Multivariate statistical analyses on the enrichment of arsenic with different oxidation states in the Quaternary sediments of the Pearl River Delta, China

• Contents of As(III) and As(V) were probably directly related to a terrestrial source.
• There were similar enrichment behaviors among As(III), As(V), V, Cu and Ni.
• As(III) showed significant correlations with Fe2O3 in the sediments.
• TOC and Mn-bearing minerals were not correlated with any oxidation forms of As.
• Strong paleo hydrodynamic conditions seemed to favor the enrichment of As.

Core sediments of boreholes from a Quaternary aquifer–aquitard system of the Pearl River Delta (PRD), China, were collected for the geochemical, particle size and X-ray absorption near-edge spectroscopy analyses. Multivariate statistical techniques of Pearson's correlation analysis and R-mode factor analysis were applied to the geochemical data (TOC, SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O, P2O5, V, Cu, Ni, Cr, Ba, Zn, Sr, Y, Zr and Pb), particle size (ɸ) and solid arsenic contents with different oxidation states (As(− 1), As(III) and As(V)) of the bulk sediments. The results demonstrated that the existence and contents of As(III) and As(V) were dominantly directly related to the terrestrial detrital sources. As(III) showed significant correlation with Fe2O3, but correlation between As(V) and Fe2O3 was insignificant. Aqueous arsenic mainly existed in the form of As(III) as indicated by Eh–pH diagram. As(III) was speculated to exist on amorphous iron oxide. Therefore, this suggested that arsenic in groundwater had the same oxidation state with arsenic adsorbed on amorphous iron oxide. This conclusion was consistent with the previous conclusion that iron oxyhydroxide reduction was an important mechanism to mobilize arsenic in the aquifer–aquitard system. Significant correlations between oxidation forms of arsenic (As(III) and As(V)) and trace elements V, Cu and Ni suggested similar enrichment behaviors among them. Sedimentary organic matter was not correlated with any oxidation states of solid arsenic. Factor analysis indicated that good hydrodynamic conditions of the fluvial dominated sedimentary environment seemed to favor the enrichment of As(III) and As(V). Reduced form of arsenic in the sediments most likely existed in authigenic pyrite in the sediments.