Slope Effects on Cadmium Load of Eroded Sediments and Runoff Water

Little information is available on the effects of slope on Cd load of runoff water emanating from the Cd-contaminated soils. A laboratory rainfall simulator study was conducted to assess the effects of slope on runoff, sediment yield, and Cd load of runoff water and eroded soil sediments. The experiment was conducted with three Cd-amended soils (20 mg kg⁻¹) of varying degrees of erodibility, i.e., Msinga clay loam (Paleudalf, low erodibility), Jozini sandy loam (Haploustalf, moderate erodibility), and Mngazi clay (Pelloxerert, high erodibility), using four slopes (10, 20, 30, and 40%) and three replicates. A Cd-adsorption experiment and sequential extraction techniques were employed to characterize Cd soil affinity and Cd partitioning in soil. Cadmium runoff losses varied from 4.6 to 58.5% of the applied Cd. Sediment Cd accounted for over 93% of the total Cd load of the runoff slurry. The Cd load of runoff increased with an increasing degree of soil erodibility, reflecting the effects of increased removal of a Cd-contaminated surface soil layer. With increasing slope, sediment yield and sediment-associated Cd increased. Slope did not have a significant effect on the soluble Cd load of runoff water. In the Jozini soil, an increase in slope resulted in a sharp decrease in the clay and Cd enrichment ratios (the ratios between clay and Cd contents of the sediment and their respective contents in the soil of origin). In the other two soils enrichment ratios were unaffected by slope. Soluble Cd load of runoff water was less than 28% of the mobile (extractable with 1 M NH₄NO₃) Cd content of the soils. Differences in soluble Cd load could not be accounted for by the differences in the soils' Cd sorption characteristics or their mobile Cd content, suggesting that these parameters are inappropriate for estimating the extent of soluble Cd losses from the soils.