Groundwater
Pollution
1
Water Pollution
III
•
Types of Aquifers
•
Groundwater Flow Physics
!
Darcy’s Law and Hydraulic Gradient
!
Movement of Underground Plumes
!
Cleanup
•
Stormwater Flows
!
Contamination Sources
!
Impacts, Solutions
2
•
1980 Comprehensive Environmental
Response, Compensation and Liability
Act (CERCLA), more commonly known
as Superfund.
•
300,000 – 400,000 sites need
remediation. Many can be cleaned up
for ~$100,000, but some require
millions. Total ~0.5–1 trillion dollars.
!
The annual Gross Domestic Product (GDP)
is currently 10 trillion dollars.
Groundwater Pollution Facts
3
• To date, roughly 10 billion has been
spent on superfund site remediation.
• 1984 Resource Conservation and
Recovery Act (RCRA). This legislation
hopes to avoid more superfund sites.
4
Groundwater Structure
5
Unconfined aquifers: typically bounded
below by a nonporous layer but are not
bounded above.
Confined aquifers: sandwiched between
two rock layers.
6
The quantity of water in an aquifer is
determined by its porosity.
!
Fails to account for the amount of water
that will be retained due to surface tension
at the solid interfaces.
• The specific yield tells the amount of
water that can actually be withdrawn
(of the total volume).
Porosity
!
( )
=
volume of voids
volume of voids and solids
7
Material
Porosity (%)
Specific Yield (%)
Clay
45
3
Sand
34
25
Gravel
25
22
Gravel and Sand
20
16
Sandstone
15
8
Limestone or Shale
5
2
Quartz or Granite
1
0.5
For an aquifer of sand with cross sectional area
of 1 m
2
and depth of 2 m, the volume of water is:
Volume of water
=
Porosity
(
)
Vol. of material
(
)
=
0.34
!
2 m
3
=
0.68 m
3
Yield of water
=
Specific yield
(
)
Vol. of material
(
)
=
0.25
!
2 m
3
=
0.5 m
3
8
Groundwater Flow
The hydraulic gradient is defined as:
Where
h
i
are the vertical heights
L
is the horizontal distance.
The gradient is dimensionless, and can be
expressed as:
Hydraulic gradient
=
h
2
!
h
1
L
Hydraulic gradient
=
dh
dL
9
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Darcy’s Law for flow through porous media
Q = flow rate (m
3
/day)
K = hydraulic conductivity (m/day)
A = crosssectional area (m
2
)
Material
Hydraulic Conductivity (m/
day)*
Clay
0.0004
Sand
41
Gravel
4100
Gravel and Sand
410
Sandstone
4.1
Limestone, Shale
0.041
Quartzite, Granite
0.0004
*Note that the actual values can vary by several orders
of magnitude, and aquifers are highly nonhomogeneous.
Q
=
KA
dh
dL
10
Example
A confined aquifer 20 m thick has two wells
spaced 500 m apart, and the difference in the
water levels at the two wells is 2 m.
The
hydraulic conductivity is 50 m/day (assume an
arbitrary width of 1 m).
What is the flow rate?
Q
=
KA
dh
dL
=
50
m
day
!
"
#
$
%
&
1 m
(
)
20 m
(
)
2 m
500 m
=
4
m
3
day
(per meter of width)
11
From Darcy’s Law,
we can get the Darcy Velocity:
Q
=
KA
dh
dL
v
=
Q
A
12
•
Darcy’s Law Velocity is for flow
through the total porous media per m
3
of aquifer volume
!
Underestimates the actual groundwater
flow in proportion to the porosity.
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 Spring '09
 Jeffery
 Aquifer, San Fernando Valley

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