Biogeochemical cycling of arsenic in coastal salinized aquifers: evidence from sulfur isotope study

As-contamination of groundwater, accompanied by critical salinilization, occurs in the coastal area of the Choushui river alluvial fan and Lanyang Plain, Taiwan. Statistical analyses and geochemical calculations indicate that a possible source of aqueous arsenic is the reductive dissolution of As-bearing iron oxyhydroxides. There are few reports of the influence of sulfate-sulfide redox cycling on arsenic mobility in brackish groundwater. This study evaluated the contribution of sulfate reduction and sulfide re-oxidation on As enrichment using δ34S[SO4] and δ18O[SO4] sulfur isotopic analyses of groundwater. In the Choushui river alluvial fan, fifty-three groundwater samples were divided into groups of high-As content and salinized (Type A), low-As and non-salinized (Type B), and high-As and non-salinized (Type C) groundwater, based on hydro-geochemical analysis. High18O enrichment factor (ε[SO4-H2O]) and 34S enrichment factor (ε34S[FeS2-SO4]) indicated that the disproportionation and dissimilatory sulfate reduction were both involved in the Type A groundwater. Sulfur disproportionation is an important process during the reductive dissolution of As-containing iron oxyhydroxides. In contrast to this, Type B and Type C groundwater samples showed high δ18O[SO4] and low δ34S[SO4] values under mildly reducing conditions. Based on 18O mass balance calculations, the oxygen sources of sulfate are from infiltrated atmospheric O2, caused by additional recharge of dissolved oxygen and sulfide re-oxidation. The anthropogenic influence of extensive pumping also promotes atmospheric oxygen entry into aquifers, altering redox conditions, and increasing the rate of As release into groundwater. For the Lanyang Plain, thirty-one groundwater samples are divided into three groups, which are similar with those of the Choushui river alluvial fan. High 18O enrichment factors and large range of sulfur isotope fractionations in Type A and Type C groundwater of Lanyang plain closely relate with the microbial disproportionation reactions and bacterial sulfate reduction. Samples of Type B groundwater are mostly located in mountainous recharge area, causing the mixing of different sources of sulfate. Based on the infiltration of marine brackish water from fish pond and affect of isolated brackish groundwater were responsible for groundwater salinization in Lanyang Plain. Hence, the highest δ34S[SO4] and δ18O[SO4] values caused by significant sulfate reduction of paleo-seawater in Type A groundwater of Lanyang Plain. Moreover, the results of geochemical simulations on the influence of various anthropogenic factors including mixing with non-saline natural water, organic-rich infiltration water, oxygenic rain water and sea water on As mobility in salinized groundwater agreed with the proposed biogeochemical cycle based on the sulfur isotopic analysis. The major As release mechanism by the reductive dissolution of As-Fe oxyhydroxide, accompanying with sulfate reduction, sulfide re-oxidation, carbon degradation and oxygen consumption, the extensive pumping, seawater intrusion and paleo-seawater residues govern As enrichment in saline groundwater of Choushui alluvial fan and Lanyang plain. Two conceptually models that summarized As cyclic redox reactions in the Choushui river alluvial fan and in the Lanyang plain were herein proposed to illustrate the complex interations of As in the saline and non-saline groundwater aquifers.