tive mathematical modeling allows geologists to wring more information from the data obtained.
As fluids flow through soils and rocks, chemical reactions inevitably occur with the minerals of the rocks, sometimes catalyzed by microorganisms. The most familiar interaction is adsorption, or ion exchange, by which ions carried in solution in water are adsorbed and desorbed from mineral surfaces. This process, which happens everywhere in nature, has been successfully exploited by humans to create water purification systems. Fluids moving through rocks also act as weak acid solutions, often due to dissolved carbon dioxide, that slowly dissolve the original minerals, which are then replaced by secondary minerals such as rusty iron oxides and clays. As rocks and soils chemically react with fluids, changes occur not only in mineralogical and chemical composition, but also in ion exchange and hydrological properties. For soils, the activities of plants, animals, and microbes are important. In deeper groundwater systems, where temperatures are higher and fluids can be more corrosive, chemical reactions can be quite fast. But because chemical reactions between fluids and minerals occur only at mineral surfaces, the structure of the fluid flow through rocks and the geochemistry are inextricably linked. If fluid flow is confined to a few fractures, it may be fast, with little contact area between fluid and minerals and little chemical interaction. If there is grain-scale porous flow, however, flow velocity will be low, the contact area large, and fluid-rock interaction extensive.
Geological studies of fluid flow, chemical reactions, and their interplay are grouped under the heading of reactive chemical transport (e.g., Steefel et al., 2005; Figure 4.10). A major goal of this subfield is to describe, with advanced computational techniques, how the characteristics of fluid-rock systems affect their physical, chemical, and biological development. The computer models require large inputs of basic materials property data, and the complexity of the interactions is a conceptual as well as a computing challenge. One crucial feature of the models is mineral surface properties and their role in chemical reaction kinetics, which are increasingly explored at synchrotron X-ray facilities. Other inputs come from benchtop experiments that produce and observe coupled processes in a realistic, controlled environment. In addition to modeling, efforts are being made to document the role of microbes in altering mineral surfaces and chemical microenvironments (Figure 4.10). And the role of hydrology in chemical reactions is being approached with a combination of numerical models, such as approaches that include multiphase flow in complex geometries and microfluidic experiments, both of which can address the roles of chemical transport and pore structure on chemical reactions. For multiphase fluids there are additional considerations because the presence of each phase interferes with the flow of the other phases, and the detailed distribution of each phase within the pores can affect the surface area that is available for fluid-rock chemical interactions. There is also partitioning of chemical elements between multiple flowing phases (e.g., gas, oil, water), which is important in many subsurface processes but difficult to model because of its dependence on the physical relationships between the phases.
Chemical reactions are not the only processes that complicate fluid flow. As fluids move through rocks they redistribute heat as well as material, and both the heat and materials affect the subsequent fluid flow. For example, buoyant upwelling of groundwater heated by magma can cause rainwater that has percolated into the ground to circulate to depths of several kilometers in areas of active volcanism and mountain building, as well as in sedimentary basins. At midocean ridges, cold seawater circulates through hot rocks to depths of several kilometers, and magma at the shallow depths of midocean ridges causes such rapid heating of water that it is expelled back into the ocean at temperatures above 350°C. Base metal ore deposits associated with magmatic intrusions in the crust are products of “fossil” hydrothermal systems where circulating water attained temperatures of 200°C to over 500°C (Hedenquist and Lowenstern, 1994; Sillitoe and Hedenquist, 2003). Some of these systems persisted for tens to hundreds of thousands of years at depths of 3 to 10 km. Any magma that makes