Page 347 - Laboratory Manual in Physical Geology

FIGURE 12.4 Water flow through fractures. Looking
east toward the Arkansas River from Vap’s Pass, Oklahoma
(15 miles northeast of Ponca City). The Fort Riley Limestone
bedrock crops out (is exposed at the surface) here. There is no FIGURE 12.5 Stalactites. These stalactites formed on part
soil, but plants have grown naturally along linear features in of the ceiling of Cave of the Winds, which has formed in Paleozoic
the bedrock. limestones near Manitou Springs, Colorado.

The Floridan Aquifer Land Subsidence Hazards Caused by

FIGURE 12.6 shows karst features developed in the Floridan Groundwater Withdrawal

Limestone Aquifer in the northern part of Tampa, Florida. Land subsidence caused by human withdrawal of ground-
Notice the abundant lakes and ponds. They are mostly water is a serious problem in many places throughout
sinkholes, which are filled with water and surrounded by the world. For example, in the heart of Mexico City, the
hachured contour lines (contours with small tick marks land surface has gradually subsided up to 7.6 m (25 ft).
that point inward, indicating a closed depression). By At the northern end of California’s Santa Clara Valley,
determining and mapping the elevations of water surfaces 17 square miles of land have subsided below the highest
in the lakes, you can determine the slope of the water tide level in San Francisco Bay and now must be pro-
table and the direction of flow of groundwater here (as in tected by earthworks. Other centers of subsidence include
FIGURE 12.1B ). Houston, Tokyo, Venice, and Las Vegas. With increasing

withdrawal of groundwater and more intensive use of the
land surface, we can expect the problem of subsidence to
become more widespread.
Subsidence induced by withdrawal of ground water com-
ACTIVITY monly occurs in areas underlain by stream-deposited (allu-

12.4 Land Subsidence from vial) sand and gravel that is interbedded with lake-deposited
(lacustrine) clays and clayey silts ( FIGURE 12.7A ). The sand-
Groundwater Withdrawal and-gravel beds are aquifers, and the clay and clayey silt beds

THINK | What can happen if ground water are confining beds.
About It is withdrawn faster than it is
replenished? Subsidence in Unconfined and Confined
Aquifers

OBJECTIVE Evaluate how groundwater withdrawal In FIGURE 12.8 , the water in the lower aquifer (“sand and

can cause subsidence (sinking) of the land. gravel”) is confined between impermeable beds of clay
and silt and is under pressure from its own weight. Thus,
PROCEDURES water in wells A and C rises naturally from the confined
1. Before you begin , read Land Subsidence Hazards aquifer to the potentiometric (water-pressure) surface . Such
Caused by Groundwater Withdrawal below. Also, wells are termed artesian wells (water flows naturally
this is what you will need : from the top of the well). The sand in the water table

____ calculator aquifer ( FIGURE 12.8 ) contains water that is not confined

____ Activity 12.4 Worksheet (p. 327 ) and pencil under pressure, so it is an unconfined aquifer (also

called a water table aquifer ) The water in well B stands
.
2. Then follow your instructor’s directions for
completing the worksheet. at the level of the water table and must be pumped up to
the land surface.
Groundwater Processes, Resources, and Risks ■ 317