Injection of liquid wastes into subsurface geologic formations is a technology that has been widely adopted as a waste-disposal practice. The practice entails drilling a well to a permeable , saline-bearing geologic formation that is confined above and below with impermeable layers known as confining beds. When the injection zones lie below drinking water sources at depths typically between 2,000–5,000 ft (610–1,525 m), they are referred to as Class I disposal wells . The liquid hazardous waste is injected at a pressure that is sufficient to replace the native fluid and yet not so high that the integrity of the well and confining beds is at risk. Injection pressure is a limiting factor because excessive pressure can cause hydraulic fracturing of the injection zone and confining strata, and the intake rate of most injection wells is less than 400 gal (1,500 l) per minute.
Deep-well injection of liquid waste is one of the least expensive methods of waste management because little waste treatment occurs prior to injection. Suspended solids must be removed from wastewater prior to injection to prevent them from plugging the pores and reducing permeability of the injection zone. Physical and chemical characteristics of the wastewater must be considered in evaluating its suitability for disposal by injection.
The principal means of monitoring the wastewater injection process is recording the flow rate, the injection and annulus pressures, and the physical and chemical characteristics of the waste. Many consider this inadequate and monitoring is still a controversial subject. The major question which arises concerns the placement of monitoring wells and whether they increase the risk that wastewater will migrate out of the injection zone if they are improperly constructed.
Deep-well injection of wastes began as early as the 1950s, and it was then accepted as a means of alleviating surface water pollution . Today, most injection wells are located along the Gulf Coast and near the Great Lakes , and their biggest users are the petrochemical , pharmaceutical, and steel mill industries.
As with all injection wells, there is a concern that the waste will migrate from the injection zone to the overlying aquifers.
[Milovan S. Beljin ]
Assessing the Geochemical Fate of Deep-Well-Injected Hazardous Wastes: Summaries of Recent Research. Washington, DC: U. S. Environmental Protection Agency, 1990.
International Symposium on Subsurface Injection of Liquid Wastes. Proceedings of the International Symposium on Subsurface Injection of Liquid Waste. March 3-5, 1986. Dublin, OH: National Water Well Association, 1986.