Hossai, and 3 co-authors, reported in 2019 that the leaching behavior of arsenic (As), lead (Pb), nickel (Ni), iron (Fe), and manganese (Mn) was investigated from subsurface core sediment of marine and nonmarine depositional environments in central Kanto Plain, Japan. A four-step sequential extraction technique was adopted to determine the chemical speciation, potential mobility, and bioavailability of metals under natural conditions in variable depositional environments. In addition, a correlation of these properties with pore water and total metal content was carried out. The concentration of As in pore water was found to be 2–3 times higher than the permissible limit (10 µg/L) for drinking water and leachate in fluvial, transitional, and marine environments.
The trend of potential mobile fractions of As, Pb, and Ni showed Fe–Mn oxide bound > carbonate bound > ion exchangeable bound > water soluble in the fluvial environment. However, in the marine environment, it showed Fe–Mn oxide bound > water soluble > carbonate bound > ion exchangeable bound for As. The leaching of As in this fluvial environment is due to the organic matter-mediated, reductive dissolution of Fe–Mn oxide bound, where Mn is the scavenger. The amount of total content of As and sulfur (S) in transitional sediment reflects an elevated level of leachate in pore water, which is controlled by S reduction. However, the leaching of As in marine sediment is controlled by pH and organic matter content.
The authors concluded that the leaching behavior of As, Pb, and Ni from soil/sediment is different, due to their individual chemical characteristics in response to various potentials in different depositional environments. Though the total contents of As and other metals were within the ranges of soil standards, the pore water concentration of As exceeded the permissible limit in some layers of the fluvial (aquifer), transitional, and marine environment. The chemical characteristics of the upper part of the fluvial sediment were influenced by the overlaying tuffaceous clay, and by oxidation. The leaching of As, Pb, and Fe in the fluvial environment was mainly controlled by the oxidation-reduction process of the Fe–Mn oxide bound percentage, which was influenced by the organic matter content. However, the leaching of As was relatively high in the transitional environment due to As enrichment, and was controlled by sulfur reduction. Mn played an important role, i.e., that of the scavenger, to leach As in the fluvial and transitional environment.
On the other hand, the leaching of As, Pb, and Ni in the marine environment was controlled by pH and organic matter content. The pore water As content in peat indicated poor leaching due to a higher amount of organic matter content. The overall risk of bioavailable percentage of all metals was less than 1%, except for Mn (RAC > 1%) in the peat layer; this indicates that there is no risk of these metals leaching into the subsurface environment. In the fluvial environment, the Mn content in the pore water was several times the secondary drinking water permissible limit; this indicated poor water quality in terms of Mn in the shallow aquifer. There was no/low risk of pollution from these metals in the subsurface environment, as most of the metals are bound to residual and complex binding processes with organic matter. However, a change of oxidation/reduction or acidic conditions may leach considerable amounts of As to the environment from the peat, transitional, and marine environments. Special measures should be taken if these sediments are excavated for underground development.
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