Ground-Water Availability in the United States

Ground water is one of the Nation's most valuable natural resources. It occurs almost everywhere beneath the Earth's surface and is a major source of water supple worldwide. Ground water has a crucial role in sustaining streamflow between precipitation events and especially during protracted dry periods. In addition to human uses, many ecosystems are dependent on ground-water discharge to streams, lakes, and wetlands. Although humans have been digging wells and tunnels for water supply for thousands of years, extensive use of ground water is relatively recent, with the advent of rural electrification and more pumping technologies during the past 75 years. A growing awareness of ground water as a critical natural resources leads to some basic questions. How much ground water do we have? Are we running out? Where are ground-water resources most stressed by human development? Where are the resources most available for future supplies? Although these questions seem simple, providing the answers is complex because a meaningful assessment of ground-water availability in the United Stated requires a multidisciplinary evaluation of the hydrologic system, as well as an understanding of the different water issues taht exist across the Nation. Furthermore, the information available to support a broad assessment of the resource varies across the Nation (Alley, 2006). During the past century, several ground-water assessments have been completed by the U.S. Geological Survey (USGS) on a national scale. The first of these assessments was completed by O.E. Meinzer (1923) who has been called the "father of ground-water hydrology" (Lohman, 1986, p. 51). Meinzer's publication was followed several decades later with State-by-State summaries on ground-water resources (McGuinness, 1951 and 1963); by summary appraisals for 21 regions of the Nation in the 1970s (U.S. Geological Survey Pro-fessional Papers 813A– U); a State-by-State summary (U.S. Geological Survey, 1985); and by the Regional Aquifer-System Analysis (RASA) Program in which 25 of the Nation's most important regional ground-water systems were evaluated (Sun and Johnston, 1994). These national and regional evaluations have improved our knowledge about the Nation's ground-water resources. Repeated evalua-tions of the resource through time are needed as new information on ground-water resources and connected surface-water systems becomes available; new methods and technologies for resource assessment are developed; and the places ground water is used, water demands, and the issues of concern change with time. Environmental decision making has grown more complex with society demanding ample water for human use along with environmental protection and preservation at the same time. When O.E. Meinzer (1923) published his first national ground-water assessment, indoor plumbing still was not commonly used, low capacity wells were sufficient for most purposes, and the population of the country was more dispersed. Today, lifestyles generally require large amounts of water and a complex infrastructure to deliver water to urban and suburban population centers. Even if water resources are abundant regionally, heavy water use in centralized areas can create local stresses. As water-related problems evolve in complex ways, an up-to-date and comprehensive evaluation of ground-water resources that builds on the foundation of previous studies is needed to meet society's ever-changing water demands. A goal of ground-water resource assessment is to provide information on the current status of the resource that provides insights about the future availability of ground water. Ground-water management decisions in the United States are made at a local level, such as the State, municipality, or a special district formed for water-resources management. Many aquifer systems cross these political boundaries. Thus, a key role of national and regional assessments is to provide consistent and integrated information across political boundaries that is useful to those who use and manage the resource. The State and local agencies manage the water-resources system and collect and analyze local data. Federal scientific agencies support this function by developing methods of analysis and analyzing the water-resources system across political boundaries. This partnership between State and local agencies and the USGS enables the resource to be understood on a multi-State, regional, and national basis. With these considerations in mind, the purpose of this report is to identify the challenges in determining ground-water availability, summarize the current state of knowledge from a national perspective, and outline an approach for developing the needed understanding of future water availability. This report is an outgrowth of a pilot study, National Assessment of Water Availability and Use, that began in 2005 at the request of Congress (Barlow and others, 2002). The report also builds on regional ground-water availability studies recently undertaken as part of the USGS Ground-Water Resources Program (Dennehy, 2005). The approach to national ground-water assessment described in the section "Regional-Scale Approach to National Assessment" of this report, is a key element of the water census of the United States, which has been proposed as a strategic science direction of the USGS (U.S. Geological Survey, 2007), as well as part of the proposed Federal science strategy to meet nationwide water challenges by the National Science and Technology Council (2007) Subcommittee on Water Availability and Quality.