Measuring heavy metal migration rates in a low-permeability soil

Abstract

Heavy metals at high concentrations are often toxic to living organisms, and their environmental toxicity depends on soil properties. It has long been thought that in clay-rich, low-permeability soils, heavy metals are bound to soil particles, and thus there are only few toxicity risks. This study questioned this perception and tested heavy metal mobility in such a soil, of the London Clay series, using a benchtop centrifuge. Soil columns were placed in the centrifuge and were infiltrated with solutions of Cu, Ni and Zn ions, while the centrifuge was running at three different gravity levels, at 5,280, 2,600 and 1,300 gravities. The measured rates of migration of Cu, Ni and Zn ions were extrapolated down to 1 gravity, which represents field conditions, the conditions for which an assessment of risk due to metal toxicity would be needed. It was found that heavy metal movement was significant in London Clay, Ni being the most mobile metal in the study, followed by Zn and then Cu ions. Centrifuge infiltration tests were proven to be a valuable tool in the study and quantification of metal mobility in low-permeability soil, because they were easy to run and precise in predicting metal movement in London Clay.