BIOGEOCHEMICAL CYCLING OF ARSENIC IN COASTAL SHALLOW GROUNDWATER OF SOUTHWESTERN TAIWAN: EVIDENCE FROM SULFUR ISOTOPE STUDY

Groundwater salinization and As enrichment are the increasing concern in costal shallow aquifer of southwestern Taiwan. However, the impact of sulfur biogeochemical cycling on groundwater As mobility is still poorly defined. The purpose of this study is to assess the oxidation/reduction processes associated with the elevated As concentration and salinization in groundwater. Hydrogeochemical data was collected to identify the characteristics of As-contained groundwater. Based on assumption, the major composition of groundwater is the surface water and (residual) sea water, conservative chloride concentration was adopted to evaluate the occurrence of sulfate in As-enriched shallow groundwater. Most of groundwater with high As and sulfate concentration was observed in iron-reducing zone. Reduction of Fe-minerals supposed to be a possible pathway of As release and it may be also involved with sulfur geochemical cycling. Moreover, sulfur and oxygen isotope compositions of dissolved sulfate were analyzed to explore the possible processes that control As mobility under Fe reducing condition. The values of sulfur isotope of dissolved sulfate in salinization area with high As are much heavier than those in non-salinization area with low As, indicating that the different sulfuric biogeochemical processes were involved in As mobilization. In salinization area, heavier sulfur isotope of dissolved sulfate may be due to bacterial reduction of abundant sulfate supplied from sea water intrusion progressly. Previous studies suggested that co-precipitation of aqueous As with sulfide minerals under reduction condition is unable to explain the occurrence of elevated As in this area. Heavier oxygen isotope of dissolved sulfate indicated that dissolved oxygen from atmosphere may accelerate the re-oxidation of As-related sulfide minerals. The biogeochemical interactions between Fe, S, and As is the main controlling factor of groundwater As enrichment. Extensively anthropogenic pumping by residents further alternated As distribution and mobility.