Transport and fixation of arsenic in geothermal eviroments - a case study of the fossil geothermal system at Sailauf

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Many active geothermal systems worldwide discharge large amounts of arsenic, which contributes significantly to the geogenic arsenic contamination of ground waters and soils. The processes responsible for transport and fixation of arsenic in geothermal systems, in particular the role of surface adsorption and complexation involving Fe and Mn oxide and hydroxide phases, are not fully understood. Substantial improvement of geochemical models for arsenic mobility in hydrothermal systems at elevated temperatures and pressures requires the integration of well-designed field-based analytical studies with novel approaches to numerical geochemical modeling. It is proposed to perform such an integrated study using the fossil geothermal system at Sailauf, Spessart, Germany, as a key example. This system is ideally suited to serve as a case study, because it is characterized by significant enrichment in arsenic and the abundance of different Fe and Mn oxide and hydroxide phases hosting minor/trace concentrations of arsenic. The study will adress the following objectives: (1) gaining detailed insight into the distribution of arsenic among various mineral phases that have coprecipitated in the geothermal system, (2) assessing the role of arsenic adsorption onto Fe and Mn oxide and hydroxide phases using an improved surface complexation model that is applicable at hydrothermal conditions, (3) evaluating the relative role of fluid-rock interaction and fluid boiling for arsenic distribution and enrichment, and (4) development of a consistent geological-geochemical model for the transport and fixation of arsenic that is applicable to other geothermal systems worldwide.