G roundwater, the water in aquifers accessible by wells, is a critical component of the U.S. water supply. It serves as a water source for domestic use and as irrigation water for agriculture, and it is used in mining, oil and gas development, industrial processes, livestock, and thermoelectric power, among other uses. Managing groundwater resources largely has been the purview of states rather than the federal government. How each state manages its groundwater resources differs and depends on a mix of common law emerging from the 19 th century, state law, court decisions, water settlements, and, to a lesser extent, federal law. The federal role in managing groundwater includes activities under federal trust responsibilities to Indian tribes and reservations. It also includes management responsibilities for certain federal reservations if the purposes of those reservations require water, such as some national monuments, national forests, military bases, and other federal land holdings. In addition, the federal government is involved in groundwater monitoring and assessment and in aspects of groundwater recharge, storage, and recovery. Much of the recent congressional interest in groundwater has been broadly related to policies for increasing water supplies generally, as a response to recent droughts, and in preparation for future droughts. In recent Congresses, some Members have introduced legislation that could affect how groundwater resources may be managed to better ensure a sufficient and reliable supply, and several such bills (or portions of such bills) have been enacted into law. Drought conditions and constrained supplies of surface water have helped to spur legislative action. These conditions continue to affect many regions in western states, although droughts can occur anywhere in the nation. Congress could continue to explore its authority to affect policy, conduct oversight, and provide appropriations for federal activities that influence groundwater supply management in the United States. This report is intended to provide context and a broad summary of federal authorities and activities affecting the supply and use of groundwater resources. Whereas the states primarily manage groundwater supply , the federal government plays a more direct role in managing the nation's groundwater quality . For example, the Comprehensive Environmental Response, Compensation, and Liability Act (42 U.S.C. §§9601 et seq.) authorizes federal cleanup and enforcement actions to respond to releases of hazardous substances to the environment, including groundwater. In addition, the Safe Drinking Water Act (42 U.S.C. §§300f et seq.) authorizes the U.S. Environmental Protection Agency (EPA) to regulate underground injection activities to protect underground sources of drinking water, including injection wells used for aquifer recharge. This report focuses on issues related to groundwater supply, not groundwater quality. This report is divided into two parts. The first part discusses general background related to groundwater supply and management, including selected major issues before Congress. The second part provides a more detailed primer on groundwater resources, including relevant federal activities and authorities. Nearly half of the U.S. population relies on groundwater to meet their everyday needs. Groundwater makes up a portion of the water supplied to about 149 million people (46% of the U.S. population in 2015) for their domestic indoor and outdoor water uses. Most U.S. citizens (approximately 282 million people, or 87%) depended on public water supplies in 2015. Because about 38% of public supply water is groundwater, that translates into about 107 million people using groundwater from public water supplies. The remaining 13% (approximately 42.5 million people) supplied their own water, and nearly all of these citizens (98%, or about 42 million) pumped the water from their private wells. The 107 million people using groundwater from public water supplies plus the 42 million people pumping groundwater from their private wells equal 149 million people total using groundwater in 2015. The greatest volume of groundwater used is for agriculture, nearly entirely for irrigation. In 2015, irrigation accounted for over 69% of all fresh groundwater withdrawals in the United States, which corresponded to about 57.2 billion gallons per day (bgpd) in irrigation withdrawals as compared to 18.4 bgpd in withdrawals for domestic use (both public supply and self-supplied groundwater). Among all states, California uses the most groundwater for irrigation, withdrawing 13.9 bgpd in 2015. Arkansas was second, withdrawing 9.28 bgpd in the same year, followed by Nebraska (5.42 bgpd), Idaho (4.9 bgpd), Texas (4.48 bgpd), and Kansas (2.56 bgpd). Overall, groundwater withdrawals for irrigation in 2015 accounted for 48% of the total water withdrawn for irrigation, an increase of 16% compared to 2010. In comparison, surface water sources supplied 52% of total irrigation withdrawals, a decrease of about 8% from 2010. Figure 1 illustrates the amount of groundwater withdrawn for irrigation by state. Generally speaking, western states tend to use the most groundwater, due in part to hydrology and other surface water supply constraints. The federal government directly and indirectly influences how groundwater is managed in the United States. Several federal agencies monitor groundwater directly or with partners—through measurements at wells and springs—and remotely using satellites or other remote sensing devices to provide information on groundwater flow, storage, depletion, and other characteristics that help inform state and local groundwater management. This includes the U.S. Geological Survey (USGS), the National Aeronautics and Space Agency (NASA), the National Oceanic and Atmospheric Administration (NOAA), and the U.S. Department of Agriculture (USDA). Congress has provided other federal agencies with the authority to make available some water delivered from or stored at federal water resource projects for groundwater recharge, storage, and recovery. This includes the two principal federal water resources agencies: the U.S. Army Corps of Engineers (USACE, which operates nationwide) and the U.S. Bureau of Reclamation (Reclamation, which operates in the 17 coterminous states west of the Mississippi River). Additionally, courts have found that when the federal government reserves lands for a particular purpose (such as for a tribal reservation or national monument), it impliedly reserves a right to water necessary to accomplish the purposes for which the reservation was created. Thus, federal land management agencies and the Bureau of Indian Affairs often are involved in water rights issues. Federal reserved water rights doctrine has long been recognized for surface water; more recently, it is also being considered for groundwater. In recent years, congressional interest in groundwater has generally been in three major areas aquifer storage, recharge, and recovery; groundwater rights (including among other things, groundwater/surface water interaction and federal reserved water rights); and groundwater supply monitoring and assessment. In some cases, these issues overlap. Numerous bills introduced in the 115 th Congress would address several groundwater-related topics. Table 1 includes bills that would affect, in some manner, groundwater topics discussed in this report, including aquifer storage and recharge, groundwater-surface water interaction, treatment of groundwater rights by the federal government, aspects of tribal water rights, Reclamation water recycling and reuse programs, drought relief, groundwater studies, and groundwater monitoring and assessment. Table 1 does not include all bills introduced in the 115 th Congress that touch on the topic of groundwater; some of the bills omitted, for example, provide definitions for groundwater but do not affect the policy issues introduced above. Other omitted bills pertain to groundwater quality (i.e., bills addressing groundwater protection or remediation), which is not addressed in this report. Background on selected topics of interest in the 115 th Congress, including relevant legislation, is discussed in more detail below. Historically, the federal government, through USACE and Reclamation, has played a prominent role in creating infrastructure related to surface water resource management (e.g., storage, control, or delivery). At the same time, the federal government has played a comparatively smaller role in creating infrastructure to develop groundwater storage, which is commonly conducted as aquifer storage, recovery, and/or recharge. The reasons for the differing levels of federal involvement are complex, tied to the long and complicated history of common law water rights, state water law, legal adjudication, federal deference to states on water supply issues, and a historically cruder understanding of how groundwater occurs and moves underground compared to surface water. Both public and congressional focus on groundwater storage has sharpened in recent years, particularly in reaction to recent major drought events. Congressional interest has increased in the potential to assist with state, local, and private groundwater management efforts, including efforts to use surface water to recharge and/or store water in aquifers and to recover (i.e., pump to the surface) the stored groundwater when needed. Some see aquifer recharge, storage, and recovery as potentially complementary to existing storage; some also see these projects as possible alternatives to building new surface water reservoirs that may prove less costly and/or pose fewer environmental issues. Authorities to provide water for irrigation (Reclamation) and store water for various purposes (USACE) exist and are one way for the federal government to promote aquifer recharge, storage, and recovery (see below section, " Federal Authority Related to Groundwater Recharge, Storage, and Recovery "). Currently, there are no general federal restrictions on the nonfederal use of water delivered by Reclamation or stored by USACE for aquifer recharge, storage, and recovery purposes; however, some state restrictions and federal environmental protection laws may affect the use of these waters for groundwater recharge. Although Congress has authorized aquifer storage, recharge, and/or recover for some individual projects, general congressional guidance in this area has been limited to date. Under the Water Infrastructure Improvements for the Nation Act (WIIN Act; P.L. 114-322 ), Congress provided general authority for Reclamation to support new and enhanced federal and state surface and groundwater storage projects under certain, limited circumstances. Thirteen bills listed in Table 1 address groundwater recharge, storage, and recovery in various ways.