The U.S. Geological Survey's Amargosa Tracer Calibration Site in southern Nevada has been used for three different recirculating tracer tests using as tracers: (1) tritium, (2) sulfur‐35, and (3) tritium and bromide together. Although the physical setup, well spacings, and thus apparent scale were the same in all tests, the recirculating rates and pore pressures were different. Apparent dispersivities found in the three tests differed considerably, revealing an inverse relationship between computed dispersivity and recirculation rate. These differences are believed to be caused primarily by changes in fissureaperture widths and thus, hydraulic conductivities, with resulting changes in flow rates and directions caused by changes in pore pressure between different tests in the presence of high ambient pore pressure. The results of this study indicate that forced‐gradient tracer tests for determination of dispersivity should not be performed when ambient pore pressure and/or testing pore pressure is a significant percentage of overburden pressure. In addition, apparent dispersivity calculated by forced‐gradient tracer tests can differ considerably from that of unstressed natural situations, and consequent solute‐transport estimates are likely to be in error.
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