Impacts of sediment compaction on iodine enrichment in deep aquifers of the North China Plain

Abstract

To understand the effects of large-scale sediment compaction on iodine enrichment in the groundwater system of the North China Plain (NCP), hydrogeochemical and isotope (2H, 18O and 87Sr/86Sr) geochemical features of groundwater, seawater and pore water of the deep aquifer sediments were characterized. Groundwater iodine concentration ranged from 5.8 to 1110 μg/L and approximately 80% of groundwater samples have iodine concentration more than 100 μg/L. High iodine groundwater (>100 μg/L) is mainly distributed in the Bohai bay area where serious land subsidence occurs. The sediments deposited during several events of marine transgression are considered as the main source of groundwater iodine. The pore solution compacted from the clayey sediments has a iodine concentration up to 830 μg/L, indicating that marine source iodine was stored in the clayey layers. The hydrogen and oxygen isotope signatures of groundwater show that the groundwater from NCP is mainly recharged by precipitation and the pore water from sediment compaction due to intense land subsidence. The Cl/Br molar ratio and 87Sr/86Sr signatures of groundwater and sediment pore water further suggest the effects of mixing with compaction-released pore solution on the hydrochemical evolution of high iodine groundwater. Based on groundwater δ2H, Cl concentration and 87Sr/86Sr, the results of two end-members mixing model calculation indicate that the compaction-released pore solution contributes approximately 49.1–68.9% iodine to high iodine groundwater, and the iodine from compaction-released pore solution ranges between 407 and 572 μg/L. The results of this study therefore suggest that the compaction of clayey sediments can be a major hydrogeological process controlling the genesis of high iodine groundwater in deep aquifers at NCP.