Assessing the Nation's Ground-Water Resources

By Thomas E. Reilly

Ground water is an important part of the hydrologic cycle and has been a significant source of water to humanity since its beginnings. In the United States, roughly 130 million people rely on ground water for their drinking water. Only since the 19^th century, however, have scientific methods existed to quantitatively describe its movement and provide useful information for the management of the resource. The USGS has been key in the description and quantitative understanding of the Nation's ground-water resources.

Principal aquifers (water-bearing rock layers) of the United States. Across the Nation, 130 million people drink ground water.

During our 125-year history, USGS scientists have completed several assessments of the state of the Nation's ground water. The first was USGS Water-Supply Paper 489 published in 1923 by O.E. Meinzer, who has been called the "father of ground-water hydrology in the United States." Meinzer's publication was followed several decades later by State-by-State summaries of ground-water resources by C. L. McGuiness (USGS Circular 114; USGS Water-Supply Paper 1800), summary appraisals for 21 regions of the Nation in the 1970s (USGS Professional Paper series 813), and the Regional Aquifer-Systems Analysis (RASA) Program, which evaluated 25 of the Nation's most important regional ground-water systems (USGS Professional Paper series 1400 through 1425). The recent map of principal aquifers of the United States is available at http://www-atlas.usgs.gov/atlasmap.html and on the interactive National Atlas Map Web site at http://www.nationalatlas.gov/. These national and regional evaluations have increased our knowledge about the Nation's ground-water resources. Assessing ground-water resources is an evolving process, because the technology, the data available, the places ground water is used, the amounts of water needed for supply, and the issues of concern all change over time.

In order to understand the status of a ground-water system, basic information on the geologic framework, boundary conditions, hydraulic head (water level) distribution, chemical distribution, transmitting properties, and storage properties must be known to a reasonable degree. Each individual ground-water system is unique in that the source and amount of water flowing through the system depend on external factors such as rate of precipitation, location of streams and other surface-water bodies, and rate of evapotranspiration. Human activities, such as irrigation and water withdrawals, change the natural flow patterns, and these changes must be accounted for in the calculation of water availability. A good summary of the issues and challenges is presented in USGS Circular 1186, Sustainability of Ground-Water Resources, available at http://water.usgs.gov/pubs/circ/circ1186/.

Assessing the impacts of ground-water withdrawals relies on mathematical equations based on the laws of physics that allow for quantities of flow to be estimated. During the past several decades, computer simulation models for analyzing flow and solute transport in ground-water and surface-water systems have played an increasingly important role in the evaluation of the development and management of the Nation's ground-water resources. Ground-water models are an attempt to represent the essential features of the actual ground-water system by means of a mathematical counterpart. The USGS has contributed greatly to the development of these methods. For example, the USGS computer model "MODFLOW" (http://water.usgs.gov/pubs/twri/twri6a1/ and http://water.usgs.gov/nrp/gwsoftware/modflow.html) is considered to be the most widely used computer program for simulating ground-water flow in the world. Quantitative scientifically based estimates (computer simulations) of the effects of various hydrologic stresses and conditions are a mainstay of USGS studies of ground-water resources. Results of these studies are useful to the water-management agencies to guide future use of the resource.

Our understanding of a specific ground-water system that is continuous in space and time will always be approximate. Understanding of the ground-water flow and transport system improves as more data are collected and better computer simulation tools become available. Data are collected and analyzed, which in turn indicates where information is lacking and which simulation techniques need improvement. Data are then collected again based on the understanding gained in the previous analysis, and improved simulation techniques are used to analyze the information to the extent possible. This increased understanding of ground-water flow through a continuing iterative approach of data collection and analysis is the heart of the scientific method.

The availability of credible scientific information leads to improved management decisions by decision makers. This scientific effort requires a long-term commitment to obtain reliable data and improved methods of analysis. The USGS will continue to be important to this long-term commitment in understanding and quantifying the Nation's ground-water resources.