Tracing Subsurface Flow on Roadcuts on Steep, Forested Slopes1
- Walter F. Megahan and
- James L. Clayton2
In situ hydraulic conductivity was determined in a forested soil in the Idaho batholith by a tracer technique, and values were compared to hydraulic conductivity determined in the laboratory on cored samples. We injected NaCl solutions into piezometers placed in soil above a roadcut, and determined time for Na+ or Cl- to appear in outflow at the roadcut during saturated subsurface flow generated from spring snowmelt. Hydraulic conductivity values determined with the NaCl tracer averaged an order of magnitude greater than laboratory derived values. This is likely due to rapid flow in macropores formed from root channels or other biological activity that cannot be sampled by conventional coring and laboratory conductivity tests. Although the field tests indicated flow was clearly non homogeneous and anisotropic, an evaluation of the Reynold's number for the test conditions indicated Darcy flow was occurring. Sodium analysis provided a more distinct peak than Cl- when both concentrations were plotted vs. time.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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