As detailed in Chapter 5, geothermal systems are complex geochemical environments in which a range of chemical processes occur. Conceptually, these processes represent interactions in which the atmosphere, the geological framework, and subsurface uids evolve toward their lowest Gibbs energy state. These processes can be schematically represented as an interacting three-part system:

Atmosphere minerals water⇔ ⇔ (15.1)

The ⇔ symbol indicates interactions that reect exchange and chemical reactions between parts of the system. These chemical changes can inuence the physical attributes of the system by changing the rock porosity, fracture apertures, and temperature, among other things. Any perturbation in one part of the system will be reected in some measurable adjustment in other parts of the system. If, for example, the temperature of the water increases, minerals with which the water is in contact will dissolve or precipitate, reecting the temperature dependence of the respective mineral solubilities, which are site specic. Likewise, the temperature dependence of dissolved gas concentrations will change in response to temperature perturbations, and this will, in turn, result in gases being liberated to or absorbed from the coexisting atmosphere. For considerations regarding gaseous emissions from geothermal power plants, the behavior of three types of gaseous emissions is particularly important-greenhouse gas emissions (especially CO2), hydrogen sulde (H2S), and toxic metals such as mercury (Hg) are discussed elaborately in three subsections. In the discussion, attention will be focused primarily on ash geothermal generating plants because binary generating plants have no emissions as they do not expose the geothermal uid to the atmosphere. However, their operation can inuence subsurface processes. These effects will be discussed at appropriate points.