EPA's Region 6 Office
Serving: Arkansas, Louisiana, New Mexico, Oklahoma, Texas, and 66 Tribal Nations
USDWs are defined broadly to include all fresh water aquifers unless they have been specifically exempted from protection. A USDW may be in current use as a source of drinking water, but that is not necessary. A USDW is simply any aquifer which contains fewer than 10,000 mg/l total dissolved solids and is currently being used as a drinking water source or which is of sufficient volume and adequate quality to be a future source for a public water system (25 or more connections).
Underground injection is the practice of placing fluids underground, in porous formations of rocks, through wells or other similar conveyance systems. While rocks such as sandstone and limestone appear to be solid, they can contain significant voids or pores that allow water and other fluids to fill and move through them. Man-made or produced fluids (liquids, gases or slurries) can move into the pores of rocks by the use of pumps or by gravity. The fluids may be water, wastewater or water mixed with chemicals. Engineering can predict the capacity of rocks to contain fluids and the technology to do so safely.
The UIC Program defines an injection well as a bored, drilled, or driven
shaft whose depth is greater than the largest surface dimension; or, a
dug hole whose depth is greater than the largest surface dimension; or,
an improved sinkhole; or, a subsurface fluid distribution system. This
definition covers a wide variety of injection practices that range from
technically sophisticated wells which pump fluids into isolated formation
up to two miles below the Earth's surface, to on-site drainage systems,
such as septic systems and storm water wells, that discharge fluids a
few feet underground.
The EPA classifies underground injection activities into five classes
for regulatory control purposes.
An injection well's classification is determined by its waste stream constituents and its depth of injection relative to the depth of any USDWs near by.
Class II wells are oil and gas related wells. They may be used for disposal of oil and gas wastes, enhanced recovery, or hydrocarbon storage.
Class III wells are wells that inject super-heated steam, water, or other fluids into formations in order to extract minerals such as salt, sulfur, and uranium.
Class IV wells inject hazardous or radioactive wastes into or above underground sources of drinking water (USDW). These wells are banned unless authorized under other Statutes [RCRA, Comprehensive Environmental Response, Compensation and Liability Act (CERCLA)] for purposes of ground water remediation.
Class V wells includes all underground injection wells that are not included in the other classes. This "catch all" well class includes technically advanced deep wastewater disposal systems and shallow "low-tech" wells, such as septic systems and cesspools. Generally, they are shallow, on-site disposal systems, such as floor and sink drains which discharge directly into the subsurface via dry wells, leach fields, and similar types of drainage systems.
Exclusions: Injection practices which are not covered by the UIC Program include individual residential waste disposal systems that inject ONLY sanitary waste and commercial waste disposal systems that serve fewer than 20 persons per day and that inject ONLY sanitary waste.