Chromate Transport and Retention in Variably Saturated Soil Columns

Abstract

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 mM 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, Kd-app The average Kd-app of all Cr(VI) experiments (water content range: 0.07–0.43 cm3 cm−3) was 0.633 mL g−1, very similar to the distribution coefficient derived from batch equilibration, Kd (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 cm3 cm−3) than the vacuum column system (0.23–0.43 cm3 cm−3).