The increasing tempo of ecologic crusades for the cleanup of lakes and streams is literally driving pollution underground. There is in prospect a veritable explosion in the use of sanitary landfills for disposal of solid wastes, in the use of spray irrigation for disposal of partly treated sewage effluent, and in the use of deep-well injection for disposal of certain industrial wastes.
Citations of the astronomical volume of storage space within the earth's crust, the very small velocity of groundwater motion, the evidence of entrapment of hydrocarbons and brines, and the presence of very fine-grained confining rocks all intrigue proponents of subsurface storage with the potential for resolving our waste-disposal problems. What gives cause for concern is the recognition that groundwater reservoirs or aquifers are not static environments, but represent dynamic flow systems that undergo change whenever a new stress is imposed.
Attendant upon the injection of fluid into an aquifer is a consequent increase in hydraulic head which ultimately influences the hydrologie regime throughout the entire flow system, however distant its boundaries may be. Disposal to shallow aquifers, which are generally sources of water supply, poses a threat not only to present and future well developments, but also to lakes and streams that are sustained by groundwater seepage. In deep-lying confined aquifers, where overburden pressures are large, the hydraulic transmis-sivity is generally small; consequently, the pressures required for significant rates of injection are large. In marked contrast to the very slow migration of the cylinder of injected waste, a transient increase is propagated outward in a confined aquifer with the velocity of sound in the medium. Thus, evaluation of the conse-quences of waste injection requires not only consideration of the effects of the advancing cylinder of waste, but also the far-recching effects of the cone-of-pressure increase.
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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.