Leaching characteristics of naturally derived toxic elements in the alluvial marine clay layer beneath Osaka Plain, Japan: implications for the reuse of excavated soils

Abstract

The contamination risks by naturally derived toxic elements must be assessed to achieve a sustainable geo-environment when utilizing excavated surplus soils. To estimate the controlling factors and risks of groundwater pollution associating with the application of recycled excavated surplus soils, the sequentially extracted fractions of major and toxic elements were analyzed and compared to the results of the simple batch leaching test. The concentrations of bulk B and Pb of the Holocene marine clay layer Ma13 were the maximum 75 ppm and 28 ppm at the middle depth and varied similar to the change of clay fraction, while the bulk As concentration was the maximum 12 ppm at the upper part of the Ma13. The B adsorbed onto the clay minerals was easily desorbed under the neutral pH condition. Arsenic was released especially from the transitional sandy silt layers at the upper and lower parts of Ma13 where contacting with oxic groundwater. The 0.45-µm filter required by Japanese regulations does not efficiently remove colloidal particles resulting in poor reproducibility of batch leaching tests, especially for Pb. Also, relative indices of metal mobility suggest that the long-term risk of groundwater contamination via the reuse of excavated surplus soils will not be accurately estimated only by the simple batch leaching test. The change of redox and pH conditions associating with relocation and preservation must be considered to fully evaluate the risk of toxic element mobilization of the excavated surplus soils.