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A deep well is a series of concentric pipes going down through the earth into a saline, porous injection zone, usually sandstone or limestone. This porous zone is protected vertically by impermeable rock strata. The well's outer casing extends well below any underground drinking water sources. Typically the wells are cased with concrete from the surface down to the underground injection zone to further prevent any seepage from the injection zone into areas above it. Inside the surface casing there is string casing containing tubing. The waste is injected under pressure into the injection tubing through perforations in the string casing, or a hole in the bottom of this casing. An inert, pressurized fluid, called the annulus, is put in the space between the injection tube and the casing. This is sealed in to prevent injected waste backing up into the fluid.
Limitations
The forms of hazardous waste usually disposed of in deep wells are saline wastes, metals, cyanides, corrosives and waste water, according to Environmental Health and Safety Online. Disposing of hazardous waste is not permitted in areas where earthquakes or any milder seismic activities are likely. Compatibility of waste with the well injection system is another requisite. For example, the waste may need specific chemical or mechanical decontamination processes that are not available at all deep well facilities. Added to this, particular waste types, such as those with suspended solids, can block the system.
Location
The United States has five commercial deep well injection facilities handling hazardous waste. These are located in Texas, Oklahoma, Louisiana and Ohio. In Vickery, Ohio, the facility is situated 3,000 feet beneath the Mt. Simon Formation, in an area of porous sandstone stretching for thousands of square miles according to WMDisposal.
Future
According to the Federal Remediation Technologies Roundtable this waste disposal technology has been in use since the 1950s. Although the method is well known, its use is under strict regulatory controls. It is the organization's opinion that any expansion in their use is unlikely. One of the reasons for this lack of expansion is the limited range of wastes that existing facilities are able to handle.