Linkage of sulfur isotopic enrichment to sulfur and arsenic release in the coastal aquifers of southwestern Taiwan

High arsenic concentration in groundwater is influenced by redox reactions in As-bearing iron (oxy)hydroxide minerals and elevated total organic carbon, resulting in arsenic contamination and enrichment in groundwater after introduction of seawater into an aquifer. In this study, hydrochemical and isotopic (S, O and C) techniques were used to relate arsenic contamination and seawater flooding/intrusion in groundwater aquifers in the Chianan Plain, southwestern Taiwan. Thirty-three samples (30 from groundwater and 3 from river water) were collected from April-June of 2014. Most of the groundwater samples collected from the Chianan Plain were dominated by As(III), indicating that reducing conditions prevailed within the aquifers. The delta O-18-delta D plots of most of the groundwater samples from the Chianan Plain are generally lined on the global meteoric water line (GMWL), but the delta O-18 and delta D isotopic compositions of the river water samples show enriched signals. Salinity and sulfate concentrations of river water and shallow groundwater have been found to increase after seawater flooding/intrusion. The delta O-18 and dD isotopic compositions of the river water showed a gradual trend toward those of seawater. The trend in the isotopic compositions of the river and groundwater samples indicated a mixing of seawater and meteoric water. Up to the present, the groundwater levels in the coastal areas of the Chianan Plain have dropped to zero meters in elevation parallel to sea level due to excessive groundwater withdrawal, resulting in seawater intrusion into the aquifers in Budai, Yichu, and Beimen in the Plain. Moreover, high salinity (48.1 parts per thousand) occurred only in the Beimen 2B shallow groundwater at a depth of 60 m is a result of seawater flooding infiltration from the nearby salinized Pachang River. The delta S-34[SO4]/delta O-18[SO4] isotopic ratios (2.63-2.82) of low-As (concentration < 50 mu g/L) river water samples were similar to those of marine sulfate minerals dissolved in seawater, but were variable (0 - 2.93) among the high-As (concentration >= 50 mu g/L) groundwater samples from the Chianan Plain. Sulfates in seawater were introduced into the coastal aquifers of the Chianan Plain under the Holocene marine transgression, wherein the mixing of paleo- and modern-sulfates due to seawater intrusion might have occurred or are occurring. The release of sulfur and arsenic (mostly arsenite) in the Chianan Plain groundwater under reducing environments indicating redox reactions in sulfates and iron (oxy)hydroxide minerals may change the isotopic (S and O in sulfate) compositions in groundwater, resulting in enrichment of delta S-34[SO4] and delta O-18[SO4]. Salinity can promote arsenic release in groundwater and thus enrich the delta S-34[SO4] and delta O-18[SO4] isotopic compositions, in which 6.0-22.8 parts per thousand delta S-34[SO4] and 9.3-15.5 parts per thousand delta O-18[SO4]occur in shallow fresh groundwater (depth < 60 m) but up to 27.7 parts per thousand delta S-34[SO4] and 20.4 parts per thousand delta O-18[SO4] occur in shallow saline groundwater, in comparison to 0 parts per thousand delta S-34[SO4] and 5.4-6.5 parts per thousand delta O-18[SO4] in deep fresh groundwater (depth between 60 and 318 m) and up to 38. 5 parts per thousand delta S-34[SO4] and 18.7 parts per thousand delta O-18[SO4] in deep brackish groundwater. This study provides insights suggesting that the As-containing iron(II) minerals are oxidized to iron(III) minerals and then re-reduced to iron(II) which leads to As release.