Chromate Transport and Retention in Variably Saturated Soil Columns

Most subsurface contamination passes through the unsaturated zone before reaching an aquifer; however, transport studies are often conducted under saturated conditions because of the difficulty in maintaining steady-state flow. Chromate migration was measured in coarse-textured, oxide-rich sediment under different water contents using vacuum and centrifuge techniques to obtain a steady-state unsaturated flow regime. Leaching solutions contained 0.5 or 1.0 m M Cr(VI) and tritium in artificial groundwater. Breakthrough curves (BTCs) were modeled using CXTFIT assuming equilibrium conditions, since evaluation of data using a “two-region” physical nonequilibrium model indicated that mobile water was >90% regardless of saturation level. Dispersivity increased nonlinearly with decreasing water content. Retardation ( R ) increased with decreasing water content, but water content had little effect on the distribution coefficient calculated from R , K d-app . The average K d-app of all Cr(VI) experiments (water content range: 0.07–0.43 cm 3 cm −3 ) was 0.633 mL g −1 , very similar to the distribution coefficient derived from batch equilibration, K d (0.684 mL g −1 ). Though results in both transport systems were similar, average solute residence times in the vacuum system were 4 to 23 times longer than in the centrifuge system at comparable water contents. The centrifuge system column experiments could also be run over a greater range in volumetric water content (0.07–0.42 cm 3 cm −3 ) than the vacuum column system (0.23–0.43 cm 3 cm −3 ).