Geochemical characteristics of the fluids and muds from two southern Taiwan mud volcanoes: Implications for water–sediment interaction and groundwater arsenic enrichment

Mud volcanoes distributed in the southern part of Taiwan are believed to be sourced from an accretionary prism located along the collision boundary between the Philippine Sea plate and the Asian Continental plate. Fluid and mud samples have been collected from Wushanting and Hsiaokunshui mud volcanoes during October 2004, March and June 2005, and analyzed for major ions, trace elements, and stable isotopes. The results show that the mud volcano fluids are enriched in Na+ and Cl− but are depleted in Ca2+ and Mg2+. The chemical composition and oxidation state of the fluids fluctuate seasonally, with ionic contents being notably higher during the dry pre-monsoon period. The enriched ionic (e.g., Na+, image, K+, Mg2+ and Cl−) compositions of the exchangeable fractions indicate that the muds were sourced from a marine depositional environment. δD and δ18O values indicate that the mud volcano fluids may have been modified by chemical exchange with 18O-rich crustal rocks and possibly originated from mixing of deep brines with circulating meteoric water. The spatial and temporal distribution of ions in the muds suggests a varied mixing regime within the mud volcanoes. Incubation study of the muds shows the leaching of soluble salts under aerobic conditions, which is generally consistent with the fluid geochemical characteristics. Moreover, elevated trace element (e.g., As, Zn, Cu and Mn) concentrations are also observed in the fluids and muds. Geochemical correlations between As, Zn and Cu in the muds suggest prevailing SO4-reducing conditions at depth. The relatively low Eh values (range: −50 to −30 mV) of the fluids near the surface would favor bacterial Fe reduction and mobilization of As. The dewatering of mud volcanoes may represent a significant source of groundwater As in the nearby Chianan plain.