Arsenic releasing characteristics during the compaction of muddy sediments

Abstract

Muddy sediments are abundant in pore water and capable of preserving a large amount of chemicals, such as arsenic. Muddy sediments would transform into aquicludes or aquitards during long-term compaction and burial. It remains unclear whether the release of arsenic from muddy sediments poses a potential contamination risk to groundwater in the adjacent aquifer. An indoor compaction simulation experiment was conducted, coupled with an investigation on vertical geochemical profiles of muddy sediments in one actual borehole. In this experiment, aqueous arsenic in released pore water ranged from 17.5 to 21.3 μg L⁻¹ and the accumulated content of the released arsenic was 17.576 μg during the compaction. As(III) was the main As species in released pore water and had good correlations with Fe²⁺ and Mn. The analysis of the solid phase showed a remarkable depletion of Fe–Mn oxide bound arsenic during the compaction. In the profiles of the actual borehole, the contents of Fe–Mn oxide bound arsenic also exhibited a gradual decreasing trend from shallow to deep. Based on both the indoor experiment and the field profile, it can be concluded that the reductive dissolution of Fe–Mn oxides took place in arsenic-rich muddy sediments and Fe–Mn oxide-bound arsenic transformed into soluble arsenic, then soluble arsenic was released into the adjacent aquifer along with the pore water in the long-term compaction and burial.