Laboratory Manual for Introductory Geology 4e

FIGURE 14.5 The water table separates the saturated and unsaturated zones.
Unsaturated zone
Water
table
Zone of saturation
Aquitard
A stream or lake forms where the water table intersects the
land surface.
pulls groundwater downward until it reaches an aquitard that it cannot penetrate,
and the water begins to fill pores in the aquifer just above the aquitard. More water
percolating downward finds pores at the bottom of the aquifer already filled, so it
saturates those even higher. The water table is the boundary between the saturated
zone (where all pore spaces are completely filled with water) and the unsaturated
zone, where some pores are partly filled with air (FIG. 14.5). You can model the
water table by adding water to a beaker filled with sand and watching the level of
saturation change.
The water table generally mimics the surface topography, lying at a higher elevation
under hills and a lower elevation under valleys (FIG. 14.6). In addition, its
position changes through natural processes. The water table drops during times of
drought, when there isn’t enough rain to recharge the groundwater supply. During
rainy periods, additional water causes the water table to rise. Both changes can
cause problems for those living in an area where the water table is normally close to
the surface, as we’ll see in Exercise 14.4.
FIGURE 14.6 Relationship between the water table and topography.
The dashed lines show positions of the water table under different circumstances: red 5 normal
water table position; blue 5 position during drought; yellow 5 position during rainy periods.
Well
Stream
Pond
14.4 THE WATER TABLE
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