This section presents information on the use of injection wells in the oil and gas industry and key provisions of the Safe Drinking Water Act and EPA regulations related to protecting underground sources of drinking water, and EPA UIC regulations and evaluations. After a long period of declining production that lasted through 2005, annual U.S. production of oil and gas—and associated wastewater—has experienced significant growth. This increase is due in large part to development of oil and gas from unconventional formations. For example, shale oil production increased more than 5-fold, from 39 million to about 217 million barrels from 2007 through 2011, while shale gas production increased approximately 4-fold, from 1.6 to about 7.2 trillion cubic feet, over the same period. Oil and gas production, including production from unconventional oil and gas bearing formations, involves removing hydrocarbon bearing fluids from geologic formations underground. Conventional oil and natural gas are found in deep, porous rock or reservoirs and can flow under natural pressure to the surface after drilling. In contrast to the free-flowing resources found in conventional formations, the low permeability of some formations, including shale, means that oil and gas trapped in the formation cannot move easily from the rock. As a result of geologic differences, the methods used to extract hydrocarbon resources, including the amounts of water used and wastewater produced during extraction, vary widely. Some oil and gas can be developed by drilling a well and relying on the natural pressure in the formation to push the oil or gas to the surface. When an oil or gas producing reservoir is depleted—that is, no longer producing oil or gas—a producer can inject fluids, such as saltwater, into the reservoir to increase the pressure in the formation and move the residual oil and gas toward a well and the surface for collection, a process known as enhanced recovery (See fig. 1.). Similarly, some hydrocarbons are produced through the use of hydraulic fracturing. Hydraulic fracturing involves the injection of liquid under pressure to fracture the rock in geologic formations such as shale to create fractures and allow hydrocarbons to flow more freely from the formation into the well for collection. The liquids used in this process consist primarily of water, but also include chemicals, and sand or other agents for holding open the fractures (proppants). When a well is no longer economically viable, a producer may decide to cease production from it. At some point after production stops, wells may be converted from production wells to disposal wells, where wastewater can be injected for disposal. If the well is not used for any of these purposes, it becomes idle or inactive and will eventually be plugged and abandoned. The process of extracting oil and gas creates several waste streams that must be managed. Key among these waste streams is the water (brine) produced along with the oil or gas during production. Produced water may include water that occurs naturally in the formation, water or other liquids that were injected into the formation to enhance recovery during the drilling or production process, and flowback water, which consists of the water, chemicals, and proppants used for hydraulic fracturing (fracturing fluids). Over 90 percent of the water produced during oil and gas operations is disposed of underground, either through injection for disposal or for enhanced recovery. The remaining water may be discharged to surface water such as streams or lakes, stored in surface impoundments, reused for agricultural purposes such as irrigation or livestock, or reused for hydraulic fracturing. Differences in geologic formations can affect the pressure and volume of wastewater injected underground for either disposal or reuse in enhancement of oil and gas production. Layers of sediment and rock deposited over time create different underground formations with different characteristics, depending on the material that was deposited and various geologic processes. These characteristics include how porous and permeable a rock is—both characteristics that determine how well a formation can accept and hold fluids such as wastewater. Generally, fluids are injected into porous, permeable reservoirs below an upper confining layer, or geologic formation with low permeability through which significant quantities of liquids cannot move. The confining layer serves as a barrier of protection between the reservoir where fluids are being injected, known as the injection zone, and underground sources of drinking water (see fig. 1). Ideally, an injection zone is sealed above and below by continuous, impermeable rock formations and is large enough to keep injected fluids from reaching pressures great enough to fracture the confining rock layers. Much of the nation relies on groundwater as a source of drinking water, and reflecting this fact, Congress included groundwater protection provisions in the 1974 Safe Drinking Water Act, as amended. The act, among other things, required EPA to establish the UIC program, including the oversight of state programs regulating underground injection activities, to prevent the endangerment of underground sources of drinking water. Before the act’s passage, there was no overall federal regulation of injection activities, although some individual states regulated injection wells. Because of concerns about leaks from injection wells and the potential contamination of underground sources of drinking water, Congress established the national program in the Safe Drinking Water Act. The Safe Drinking Water Act requires EPA to establish regulations for state programs with minimum requirements to prevent injection of fluids that endanger underground sources of drinking water. These minimum requirements are to prohibit the injection of fluids underground unless permitted and are to include, among other things, requirements for monitoring of and reporting on injection wells. In implementing the program, EPA has defined underground sources of drinking water as an aquifer, or part of an aquifer, that has not been exempted from regulation and either supplies a public water system or contains a sufficient quantity of water to supply a public water system and (1) currently supplies drinking water for human consumption or (2) which, among other things, contains fewer than 10,000 milligrams per liter (mg/l or parts per million) total dissolved solids.the act and used for injection. Operators may apply for an exemption for a particular aquifer from EPA and, if granted, operators may inject fluids into the aquifer. Aquifers can be exempted from protection under The act, as amended through 1977, placed three conditions on the UIC program. First, EPA’s program regulations must not interfere with or impede the underground injection of brine or other fluids brought to the surface in connection with oil or natural gas production or natural gas storage operations, or any underground injection for the secondary or tertiary recovery of oil or natural gas, unless such requirements are essential to protecting underground sources of drinking water. Second, EPA’s regulations of class II wells should provide for varying geological, hydrological, and historical conditions within and among the states. Third, EPA’s program must not unnecessarily disrupt existing state UIC programs. The act also included provisions for states to apply for and receive approval from EPA to manage one or more of the UIC programs in their state (each class of well is managed as a program).gain EPA approval, a state generally must adopt and implement a program that meets the minimum requirements established under EPA regulations and conduct reporting as EPA requires. EPA must approve or disapprove a state’s program through a rulemaking process, and EPA’s practice is to enter into a memorandum of agreement with each state, outlining state and federal responsibilities for program management and oversight. States with approved programs—state programs—manage the permitting, inspection, and enforcement of injection wells in their states under EPA oversight. Under the act, to Congress amended the act in 1980, creating an alternative process for states to receive approval for their UIC class II programs specifically. Using this alternative, in lieu of adopting EPA’s minimum requirements, a state can seek approval to manage its own program by demonstrating to EPA that its program is effective in preventing the contamination of underground sources of drinking water. Similar to the conventional approval process, states are required to submit an application to EPA and enter into a memorandum of agreement with EPA laying out the components of their program. However, EPA has discretion when making the determination that the state’s program is effective at protecting underground sources of drinking water. To help states seeking approval for their programs under this approach, EPA developed guidance outlining what would constitute a program that is effective in preventing contamination of underground sources of drinking water. Most states have class II programs that have been approved by EPA under one or the other process, although some of these states do not have class II injection wells. EPA has approved 16 states under the conventional process (2 with class II injection wells), and 23 state programs under the alternative process (all 23 with class II injection wells), with most states approved in the 1980’s soon after the program’s inception (see app. I for a list of states). Eleven states do not have program approval (eight with class II injection wells), and the programs in these states are managed by six EPA regional offices. In 1980, EPA issued regulations that established minimum requirements for state programs, and through the mid-1990s, issued guidance for the class II programs. Until recently, EPA has made few significant additions or updates to the regulations and guidance, which have remained in effect to govern EPA’s class II program. In 1988, about a decade after EPA issued its regulations, the agency initiated a midcourse evaluation of the class II program to determine whether any changes were needed to improve the program. EPA evaluated the adequacy of program regulations and safeguards, including various safeguards such as mechanical integrity tests, and found that most of the regulations and corresponding safeguards were adequate to protect underground sources of drinking water and needed no changes. EPA’s evaluation also identified some areas of the program that could be improved, such as the temporary abandonment of injection wells and construction requirements for injection wells.