Comparison of vacuum and centrifuge-based techniques for evaluating solute transport processes under unsaturated conditions
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Most subsurface contamination passes through the unsaturated zone before reaching an aquifer; however, many transport studies are conducted under saturated conditions that may not approximate the natural system. Chromate migration was measured in sediment from the Savannah River Site, SC under different water contents using vacuum and centrifuge techniques to obtain a steady-state flow regime. Leaching solutions contained 0.5 or 1.0 mM Cr(VI) and tritium in artificial groundwater. Breakthrough curves were modeled using CXTFIT assuming equilibrium conditions. Dispersion, as indicated by Peclet number, increased with decreasing water content. Retardation increased with decreasing water content with no trend evident when Kd was calculated from R. Average Kd of all Cr(VI) experiments was 0.551 mL.... g -1 , similar to Kd derived from batch equilibration (0.599 mL.... g -1 ). Though results in both systems were similar, experiment duration in the vacuum system was 4 to 23 times longer than in the centrifuge system at comparable water contents.