Water, through its unique or extreme properties, is the fundamental fluid genetically relating all mineral deposits in sedimentary rocks. It is the vehicle for the transportation of materials in solution and suspension throughout the hydrologic cycle, and it takes part in reactions during the dissolution of minerals in chemical weathering, diagenesis, and metamorphism of sediments and sedimentary rocks. Understanding of the significance of water-rock interaction requires knowledge of hydrodynamics and hydrochemistry, both of which are controlled by well-recognized chemical and physical principles. In sedimentary rocks economically important mineral deposits which are the result of water-rock interaction include petroleum and Mississippi-type lead-zinc deposits. Both of th se owe their origin to complex interaction of fluid flow and creation or reduction of porosity by mineral solution or deposition. The recovery of some of these mineral deposits through the use of water (e.g., waterflooding of petroleum reservoirs or steam injection to recover crude bitumen from oil sands) involves man-imposed water-rock interaction. During in situ steam injection, some of the subsurface reactions may involve the production of toxic or deleterious substances which require removal before reuse of the produced water. These need to be monitored to avoid contamination of the local potable groundwater, or, if produced and subsequently disposed of into waste-injection wells, they again may take part in further water-rock reactions, possibly causing problems in disposing of the aste water. A comparable scenario exists for underground coal gasification. Although we understand some of the principles involved, considerably more thought and additional research effort are needed for these and other economic aspects of water-rock interaction.

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