Pumping-accelerated soft ground compaction triggers iodine enrichment in saline groundwater of coastal plains

Accelerated land subsidence within coastal plains due to overexploitation of saline groundwater for aquaculture triggers groundwater quality deterioration by abnormally high iodine (I). The underlying processes of I enrichment and mechanisms driving evolution of I-contaminated groundwater, however, remain elusive. By combining hydrogeological-biogeochemical investigations in Pearl River Delta with SBAS-InSAR remote sensing and ultrahigh-resolution organic molecular characterization, this research presents new evidence that soft clayey sediments compaction and dissolved organic matter (DOM) biotransformation play synergistic role in I transport and subsequent enrichment in saline groundwater. On the regional scale, groundwater I hotspots correlated significantly with subsiding centers and surrounding haloes. Geochemically-active I preferentially accumulates in compressible, organic-rich clay layers. Porewater release upon clay compaction leads to I mobilization, with DOM acting as a key transporter, into underlying groundwater. The high-salinity, reducing groundwater environments favor I enrichment by facilitating biotransformation of DOM-I complexes into highly mobile I^- and organic I compounds. The joint migration of biodegradable DOM and I following soft ground compaction establishes previously overlooked feedbacks between geomechanical deformation and genesis of I-contaminated groundwater in sinking coastal regions. These results highlight the critical role of land subsidence in exacerbating groundwater health hazards and alert imperative need for sustainable groundwater quality management.