Arsenic releasing mechanisms during clayey sediments compaction: An experiment study

Groundwater with low levels of arsenic (As) in deep aquifers has been overexploited for decades in many regions (such as South Asian and American countries), resulting in the compaction of clayey aquitards and the release of pore water with As into deeper aquifers. However, the release mechanism of arsenic during clayey aquitards compaction is poorly understood. An indoor compaction experiment using muddy sediments was conducted to identify the As-releasing mechanisms during clayey aquitards compaction. The chemical characteristics and As species in pore water and sediment samples collected at different stages during the compaction experiment were analyzed. Initially, the reductive dissolution of iron oxides was a key process controlling As release during compaction. With increased pressure, As desorption from Fe (hydr)oxides became more important than the reductive dissolution as the main driver for As release. At the end of the compaction, the release of As was weak and the dominant process was desorption of As from clay or carbonate minerals. Our estimate result in the Jianghan Plain suggested that As concentration release from aquitard compaction by overexploited into groundwater would be 9.33-118.09 mu g/L.

Automated summary

Groundwater with low levels of arsenic in deep aquifers has been overexploited, leading to the compaction of clayey aquitards and the release of arsenic through processes such as reductive dissolution of iron oxides and desorption from Fe (hydr)oxides. The dominant release mechanism of arsenic during clayey aquitards compaction shifts from reductive dissolution to desorption as pressure increases, with eventual weak release from clay or carbonate minerals. Our estimate suggests that arsenic concentration released from aquitard compaction into groundwater ranges from 9.33 to 118.09 μg/L in the Jianghan Plain.