The Geochemistry and Bioavailability Of As and Sb in Geothermal Waters of El Tatio Geyser Field, Chile

Lindsey R. Johnson
Department of Geology & Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803 USA
[email protected]

Water quality and policy in Northern Chile are impacted by high arsenic (As) concentrations in the geothermal waters of El Tatio geyser field. Arsenite [As(III)] predominates upstream at geyser discharges while arsenate [As(V)] occurs mainly downstream. Photic As(III) oxidation is unlikely due to similar experimental day and night oxidation rates, however reduction rates are higher during the day, possibly linking As(V) reduction to photosynthesis. The goal of this study is to understand the geochemical and microbial processes affecting metalloid behavior. X-ray absorption spectroscopy was used to understand As speciation in microbial mats and sediments from geyser discharge channels. As(V) was the predominate oxidation state in solids, with appreciable organo-As compounds and As(III) associated with organic matter and hydrous ferrous oxides downstream. Microbial diversity was evaluated with 16S rRNA and As functional genes because changes in As speciation may be linked to microbial activity. From 16S rRNA gene sequences, communities near geyser pools were dominated by Thermus and Deinococci, whereas downstream communities were dominated by Chloroflexi, γ-Proteobacteria, and Verrucomicrobia. Arsenite oxidase (aroA) genotypes belonged mainly to Chloroflexi and classes within Proteobacteria, which expands our understanding of microbial As(III) oxidation because few have shown that Chloroflexi can be linked to geothermal As(III) oxidation. The degradation of humic substances produces reactive OH groups that influence As oxidation. Continued geochemical and microbiological investigations of the As cycle at El Tatio will help us to understand remediation efforts in As-contaminated groundwater and the consequences of elevated As levels in fluids during geothermal development and operations.

AAPG Search and Discovery Article #90094 © 2009 AAPG Foundation Grants in Aid