Fate and transport of arsenic and antimony in the El Tatio Geyser Field, Chile

El Tatio Geyser Field (ETGF), northern Chile, hosts widespread geothermal activity, with very high aqueous concentrations of arsenic and antimony, higher than any other known geothermal system. Boiling springs (86°C) discharge circum-neutral pH, Na-Cl type waters with low organic carbon. Net discharge of a stream draining the ETGF basin is approximately 10 cfs. As(III), the dominant As species in discharge waters, rapidly oxidizes to As(V) at an estimated first order rate of 0.35 min-1, determined in the field by first arrival of a tracer. As and Sb concentrations and speciation in hydrothermal waters, deposits, and microbial biomass are evaluated as a product of microbial metabolism, sorption to metal-oxyhydroxides, and co-precipitation. Mechanisms controlling these reactions ( i.e. cooling, evaporation, changes in redox and pH) are evaluated and modeled. Sequential extractions reveal that As, sorbs to Fe and Mn oxy-hydroxide complexes. In contrast, Sb solid phase partitioning is influenced primarily by the co-precipitation of Sb-oxide minerals with siliceous sinter (up to 2% wt. Sb). Diurnal variations occur in spring water chemistry and may cause Sb-rich laminations in siliceous hydrothermal deposits. Microbial energetics calculations and enrichment experiments suggest that microbial activity influences the mobility of As, and probably Sb in the ETGF basin by altering redox speciation and sorption to microbial biomass.