Scientists with the US Geological Survey have proposed a strategy to study the U.S. ground-water supply as part of the federal government's effort to address the nation's increasing competition for water.
Declines in ground water levels have led to concerns about the future availability of ground water, which provides half the country's drinking water and is essential to the vitality of agriculture and industry, as well as to the health of rivers, wetlands, and estuaries throughout the country.
The pumpage of fresh ground water in the United States in 2000 was estimated to be approximately 83 billion gallons per day, which is about 8 percent of the country's estimated 1 trillion gallons per day of natural recharge. A USGS study from 1960 estimated there are about 60,000 trillion gallons of ground water in storage beneath the 48 contiguous states. Assuming net recharge of 1 trillion gallons a day, about 160 years of recharge is stored in the ground.
Despite the seeming water wealth, availability varies widely across the country.
The High Plains aquifer extending from Texas north into Nebraska supplies the most water of any ground water source in the US, at 17,000 mgd, according to 2005 figures. California's Central Valley aquifer is not even a close second, at 9,800 mgd. Most of the water drawn from the two aquifers goes for irrigation. The largest public water supply is drawn from the glacial sand and gravel aquifers that extend across the northern tier of states, at just under 2,300 mgd. The California Coastal Basin aquifer provides 1,500 mgd for public water supply.
The USGS report, "Ground-Water Availability in the United States" examines what is known about ground-water availability in the United States and outlines a strategy for future national and regional studies. View the report on-line at http://pubs.usgs.gov/circ/1323/.
"An assessment of ground-water availability is critical for state and local agencies to make decisions about important issues such as drinking water, industrial and energy production, and agricultural uses," said William Alley, USGS Office of Ground Water Chief.
The approach outlined in the report is designed to provide useful regional information for state and local agencies who manage ground-water resources, while providing the building blocks for a national assessment. The report places the regional studies by the USGS Ground-Water Resources Program as the base for a long-term effort to understand ground-water availability in major aquifers across the nation.
The report contains information about 30 regional principal aquifers and five case studies to illustrate the diversity of water-availability issues. The report is written for a wide audience interested or involved in the management, protection, and sustainable use of the nation's water resources.
Determining ground-water availability is a complex process. Issues affecting ground-water vary from location to location and commonly require analysis in the context of ground-water flow systems to achieve a meaningful perspective. 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.
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. The report also builds on regional ground-water studies recently undertaken as part of the USGS Ground-Water Resources Program. The approach to national ground-water assessment is a key element of the water census of the United States, which has been proposed as part of a federal science strategy by the National Science and Technology Council Subcommittee on Water Availability and Quality.
