Geothermal arsenic: Occurrence, mobility and environmental implications

Arsenic (As) contamination in geothermal systems has been identified in many areas of the world. Arsenic mobilization from rocks and mineral phases into geothermal fluids depends on available As sources, geochemical conditions and microbiological activity. In deep geothermal reservoirs As mobilization is predominantly from As-bearing pyrite at temperatures of 150-250 °C, and at higher temperatures also from arsenopyrite. Highest As concentrations, mostly in the range of thousands to tens of thousands of µg/L and in case of Los Humeros (Mexico) even of up to 162,000 µg/L are found in volcanic geothermal systems whereas in low- and high-enthalpy sedimentary geothermal systems they reach only about 2000 µg/L. At many sites, uprising geothermal waters contaminate shallow water resources. From the geothermal springs, those with NaCl water type have the highest As concentrations; these waters correspond to original reservoir waters which were not significantly altered during its ascent. In the geothermal reservoir and deeper parts of hydrothermal system, As is predominantly present as neutral H3As(III)O3 (arsenius acid) and under sulfidic conditions also as thioarsenites; close to the earth׳s surface oxidation through atmospheric oxygen to As(V) species may occur; however, this is a slow process. As(III) emerging in geothermal springs is oxidized quickly through microbial catalysis and often most As is present as As(V), at a distance of few meters from the spring outlet. This review highlights the occurrence and distribution of geothermal As worldwide, its sources and its mobilization and the presence of different As species in geothermal fluids considering different geological settings and processes involving geothermal fluids rising from deep geothermal reservoirs to the earth׳s surface where it may mix with shallow groundwater or surface waters and contaminate these resources. The microbial diversity of hot spring environments which plays an important role to mobilize the As by oxidation and reduction process in the geothermal system is also addressed.