Page 294 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

258
G roundwater forms when rainfall and snowmelt seep down through the soil and accumulate in the pore spaces in fractured bedrock or sediment. GIA 9.1
shows the structure of groundwater deposits, called aquifers, and threats to this resource from pollution and overuse.
GIA 9.2 shows how groundwater helps to maintain streamflow.
9.1
THE WATER TABLE AND AQUIFERS
The water table is a boundary between the zones of aeration and saturation. Beneath the zone of saturation, an impermeable layer of rock blocks further downward movement of water. An aquifer contains groundwater stored in the zone of saturation.
An artesian spring in Manitoba,
[Gilles DeCruyenaere/Shutterstock.]
(3) Slope and flow
(1) Zone of aeration
In this layer, some pore spaces contain air.
Dead vegetation Landfill
(2) Zone of saturation
In this layer, water fills the spaces between particles of sand, gravel, and rock.
The water table follows the slope of the land surface above it. The water in an aquifer flows toward areas of lower elevation and lower pressure. A plume of water pollution from septic systems or landfills can flow through an aquifer, contaminating wells.
Seepage
1 Zone of aeration
Dry well
6
Impermeable rock
(6) Dry wells and aquifer overuse
Wells pump groundwater to the surface, lowering the water table. Overuse, or groundwater mining, occurs when drawdown exceeds an aquifer’s recharge capacity. A dry well results if a well is not drilled deep enough or if the water table falls below the depth of the well.
3
Septic system
Septic system
(4) Unconfined aquifer
An unconfined aquifer has a permeable layer above, and an impermeable layer beneath.
Perched water table
Original water table
Spring
Stream
Effluent condition
2
Zone of saturation
Active well soon to go dry
4
Cone of depression 5 Active well
Aquiclude
(5) Wells in an unconfined aquifer
Unconfined 7 aquifer
(7) Aquicludes and Springs
An aquiclude is a layer of impermeable rock or unconsolidated material that prevents water from seeping farther down. Springs form where the perched water table intersects the surface.
8
(8) Water table at the surface
The water in an unconfined aquifer is not under pressure and must be pumped to the surface.
Streams, lakes, and wetlands form where the water table intersects the surface.
Infer: In which direction does the water flow in this aquifer? How can you tell?
Visit the Study Area in MasteringGeographyTM to explore groundwater.
Visualize: Study a geoscience animation of Assess: Demonstrate understanding of groundwater, the water table, and aquifers. groundwater (if assigned by instructor).
geosystems in action 9GROUNDWATER
d
e
r
e
W
a
t
w
e
r
o
l
t
a
b
l
e