COLUMN-SCALE AND CATCHMENT-SCALE TRANSPORT OF CADMIUM - EFFECT OF DISSOLVED ORGANIC-MATTER

In the Ellen catchment on the Pinjarra Plain, NE of Perth in Western A ustralia, cadmium from fertilisers is starting to leach from soils. Ab out 70% of surface soils in the Ellen catchment are sandy and often on top of a shallow ephemeral water table. Adsorption of Cd in the sandy soils of the Ellen catchment was studied by batch adsorption and by l eaching small columns of soil, Adsorption of Cd increases linearly wit h increasing soil organic matter content and exponentially with increa sing pH. Cadmium is significantly mobilised in the sandy soils by diss olved organic matter. The capacity of most of the sandy soils in the E llen catchment to adsorb phosphate from fertiliser hhs been saturated. Resulting concentrations in Ellen Brook average similar to 500 mu g L (-1) P, Cadmium is adsorbed more strongly in the sandy soils than phos phate and is just starting to leach into Ellen Brook. From a compariso n of Cd/P ratios in water, soils and fertiliser, cadmium concentration s in Ellen Brook are estimated to be at 10-30% of their maximum for co mplete breakthrough from soils. Present concentrations of Cd in Ellen Brook average similar to 0.1 mu g L(-1) and are estimated to approach the maximum for complete breakthrough in similar to 100 yr, Maximum Cd concentrations in Ellen Brook could range from 0.6 to 2 mu g L(-1), d epending on rates of input with fertiliser and future increases in agr icultural land use in the catchment. Breakthrough curves, resulting fr om leaching Cd through small columns of sandy soil, indicate that adso rption significantly increases the effective hydrodynamic dispersion o f Cd. Longitudinal dispersivities, measured at pore-water velocities o f 0.7-14 m day(-1), were similar to 5 cm for Cd and similar to 0.1-0.2 cm for chloride. The much greater dispersion of Cd in the sandy soils than of chloride is shown not to be caused by non-equilibrium adsorpt ion.