Page 349 - Laboratory Manual in Physical Geology

Lowered land surface
Water table Water table
Well

Sand and
gravel

Artesian aquifer
Clayey silt
Confining bed

A. Before extensive pumping of well B. After pumping
FIGURE 12.7 Before (A) and after (B) extensive pumping of a well. Note in B the lowering of the water-pressure surface, compaction
of confining beds between the aquifers, and resulting subsidence of land surface. Arrows indicate the direction of compaction caused by the
downward force of gravity, after the opposing water pressure was reduced by excessive withdrawal (discharge) of groundwater from the well.

Land surface
Well A Stream
Well B
Well C
Potentiometric (water-pressure) surface
Sand
Water table

Sand UNCONFINED AQUIFER
(Water table aquifer)

Clay and silt
CONFINING BED
Sand and gravel
CONFINED AQUIFER
CONFINING BED Clay and silt
Consolidated rock

FIGURE 12.8 Geologic cross section of unconfined and confined aquifers. Vertical scale is exaggerated.

Land subsidence ( FIGURE 12.7B ) is related to the Subsidence in the Santa Clara Valley

compressibility of water-saturated sediments. Withdrawing
The Santa Clara Valley ( FIGURE 12.9 ) of California is a very

water from wells not only removes water from the system,
important center of agriculture that depends on ground-
but it also lowers the potentiometric surface and reduces
water for irrigation. It was one of the first areas in the
the water pressure in the confined artesian aquifers. As
United States where land subsidence due to withdrawal of
the water pressure is reduced, the aquifer is gradually
groundwater was recognized. The Santa Clara Valley is a
compacted and the ground surface above it is gradu-
large structural trough filled with alluvium (river sedi-
ally lowered. The hydrostatic pressure can be restored by
ments) more than 460 m (1500 ft) thick. Sand-and-gravel
replenishing (or recharging ) the aquifer with water. But
aquifers predominate near the valley margins, but the
the confining beds, once compacted, will not expand to
major part of the alluvium is silt and clay. Below a depth
their earlier thicknesses.
of 60 m (200 ft), the groundwater is confined by layers of
clay, except near the margins.
Groundwater Processes, Resources, and Risks ■ 319