How Do Hydrologists Locate Ground Water?

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.

Geology is the key

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.

Existing wells provide clues

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.

How ground water occurs in rocks

Ground water is simply the subsurface water that fully saturates pores or cracks in soils and rocks. Aquifers are replenished by the seepage of precipitation that falls on the land, although they can be artificially replenished by people, also. There are many geologic, meteorologic, topographic, and human factors that determine the extent and rate to which aquifers are refilled with water.

Can you dowse?

Do you recognize this? It is a dowsing stick, one of a number of items that some people use in an attempt to locate underground water. Find out how these methods are used.

Sources and more information

♦ Water Dowsing, USGS General Information publication
♦ Appraising the Nation's Ground-Water Resources, USGS General Information publication

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