Impact of clayey sediment compaction on pore water evolution and the release of iron

Abstract

The groundwater contaminants caused by the clayey sediment compaction have been reported in numerous groundwater overextraction regions. But little is known about the impact of compaction on the evolution of pore water compacted from the clayey sediments. In this study, we collected clayey sediments and conducted a low-pressure indoor experiment to identify the pore water evolution and iron releasing mechanism. Pressure demonstrates a significant influence on the release and mobilization of iron in pore water and clayey sediments. Initially, Fe2+ concentration depletion in pore water is regulated by the reduction of Fe(III) (oxy) hydroxides and precipitation with S(-II) producing Fe–S phases. Subsequently, the increase of Fe2+ with the increased pressure is attributed to the transformation of FeS to FeS2. In the final stage of the pressure experiment, more formation of FeS2 and dissolution of siderite result in the fluctuation of Fe. In addition, a release of the trace metals (Fe, Mn, Cr, Cu, Ni and Zn) detected in pore water can be what creates a potential contaminant threat in the adjacent aquifers. The compaction of the clayey sediments results in a decrease of the water content and releases the pore water into the adjacent aquifers. This study reveals the pore water evolution, highlights the complex biogeochemical reactions of iron during clayey sediments compaction and reinforces the importance of clayey sediments compaction for the aquifers water quality.