Groundwater
Diana L. Duckworth
Rustburg High School
Campbell County Schools
Key Concepts
• Porosity: percentage of the rock that is
empty space
– Depends on grain size, shape, & sorting
• Permeability: ability of a rock to yield
water
– Depends on grain size (larger grains have
larger pores), grain shape, & sorting
• Large pores, less impact of friction
• Rounded grains, larger pores
• Poor sorting, small grains fill in pores & reduce
permeability
Distribution of Water Underground
Zone of Aeration
Pores filled with air
Water Table
Surface of Zone of
Saturation
Pore space filled
With water
Water infiltrates ground when it rains. Percolates down
under the force of gravity.
Water table reaches surface at springs,
rivers, & lakes
• Water table follows the shape of the land.
• Water flows downhill underground.
Lakes
River
Side View
Water flows from high ground into
streams, lakes & marshes.
Man’s Usage of Groundwater
• Two kinds of wells:
– Ordinary well that requires pumping
– Artesian well where water rises to surface
without pumping
Recharge Area
Aquaclude
Artesian Aquifer
Ordinary Wells
Static
• Two water levels
– Static water level
– Pumping water level
Pumping
Drawdown
Side View
Cone of
depression
Problems with Overexploitation
• Withdrawals in excess of recharge rates:
– Artesian wells lose pressure, must be pumped
– Ordinary wells can produce overlapping
cones of depression
• Lead to dewatering of aquifer
• Lower stream levels in drought
• Salt water intrusion in coastal areas
island
lagoon
ocean
Salt water table
Fresh water
floats on salt water
Perched water table
Spring
Aquaclude
Hillside Springs
Roaring Springs, North Rim
Grand Canyon
Groundwater Budget
• Income = precipitation
• Outgo = runoff + evapotranspiration
• Surplus: precipitation is greater than evapotranspiration & runoff
– Water table rises
• Deficit: precipitation is less than evapotranspiration & runoff
– Water table falls
• Steady state: precipitation = evapotranspiration &
runoff
• Water table stays steady
• Water Balance Equation P = E + R + G
– Where G = change in water table
Caves
• CO2 dissolves in
groundwater
– Becomes acidic
– Dissolves limestone
– Joints enlarged into
caves
• Dry caves
– Dripping water
evaporates
– Creates stalactites &
stalagmites
Sinkholes
• Roof of cave collapses
Map view
– Leaves circular
depression
– May fill with water
sinkhole
cave
SIDE VIEW
Groundwater Topography
• Karst topography
–
–
–
–
Sinkholes
Caverns
Disappearing streams
Ponds with no surface
outlet
– Limestone terrain
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