Heavy metal leaching in some free-draining Canterbury soils

An ion sorption and lysimeter leaching study was conducted on the surface and sub-surface horizons of a Selwyn and a Templeton soil. Three heavy metals (HM's), copper, chromium and arsenic were examined using solutions containing cupric (Cu²⁺), dichromate (Cr₂O₇²⁻), and arsenate (AsO₄³⁻) ions (CCA solution). These HM's ions are used extensively in the timber preservation industry. The sorption studies were conducted over a range of pH values using a 10:1 ratio of solution to soil. The ions of interest were added to the soils separately and together, in a background electrolyte and equilibrated for 24 hours by end-over-end shaking. A wide range of sorption strength was observed in the sorption studies between soils and HM ions over the range of pH values. The sorption strength of the three ions in all soil horizons generally increased in the following order: Cr₂O₇²⁻ < H₂AsO₄⁻/HAsO₄²⁻ < Cu²⁺ Copper and dichromate ions were more highly sorbed by the Templeton A horizon while arsenate was more highly sorbed by the Templeton B horizon. Dichromate and arsenate ions were sorbed least by the Selwyn C horizon. While copper was generally sorbed in greater amounts than both dichromate and arsenate ions, sorption was highly pH-dependant. A number of small undisturbed soil lysimeters (6.8 cm diameter by 12 cm deep) were extracted from an 18 month pasture (Templeton soil) and a non-agricultural grassland (Selwyn) from surface and sub-surface horizons and removed to the laboratory. The edge of each lysimeter was sealed to prevent preferential water flow. Two lysimeter leaching experiments using saturated-flow conditions were conducted. The first observed the leaching behaviour of the three HM ions using a 2% w/v CCA solution. The second experiment comprised two main parts. The first part examined the leaching of a pulse of applied CCA compounds in solution to the surface of a core, followed by leaching 18-20 hours later. The second part of the experiment repeated the same procedure with only the Templeton A and B horizons but stored the lysimeter cores for increasing periods of time before commencing leaching. Breakthrough curves (BTC's) plotted from the leaching experiments showed solute peak breakthrough occurred consistently early for lysimeter cores for both experiments and was attributed to preferential leaching of solutes. Quantities of HM's leached from each experiment depended to some extent on the length of time the soil was in contact with solution ions before leaching commenced. Chemical sorption data became increasingly more important the longer the soil remained in contact with the solute ions. Breakthrough curves were described for HM leaching using the non-linear regression Gompertz model. Gompertz parameter values were used to test for differences between amounts (or maximum concentrations) of leached HM's as well as different soil physical and chemical features affecting leaching. High concentrations of solutes found in some leachates have important implications for health and environmental issues related to the commercial use of CCA compounds. The accuracy of prediction of HM leaching was tested by comparing effluent content of HM's measured in the pulse leaching experiment with that predicted by the Rose model.