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Hydrogeology or geohydrology is the study of the interactions between water and rocks, particularly with respect to groundwater.

Hydrogeology is a complex subject, as the chemical and physical interactions between rocks and water are intricate, and difficult to quantify. Although the basic principles of hydrogeology are not complex or difficult to understand, the quantification of some of the most important aspects of this science are very complex, multi-component systems that are difficult to determine through direct measurement, and are therefore determined through modeling which presents questions as to the accuracy of these techniques.

For example, it may be desirable to determine the conditions of flow for a chemical plume from a spill or leak, such as that produced by an underground gasoline tank. If the contaminant reaches the groundwater, and that groundwater moves, then the gasoline may be entrained and carried with it. It is possible to determine the concentration of gasoline in the water through sampling by drilling a hole and pumping out water; it may even be possible to determine roughly which direction the gasoline is flowing. However, to determine where that plume may be headed, and where it will be in the future, requires extensive modeling and large amounts of computer time in conjunction with as much field sampling as possible.

Hydrogeology has particular importance as regards to issues such as burial of hazardous waste as well as finding out where that waste went when it escapes from such facilities as the Hanford Nuclear Waste Dump and other Department of Energy facilities, such as Los Alamos National Lab and Sandia National Labs. An understanding of the hydrogeology surrounding the Waste Isolation Pilot Plant or WIPP where tons of low level radioactive waste will be interred in salt mines is of utmost importance to help determine the chances of leakage or escape of material over hundreds or thousands of years. The hydrogeology of the Yucca Mountain, Nevada nuclear waste site, for storage of high level radioactive waste has been studied intensely in anticipation of receiving tens of thousands of glass log or similar cannisters of extremely radioactive waste less than 80 miles from Las Vegas.