Leaching of copper, chromium, and arsenic through some free-draining New Zealand soils

Concentrated toxic solutions of copper, chromium, and arsenic (CCA) are used extensively in the New Zealand timber preservation industry. A 2% w/v solution containing cupric, dichromate, and arsenate ions was leached through undisturbed soil monolith lysimeters containing the surface and subsurface horizons of 2 free-draining New Zealand soils. The resulting breakthrough curves were successfully modelled using the Gompertz equation for biomass growth Differences between soil horizons in the fitted Gompertz parameter values were related to differences in soil physical and chemical properties affecting leaching. Results were compared to those from a previous study investigating pulse leaching of a concentrated CCA solution of similar ion ratio using the same soils. Generally, solution leaching produced breakthrough curves influenced more by the soil's physical structure and less by kinetic processes (i.e. sorption and diffusion). This was in part due to the high metal concentrations of the CCA solution and the decreased contact time between soil and solute in the solution leaching experiment. The relative ease of solute ion breakthrough increased in the order copper < arsenate < dichromate. The study suggests that further research is required to assess management implications for spills of these heavy metal ions to soils to prevent leaching or runoff to ground and surface water.