Enrichment and mechanisms of heavy metal mobility in a coastal quaternary groundwater system of the Pearl River Delta, China

The risks posed by heavy metal mobilization strongly depend on the pathways that the metals follow, with the sediment-water pathway representing a direct risk to groundwater contamination. Monitoring and sequential extraction experiments in the laboratory generally have limitations with respect to understanding the mechanisms of heavy metal mobilization in the field. The Quaternary coastal groundwater system of the Pearl River Delta, China was chosen as the study area to understand heavy metal enrichment and mobility. Heavy metals including V, Cr, Co, Ni, Cu, Zn, Ba, Pb, Mo, Cd, Sr, Ga, Ge, Rb, and Cs in both sediments and groundwater were analyzed. Geochemical parameters including Fe2O3, MnO, sedimentary organic matter, and carbonate content as well as hydrochemical parameters including K+, Na+, Ca2+, Mg2+, NH4+, SO42−, Cl−, HCO3−, pH, TDS, and dissolved organic carbon were also measured. The enrichment of heavy metals in the solid sediment phase as well as the mobilization mechanisms of heavy metals in groundwater are discussed as informed by Pearson's correlation analysis. Hydrochemical analyses demonstrated that the mobility of V, Ba, Cr, Rb, and Cs is closely related to the decomposition of buried sedimentary organic matter; the mobility of Co, Ni, Cu, Zn, Pb, and Cd is closely linked with the reductive dissolution of Fe–Mn oxides; and the mobility of Co, Ni, Cu, Ba, Zn, Pb, Cd, Mn, Sr and Ga is probably controlled by ion exchange processes. This study demonstrates that heavy metal mobility in the field is not entirely consistent with the potential mobility as indicated by sediment analysis, due to the complicated hydrogeochemical conditions in the groundwater system, and suggests that comprehensive geochemical and hydrochemical studies are useful ways to understand the mobility mechanisms of heavy metals in the field. [Display omitted] •Heavy metals enriched in sedimentary organic matter, carbonate and Fe-Mn oxides.•Heavy metal mobility is significantly influenced by hydrogeochemical conditions.•Mobility of V, Ba, Cr, Rb and Cs is related to organic matter decomposition.•Mobility of Co, Ni, Cu, Zn, Pb and Cd is due to reducing dissolution of Fe-Mn oxides.•Mobility of Co, Ni, Cu, Ba, Zn, Pb, Cd, Mn, Sr and Ga may be due to ion exchange.