Application of Transport Equations to Groundwater Systems1
Predicting changes in groundwater quality in a complex hydrologie system generally requires simulation of the field problem through the use of a deterministic model. In the most general case, a complete physical-chemical description of moving groundwater includes chemical reactions in a multicomponent fluid and requires the simultaneous solution of the differential equations that describe the transport of mass, momentum, and energy in porous media.
The difficulties in solving this set of equations for real problems have forced hydrologists and reservoir engineers to consider simplified subsets of equations for the general problem. The equation of motion for single-component groundwater flow, which describes the rate of propagation of a pressure change in an aquifer, has been solved for many different initial and boundary conditions. To describe the transport of miscible fluids of differing density, such as salt water and fresh water, the mass-transport equation and the equation of motion have been coupled and solved numerically. Numerical solutions have also been obtained for the heat-transport equation and the equation of motion, particularly for convection problems.
A case history of groundwater contamination at Brunswick, Georgia, illustrates the use of the mass- and momentum-transport equations in predicting and control-ling movement of contaminants.
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This publication consists of papers based on oral presentations at a symposium of the same name co-sponsored by the United States Geological Survey and the American Association of Petroleum Geologists. A wide range of technical issues are covered, as well as regulatory and liability concerns. Documentation of two areas in Colorado where earthquakes had resulted from subsurface fluid injection set the stage for modern debates regarding possible similar results elsewhere. A wide range of fluid compositions are subject to subsurface waste disposal. The largest volumes are brines separated during the production of oil and gas wells, but acid-water and industrial wastes of all types can be disposed in significant quantities in local areas. Large hydraulic fracture treatments never recover all of the injected fluids, and the chemical additives in the fluid that remains underground can be a concern. The subsurface injection of radioactive waste is a topic for three of the papers. The possible need for sequestration of carbon dioxide was not a significant concern at the time and was not covered, but many of the papers provide insight into the issues related to modern proposals. When fluids are injected under pressure into subsurface aquifers, they interact in numerous ways. The fluids can potentially migrate for long distances and potentially interfere with other uses for the native aquifer fluids. If the aquifer cannot transport all of the fluids away, the buildup in pressure can cause fracturing of the rock. Differences in composition between the injected and native fluids can cause chemical reactions to occur; in some cases these can be desirable in that they can immobilize certain solutes in mineral form. The long-term environmental consequences are a common theme in many of the papers because of the recognition that the disposed fluids would become a permanent fixture in subsurface aquifers and could have long-term consequences for their future utilization.