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.