Soil structure and flow rate-controlled molybdate, arsenate and chromium(III) transport through field columns

Preferential flow in soil causes rapid movement of chemicals and contamination at the tile-line level. Soil structure and flow rate effect on transport velocity of MoO42−, H2AsO4−1, and Cr+ 3 was assessed for three soils differing in vadose structure. Triplicate, 50 cm long, 35 cm diameter field soil columns of Lamellic Hapludalf, Typic Udorthent, Typic Hapludalf from southwestern Germany were obtained. The metals solute prepared in 3.5 mM LiCl was applied as pulse, and flushed with synthetic rainwater of pH 5.5 at constant flow rates generated by + 10, −10, and −40 mm water heads at the column surface. The tracers' breakthrough data were fitted to a convective–dispersive flow equation with mobile and immobile soil water partition. Additionally, blue dye was rained on site, and X-ray tomography of one representative column from each soil was carried out to identify preferential flow paths in the field soils. Lamellic Hapludalf (sandy loam) had a dominantly massive structure, and blue dye flowed as vertical fingers. Typic Udorthent (silt loam) had well-connected macropores, and dye moved as macropore flow with radial diffusion. Typic Hapludalf (silty clay) had well-connected pores throughout the profile, and dye moved the deepest with smallest dyed area. Chloride moved sampling 0.30 to 0.33 fraction of porosity in the massive soil irrespective of flow rate; and depending on flow rate, it moved through only 0.05 to 0.15 fraction of porosity in the structured silt loam/silty clay soil columns. MoO42−, H2AsO4−1 and Cr+ 3 leached as fast as Cl−1 under ponded flow and with large retardation under unsaturated flow suggesting macropore flow of the adsorbing solute in the well-structured soils. The massive soil effectively retained H2AsO4−1 and Cr+ 3 but allowed MoO4−2 to leach at all the flow rates. Retardations of the absorbing tracers were in the order of Cr+ 3 > H2AsO4−1 > MoO4−2 > Li+ 1. Depending on soil structure, the effective porosity and retardation increased, and dispersion decreased with reduced flow rate. The study concludes that strongly graded, coarse to medium angular/prismatic soil peds allow adsorbing solute to leach at reduced flow rate, and the field description of soil structure helps discern areas where preferential flow can potentially cause subsoil contamination.

Research Highlights
► We investigated the effect of soil structure and flow rate on breakthrough of metals at near-saturation using replicated intact columns from three field soils.
► Blue dye application marked parallel vertical fingers in a massive loamy sand and macropores in well structured silty clay/clay loam soils.
► CT-Scan of intact soil columns revealed fairly well-connected pores in the silty clay soils, and massive structure of the sand soil.
► Massive sandy soil had preferential flow of Chloride, and flow rate had no effect on the transport of parameters. The well structured silty clay soils exhibited macropore flow at saturated flow; but as flow rate decreased, breakthrough was delayed and peak concentration was reduced due to greater interaction with soil matrix.
► The massive loamy sand effectively retained H2AsO4−1 and Chromium(III) but allowed MoO4−2 breakthrough. Molybdate, H2AsO4−1 and Chromium(III) arrived with macropore flow through the well-structured silty clay/clay loam soils under saturated flow although percolate concentration was generally low.
► Molybdate and H2AsO4−1, both as anions, had different breakthrough pattern and transport parameters, which varied with soil structure and percolate rate. The mobility of metals was in the order MoO4−2 > H2AsO4−1 > Chromium(III).
► The study concludes that field description of soil structure may help identify potential sites for the metals' transport at tile-line level, especially in area with intense precipitation.
► Importance of column experiments to study preferential flow for a comprehensive assessment of retardation capacity of various elements under field conditions was highlighted.