Environmental Protection Agency
More than 700,000 different wells are currently used for the underground injection of fluids in the United States and its territories.2 Underground fluid injection began in the 1930s in order to increase production from existing oil and gas fields and was used in later years to dispose of industrial waste, but it was unregulated until 1974 when Congress passed the Safe Drinking Water Act (SDWA). The SDWA ensures safe drinking water for the public and establishes regulatory authority over the underground injection of fluids. In accordance with the act, the EPA is required to set standards for drinking water quality and to oversee all states, localities, and water suppliers that implement these standards. The EPA also regulates the construction, operation, permitting, and final plugging and abandonment of injection wells that place fluids underground for storage or for disposal under its Underground Injection Control (UIC) program.3 It is important to note that the SDWA gives authority to the EPA to protect underground sources of drinking water from contamination due to underground injection and does not explicitly address the issue of seismicity induced by underground injection. UIC regulations requiring information on locating and describing faults in the area of a proposed disposal well are concerned with containment of the injected fluid, not the possibility of induced seismicity.
Developers applying for a permit to inject fluids underground must demonstrate to the EPA that the operation will not endanger any underground sources of drinking water (USDWs). This regulatory scheme allows for six classes of injection wells, which are classified by the type of fluid injected and the specific injection depth (e.g., above or below sources of drinking water). Under this program, oil and gas industry injection wells are regulated as Class II injection wells, which also generally cover enhanced oil recovery projects or projects involving the disposal of exploration and production wastes (NRC, 2010). Table 4.1 provides an explanation of the distinction among classes of wells regulated under the SDWA.
Although the number and distribution of the different classes of injection wells vary by state, Class V wells are by far the most numerous, accounting for almost 79 percent of the total number of reported UIC wells. Because Class V wells normally inject fluid into formations above USDWs, these wells are usually too shallow to be considered a source of induced seismicity. This does not hold true in all cases, however, because wells used for fluid injection associated with the extraction of geothermal energy are included in this class of injection wells and are often the source of seismic events. The total number of geothermal wells in the United States was estimated to be approximately 239 wells, with 153 of these wells located in California and 53 located in Nevada (EPA, 1999). Although Class VI wells