Selecting an As treatment technology for remediation of geothermal waters depends on several key factors. Among these, speciation of As, initial As concentration, regulatory requirements and target treatment levels must be considered. Due to variations in As speciation and large differences in the chemistry and physical properties of geothermal waters, no single technology will adequately meet the needs of every project. Furthermore, successful remediation often requires a combination of two or more treatment technologies. There are several inorganic arsenite species [inorganic As(III); e.g., H3AsO3, H2AsO3

Table 8.1 lists the As content of several geothermal waters in the world. Though large variation of As content is indicated in geothermal sites, extremely high As content is detected in some hot spring areas. This is linked to the mineralogical, chemical and physical characteristics of the soils, sediments and rocks in contact with these waters. In addition, the As is significantly leached from aquifers under the extremely high temperature and pressure conditions. Depending on oxidation-reduction (redox) conditions and biological activity, groundwater and geothermal water may contain As(V) and the more toxic As(III) forms (US EPA 2002). Considering that As contamination can originate from geological materials, the remediation of these materials is usually necessary to reduce As concentrations in associated geothermal waters. In some cases, however, geothermal water contamination is so severe that affordable and effective remediation is not possible. The physical and chemical characteristics of geothermal waters will affect the selection of reliable treatment technologies to work effectively under the high temperature condition. Alaerts and Khouri (2004) identified several factors

that affect the costs and feasibility of treating As in geothermal water. The lowering of As drinking water standards (maximum contaminant level, MCL) from 50 to 10 µg L−1 in many countries has resulted in increasing demands for additional removal technologies when geothermal waters are used for drinking and cooking.