Underground Injection

Underground injection involves using specially designed wells to inject liquid hazardous waste into deep earth strata containing non-potable water. Through this method, a wide variety of waste liquids are pumped underground into deep permeable rocks that are separated from fresh water aquifers by impermeable layers of rock above, below, and lateral to the waste layer. The depth of an injection ranges from 1,000 to 8,000 ft and varies according to the geographical factors of the area. HSWA prohibits the disposal of hazardous waste within Af mi of an underground source of drinking water.

Figure 11.12.4 is a cross-section of a typical injection well. To prevent plugging of the injection equipment, wastes are usually pretreated to remove solids greater than one micron. The well must be constructed to assure that potable water zones are isolated and protected. At minimum, well casings must be cemented and must extend through all potable water zones.

Deep-well disposal uses limited formation space, is expensive in construction and operation, and the subject of ever-tightening regulations. For hazardous liquid waste to be deep-well injected, the following criteria must apply: the hazardous liquid waste must have a low volume and a high concentration of waste, cannot cause an unfavorable reaction with material in the disposal zone, must be biologically inactive, must be noncorrosive, and must be

Injection Gauge

Waste Stream

Annulus Pressure -Gauge

Injection Gauge

Waste Stream

Annulus Pressure -Gauge

Source: Adapted from Wentz

FIG. 11.12.4 Schematic cross section design of a hazardous waste injection well. (Reprinted and adapted, with permission, from C.A. Wentz, 1989, Hazardous Waste Management [McGraw-Hill, Inc.].

Source: Adapted from Wentz

FIG. 11.12.4 Schematic cross section design of a hazardous waste injection well. (Reprinted and adapted, with permission, from C.A. Wentz, 1989, Hazardous Waste Management [McGraw-Hill, Inc.].

difficult to treat by other methods. Thus, the method should be used only for those wastes with no other feasible management options.

Due to faulty construction or deterioration, there is a potential for leakage from some old wells. Detection of a leak and remedial action may not be feasible because of the nature and location of the leakage. Because of the difficulty associated with monitoring subsurface migration of liquid waste, the potential for geographical disturbances to the underground injection system, or the geographical nature of the land, underground injection wells are severely restricted in most states.

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