Appraising the Nation's Ground-Water Resources
Although there are sizable areas where ground water is being withdrawn at rates that cause water levels to decline persistently, as in parts of the dry Southwest, this is not true throughout the country. For the Nation as a whole, there is neither a pronounced downward nor upward trend. Water levels rise in wet periods and decline in dry periods. In areas where water is not pumped from aquifers in excess of the amount of recharge to the aquifer--particularly in the humid central and eastern parts of the country--water levels average about the same as they did in the early part of the twentieth century.
A major responsibility of the U.S. Geological Survey is to assess the quantity and quality of the Nation's water supplies. The Geological Survey, in cooperation with other Federal, State, and local agencies, maintains a nationwide hydrologic-data network, carries out a wide variety of water-resources investigations, and develops new methodologies for studying water. The results of these investigations are indispensable tools for those involved in water-resources planning and management. Numerous inquiries concerning water resources and hydrology are directed to the Survey and to State water-resources and geological agencies.
To locate ground water accurately and to determine the depth, quantity, and quality of the water, several techniques must be used, and a target area must be thoroughly tested and studied to identify hydrologic and geologic features important to the planning and management of the resource. The landscape may offer clues to the hydrologist about the occurrence of shallow ground water. Conditions for large quantities of shallow ground water are more favorable under valleys than under hills. In some regions--in parts of the arid Southwest, for example--the presence of "water-loving" plants, such as cottonwoods or willows, indicates ground water at shallow to moderate depth. Areas where water is at the surface as springs, seeps, swamps, or lakes reflect the presence of ground water, although not necessarily in large quantities or of usable quality.
Rocks are the most valuable clues of all. As a first step in locating favorable conditions for ground-water development, the hydrologist prepares geologic maps and cross sections showing the distribution and positions of the different kinds of rocks, both on the surface and underground. Some sedimentary rocks may extend many miles as aquifers of fairly uniform permeability. Other types of rocks may be cracked and broken and contain openings large enough to carry water. Types and orientation of joints or other fractures may be clues to obtaining useful amounts of ground water. Some rocks may be so folded and displaced that it is difficult to trace them underground.
Next, a hydrologist obtains information on the wells in the target area. The locations, depth to water, amount of water pumped, and types of rocks penetrated by wells also provide information on ground water. Wells are tested to determine the amount of water moving through the aquifer, the volume of water that can enter a well, and the effects of pumping on water levels in the area. Chemical analysis of water from wells provides information on quality of water in the aquifer.
Evaluating the ground-water resource in developed areas, prudent management of the resource, and protection of its quality are current ground-water problems. Thus, prediction of the capacity of the ground-water resource for long-term pumpage, the effects of that pumpage, and evaluation of water-quality conditions are among the principal aims of modern-day hydrologic practice in achieving proper management of ground water.
Ground water, presently a major source of water, is also the Nation's principal reserve of fresh water. The public will have to make decisions regarding water supply and waste disposal-decisions that will either affect the ground-water resource or be affected by it. These decisions will be more judicious and reliable if they are based upon knowledge of the principles of ground-water occurrence.