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The increasing tempo of ecologic crusades for the cleanup of lakes and streams is driving pollution underground, in more than a manner of speaking. 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, 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 hydrologic regime throughout the entire flow system, howsoever 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 transmissivity is generally small and 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, the cone of pressure increase is propagated outward in a confined aquifer with the velocity of sound in the medium. Thus, to evaluate the consequences of waste injection requires not only consideration of the effects of the advancing cylinder of waste, but also the far-reaching effects of the cone of pressure increase.
Profile sections of hypothetical systems illustrate the mature, form, and extent of hydrologic system response to the stress of subsurface waste storage. Representative values of travel time for advances of the waste cylinder versus the cone of pressure increase are given for selected values of injection rate and aquifer diffusivity.
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