Arsenic in hot springs of Bahía Concepción, Baja California Peninsula, México

High arsenic concentrations (0.15 to 0.78 mg/L) were measured in water discharged from hydrothermal vents at Bahía Concepción, Baja California Sur. Additionally, concentrations above the average for the continental crust (1.8 mg/kg) were determined in rocks and marine sediments. Rock samples collected around intertidal and submarine hydrothermal discharges are encrusted by deposits of opaline silica, barite, calcite, and Mn oxides, and in subordinate amounts of pyrite and ferrihydrite. Sequential extraction showed that arsenic is adsorbed mainly on amorphous and crystalline iron-oxides, and in lower concentrations, to the easily extractable phase. When hydrothermal fluids mix with oxidizing and alkaline seawater (pH about 8.3), arsenic in H3AsO3 is oxidized to form H2AsO4− or possibly a deprotoned form such as HAsO42 −. In addition reduced iron from the hydrothermal fluid is oxidized to ferric ion and forms non-crystalline ferric hydroxide Fe (OH)3, which is able to adsorb arsenic. X-ray elemental maps of the surface of the rock samples located near the submarine discharges, show evidence that arsenate is adsorbed onto calcite surface. Iron-oxyhydroxides with adsorbed As could be present on the seafloor forming deposits of flocculent material, which can be incorporated into the tissue of some marine organisms (e.g. holothurians) that feed in the vicinity of the hydrothermal manifestations. Oxidation and adsorption processes control transport and mobilization of arsenic in Bahía Concepción hydrothermal fluids.

► We studied hydrothermal fluids in some coastal springs.
► High arsenic concentrations are bioavailable to benthic organisms near the vents.
► Mixing of hydrothermal fluids with seawater produces the oxidation of As.
► The main chemical species of the arsenic is H2AsO4− or possibly HAsO42 −.
► The adsorption of arsenic occurs onto calcite and non-crystalline Fe oxyhydroxides.