte River tributary si tes, the upper 0.3 m of the streambed typically had greater K than sediment located 0.3 to 2.5 m below the streambed surface, indicating that deposits below the streambed may limit ground water/surface water fluxes. The Hvors lev permeameter tests are not a practical measurement approach for these gr eater depths. Thus, selection of a method for measuring streambed K needs t o consider the vertical location of the sediments that are most likely to l imit the rate of ground water/surface water interaction.