On March 22, 2014, a major landslide occurred near Oso,
Washington. Death toll currently at 30, with 15 still missing.
Groundwater
Before and After Swipe
http://bit.ly/PeN1JT
N. Fork Stillaguamish River
Groundwater: Outline
1. Groundwater intro and activity
Thunder River, Grand Canyon
Oct 2006
The Water Table
Top of the saturated zone
2. Water table and groundwater flow
Configuration similar to
ground surface
3. Darcy’s Law and the driver for groundwater
movement
Water level of most lakes
and rivers corresponds to
the water table
4. Groundwater storage: aquifers & aquitards
Varies with time
(season, rainfall, ...
4. Groundwater problems (e.g., contamination,
pumping, etc.)
Gradient: change in water level
elevation/change in distance
Groundwater Movement
Recharge: the infiltration of water into any subsurface formation
Groundwater Movement
Water table mimics topography, it is not flat
Its elevation varies with time
Discharge: the opposite of recharge; the exit of groundwater to
the surface
1
Draw the water table
How might you draw the water table in this region?
What drives groundwater flow?
Hydraulic gradient
?
Horizontal hydraulic gradient
610 ft
h1=550 ft
Well 1
Well2
Friday’s in-class activity
610 ft
60 ft
dh
100 ft
(depth
to water)
L
h2=500 ft
200 ft
Hydraulic gradient = water level difference / distance
dh/L
= (550 – 500) / 200 = 0.25
Flow from well 1 to well 2
How fast does groundwater move?
Groundwater: Outline
2. Water table and groundwater flow
Rate controlled by 2 things:
1. Driving force
 “gradient of potential energy”
 think of the water table’s slope
3. Darcy’s Law and the driver for groundwater
movement
2. permeability
 How easily water moves through subsurface
1. Groundwater intro and activity
4. Groundwater storage: aquifers & aquitards
4. Groundwater problems (e.g., contamination,
pumping, etc.)
Rates extremely variable
< 1 cm/year to km’s/year
2
Porosity - the percentage of rock or sediment
that are voids or openings…A measurement of a
Porosity - the percentage of rock or sediment
that are voids or openings
rock’s ability to hold water
Permeability – rocks’ ability to transmit
fluid…degree of interconnectedness of
pore spaces
A measurement of a rock’s ability to hold water
– Loose sand may have 30-50% porosity
– Compacted sandstone may have only 10-20%
porosity
– Low in metamorphic and igneous rocks
Permeability – rocks’ ability to transmit
fluid
Depends on
grain size/arrangement
interconnectedness of pore spaces
High permeability: gravels, sands, limestones with
dissolution channels, fractured rocks
Low permeability: clays, unfractured metamorphic and
igneous rocks, limestones without dissolution
Porosity (and permeability) can be increased
by fracturing and dissolution
- secondary porosity
Clicker question
Vesicular rhyolite is likely to be ________.
A. Both porous and
permeable.
B. Porous but not
very permeable.
C. Permeable but
not very porous.
3
How fast does groundwater move?
Rate controlled by 2 things:
1. Driving force
 “gradient of potential energy”
 think of the water table’s slope
Darcy’s Law
Q = discharge (per unit area) (m/day)
K = coefficient of permeability (m/day)
h1 = beginning height (m)
(h – h )
Q=K 1 2
L
h2 = ending height (m)
L = distance (m)
Flow rate
2. permeability
 How easily water moves through subsurface
Rates extremely variable
< 1 cm/year to km’s/year
L
Groundwater: Outline
How does most groundwater exist
within the subsurface?
1. Groundwater intro and activity
2. Water table and groundwater flow
A. As large pools of water within large
pockets (caverns) in rocks.
3. Darcy’s Law and the driver for groundwater
movement
B. Within cavernous regions occupied by
rivers of water that flow underground.
4. Groundwater storage: aquifers & aquitards
C. Within the small open spaces between
the grains of rocks and sediments.
4. Groundwater problems (e.g., contamination,
pumping, etc.)
D. Both A and B
E. None of the above
Aquifer: geologic unit that stores and is capable of
Unconfined Aquifer
transmitting significant water, must be both porous
and permeable
At shallow depth
Water table is the top boundary
Recharged by precipitation infiltrating
Aquitard: low permeability rock formations that
retard (or slow down) the flow of water to or
from an adjacent aquifer
Confined Aquifer
Deeper
Confined on top by a less permeable layer
Recharged slowly
4
Perched water table
An aquifer must be:
a. both permeable and porous.
b. neither permeable nor porous.
c. permeable, but not necessarily porous.
d. porous, but not necessarily permeable.
Above the main water table
In unsaturated zones
Due to lenses of less permeable rocks (e.g., shales or
clays)
Artesian (confined) aquifer:
City water systems can be considered artificial
artesian systems.
under high pressure
Water in wells rises above the top of the aquifer.
Well #___ is a flowing artesian well and
well #___ is a well that must be pumped
from an unconfined aquifer?
Spring
Location where the
water table intersects
the surface or where
groundwater is
discharged in general
(Assume that only the very bottom of the well is open)
A) 1, 2
B) 2, 3
C) 3, 4
D) 1, 4
E) 2, 4
Marble Canyon
Grand Canyon National Park
5
Groundwater: Outline
1. Groundwater intro and activity
Problems Associated
with Groundwater
2. Water table and groundwater flow
3. Darcy’s Law and the driver for groundwater
movement
4. Groundwater storage: aquifers & aquitards
4. Groundwater problems (e.g., contamination,
pumping, etc.)
Drawdown due to Over Pumping
If pumping > recharge
Subsidence due to groundwater
withdrawal in Pahrump Valley, Nevada
• Overuse
• Subsidence
• Contamination
- Salt water intrusion
- Hard water, pollutants
Friday’s in-class activity
If pumping > recharge
Subsidence due to groundwater
withdrawal in Pahrump Valley, Nevada
6
If pumping > recharge
If pumping > recharge
Subsidence due to groundwater
withdrawal in Pahrump Valley, Nevada
Subsidence due to groundwater
withdrawal in Pahrump Valley, Nevada
If pumping > recharge
If pumping > recharge
Subsidence: when water is
removed, sediment compacts,
lowering ground surface.
San Joaquin Valley
subsidence
Subsidence due to groundwater
withdrawal in Pahrump Valley, Nevada
Denver Basin
Cross Section
Denver
Basin
Aquifers
West
As of Feb 2001:
~34,000
water wells were
drilled in the
Denver Basin
East
Non-renewable
resource
Barkmann, 2004
Barkmann, 2004
7
Ogallala Aquifer: Largest in the U.S.
Setting of the Ogallala Aquifer
First used for irrigation
in the late 1800s
1950s: large-scale
exploitation
Unconfined aquifer; water levels
dropping in south; some parts
may run dry (graphs below)
Today: ~170,000 wells
used for irrigation
Water table ~200’ lower
in some areas!
17.11.a
Ogallala Water Supplies and Usage
17.11.b-c
Water Quality
and Groundwater Movement
More rain in east
Thickest in north
Water table decline
greatest in south
(Kansas & Texas)
Groundwater Contamination
Sources:
Pesticides/herbicides/fertilizers
Landfill pollutants
Sewage
Acid mine drainage
Radioactive waste
Oil and gasoline
Salt water intrusion
Saltwater Intrusion
8
Question: Why is ground water
flow direction changing
here?
A) Pumping lowers water table so
that it is tilted towards
community well
B) Bacteria expand and push
water away from septic tank
C) Drought causes water table
to become lower
D) Septic wastes always flow
towards wells open to the air
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