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Groundwater
Chapter 10
I. Importance of groundwater
A. Groundwater is water found in the pores of
soil and sediment, plus narrow fractures in
bedrock
B. Where is fresh water - 2 % of all water is fresh
water. Of this Fresh Water:
– Glaciers – 85% (not really accessible)
– Groundwater – 14%
– Surface water - < 1 %
I. Importance of groundwater
C. Importance of groundwater
• Important source of drinking and irrigation water
– In CA – 80% of used fresh water is for irrigation.
– In CA – almost half comes from groundwater.
• Easily contaminated – Ex. Gas stations, dry
cleaners, Navy bases.
• Contributes water to streams and lakes
• Over pumping causes subsidence (lowering of
land)
• Erosion – makes: caves and sinkholes (collapsed
roofs of caves).
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II. Distribution of groundwater
A. Belt of soil moisture – water held by soil
particles at the near-surface. (plants)
B. Zone of Aeration (Unsaturated Zone)
• Area under Belt of Soil Moisture but above the
water table (PLINKO!!!!!!)
• Air and water in pore spaces (voids between
the grains)
• Water cannot be pumped by wells
II. Distribution of groundwater
C. Zone of Saturation
• Formation - Water not held as soil moisture
percolates downward
• Water reaches a zone (place) where all open
pore spaces are completely filled with water
(no air)
• Water within the pores is called groundwater
• Water table – the top of Zone of Saturation
Figure 10.3
fig_11_02a
II. Distribution of groundwater
D. Water Table
• Level of water table changes.
– Varies season to season
– Year to year depending on amount of precipitation received.
• Water table has a slope.
• Groundwater flows down slope. (just like
surface water)
• This slope usually loosely mimics surface
slope. (huge implications for groundwater
contamination)
II. Distribution of groundwater
E. Groundwater Flows into Surface Water
• In wetter climates, water table is higher than
streams – so groundwater flows into the
streams and streams get bigger downstream.
• These are called Gaining Streams.
• Most streams in CA are Gaining Streams –
except in dry southeastern deserts.
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II. Distribution of groundwater
Gaining and losing streams
• In dry climates, like CA’s deserts, water table is
lower than stream, and stream gets smaller
down stream, eventually drying out.
• Called Losing Streams
Figure 10.5
III. Movement of groundwater
A. Groundwater Flow - Darcy’s Law
– Groundwater flows very slowly – about 6
feet per day in CA’s Central Valley.
– Hydraulic gradient – the water table slope
– Energy for the movement is provided by the
force of gravity
– Darcy’s Law – if permeability remains uniform,
the velocity of groundwater will increase as the
slope of the water table increases
III. Movement of groundwater
B. Factors Affecting Flow
– Aquifer –rock or sediment that holds groundwater
and can provide groundwater if pumped from a well
– Porosity – percentage of aquifer that are pore
spaces. High = 50%; Low = 15%
• Determines how much water can be held there.
– Permeability – the ability of groundwater to flow
through aquifer.
– Aquitard – an impermeable layer that hinders or
prevents water movement (such as clay)
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III. Movement of groundwater
C. Springs
– Occur where the water table intersects
Earth’s surface
– Natural outflow of groundwater
IV. Groundwater Extraction and
Contamination
A. Wells
– Drill hole, place pipe with slots, pour
sand
– A well must penetrate below the water
table
– Pumping of wells can cause
– Drawdown (lowering) of the water table
– Cone of depression in the water table – localized
drawdown near a well
Formation of a cone of depression
IV. Groundwater Extraction and
Contamination
B. Withdrawal and Recharge
– Recharge – water flowing into the
aquifer.
– Withdrawal – water pumped out
– When withdrawal exceeds recharge =
drawdown.
– To0 much drawdown, can:
•
•
IV. Groundwater Extraction and
Contamination
1) Subsidence
• Ground sinks when water is pumped from
withdrawal exceeds recharge.
make well go dry
Cause subsidence
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Subsidence in the
San Joaquin Valley
of California
Subsidence in Central
Valley
Figure 10.15
IV. Groundwater Extraction and
Contamination
Groundwater contamination
C. Groundwater Contamination
1. From Gas Stations and Dry Cleaners
• Leaking underground storage tanks of fuel and
chemicals into soil and groundwater
2. From Industry (ex. Chevron Refinery)
•
Leaking chemical tanks
3. From Agriculture – fertilizers and pesticides
V. Hot springs and geysers
A. Hot springs
• The water for most hot springs is heated by
cooling of igneous rock
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Mammoth Hot Springs, Yellowstone
National Park
V. Hot springs and geysers
B. Geysers
•
Hot water erupts with great force
• Found over hot igneous rocks
Figure 10.20
V. Hot springs and geysers
VI. Caves and Karst Topography
C. Geothermal Energy –
–
–
Use the hot water to generate electricity
Renewable clean energy resource. But not much of it
and hard to find.
•
•
Ex. The Geysers Power Plant north of the Bay.
Link to QUEST video
A. Caves
– Rain water and groundwater are slightly acidic so
they dissolve limestone rock making a “hole” in rock
– cave or cavern.
– Often form in the Zone of Aeration
– Limestone is precipitated in caves as stalactites
(hanging from the ceiling) and stalagmites
(form on the floor of a cavern)
Geologic work of groundwater
B. Karst topography
• Landscapes on top of limestone dissolved by
groundwater
• features include
– Sinkhole or sinks - collapsed cave roofs
– These can makes depressions or small lakes
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