HYDROGEOCHEMICAL AND MULTIPLE ISOTOPIC INVESTIGATION OF ARSENIC MOBILIZATION RELATED TO OXIDATION OF IRON SULFIDES UNDER THE REDUCING ENVIRONMENT IN AS-RICH GROUNDWATER OF BLACKFOOT DISEASE AREA

High arsenic concentration in groundwater was influenced by redox reactions in As-bearing iron sulfide minerals and elevated total organic carbon, resulting in arsenic enrichment and contamination in groundwater. In this study, hydrogeochemical and multiple (O, S and C) isotope techniques were used to elaborate arsenic mobilization and enrichment in groundwater aquifers of Chianan plain, southwestern Taiwan, where Blackfoot disease occurred before 1990. Fifteen water samples (12 from groundwater and 3 from river water) were collected during April - June in 2014. Most of the groundwater samples collected from Chianan plain were dominated by As(III), indicating reducing conditions prevailed within the aquifers. The δ¹⁸O-δD plots of most groundwater samples in Chianan plain are generally lined on global meteoric water line (GMWL), but the values of δ¹⁸O and dD of river water approached those of seawater. The trend of isotopic compositions of river water and groundwater indicated a mixing of seawater and meteoric water, suggesting that the seawater encroachment was likely to be occurred. The δ¹⁸O_[SO4]/δ³⁴S_[SO4] ratio of low-As river water were similar to paleo-marine sulfate minerals, but no significant differences among high-As groundwater samples in Chianan plain was observed. paleo-marine sulfate minerals gradually dissolved under Holocene marine transgression. The mixing of paleo- and modern-sulfates due to seawater encroachment and flooding have occurred or are occurring in the coastal belt of Chianan plain. The wide range of δ³⁴S_[SO4] and δ¹⁸O_[SO4] values and the positive correlation between δ³⁴S_[SO4] values and total organic carbon (TOC) indicated the SO₄^2- reduction coupled with biodegradation of organic matter. Relatively low δ³⁴S_[SO4] and high δ¹⁸O_[SO4] along with low Eh values in the Chianan plain groundwater were likely by the sulfate reduction. Arsenic mobilization in the groundwater environment indicating for redox reactions in iron sulfide minerals may change the multiple isotopic (O, S, and C) concentrations in groundwater. This study provides insight that the oxidation of As-bearing FeS minerals followed by As dissolution under reducing environments is the processes responsible for As enrichment and contamination in the Chianan plain groundwater.