ERT 246- HYDROLOGY AND WATER
RESOURCES ENGINEERING
GROUNDWATER
Siti Kamariah Md Saat,
PPK Bioprocess 2010
Groundwater
• Groundwater is part of the water cycle. It is
water that is located beneath the earth’s surface
in pores and crevices of rocks and soil.
• The process of water entering the ground to
become groundwater is called recharge.
• The volume of recharge and how fast it enters
groundwater is dependent on climate, the depth
to the water table, the types of plants that use
water in the soil and the types of soil and rock it
must pass through.
Groundwater flow
• Because groundwater has to move between
pores and crevices in soil and rock, it moves
much more slowly than surface water.
• Water can move down a river in hours, days or
perhaps weeks.
• Groundwater in an aquifer may take ten, one
hundred or many thousand years to flow through
an aquifer.
• Management of groundwater needs to consider
the amount of water going into the aquifer, how
much is stored in the aquifer, and how long it
takes to move through it.
Groundwater Flow
• Groundwater velocity
– Depends on permeability and hydraulic
gradient (slope of water table)
– Ranges from 100 m/day to mm/day
Groundwater problem and issue
• Groundwater is a complex resource. The unseen
nature of groundwater makes it difficult to
quantify, however careful monitoring and
management of groundwater resources helps to
guard against over-extraction and ensure
reserves do not become stressed or drop below
sustainable levels.
• It may also be impacted by climate change.
However, by recording water levels and
assessing recharge rates, it is possible to betterunderstand the resource and the impacts that
climate change may have upon it.
Groundwater problem and issue
• While groundwater can be a reliable
source of water, its overuse can result in
failure of supply.
• The risk of loss of supply or changes to
groundwater quality is increased as
surface water systems become fully
allocated and more people access
groundwater to meet their water needs.
Soil Water
Intermediate Vadose Water
Capillary Water
Groundwater
Water in Unconnected Pores
Bound Water in Minerals
Interstitial Zone
Saturated
Unsaturated
Water Profile
Subsurface Flow
• Infiltration
– flow entering at the ground surface
• Percolation
– vertical downward unsaturated flow
• Interflow
– sub-horizontal unsaturated and perched saturated flow
• Groundwater flow
– sub-horizontal saturated flow
Groundwater zones
• Water in the ground is found in three
general zones
– Vadose, or unsaturated zone (saturation <1)
– Saturated zone (saturation = 1)
– Capillary zone, lies above the water table
Aquifers
• Groundwater is the water found in an aquifer
• Aquifer:
– The saturated underground formation that will
yield usable amounts of water to a well or spring.
The formation can be sand, gravel, limestone or
sandstone
– Any geologic unit through which water can move
easily (i.e. it’s permeable)
(= high permeability)
Permeability: The ease with which water flows through
porosity. Most important variable is grain / pore size.
Confined aquifer
– The saturated formation between low
permeability layers that restrict movement of
water vertically into or out of the saturated
formation
– Water is confined under pressure similar to
water in a pipeline
– In some areas confined
aquifers produce water without pumps
(flowing artesian well)
Unconfined aquifer
– The saturated formation in which
the upper surface fluctuates with addition or
subtraction of water
– The upper surface of an unconfined aquifer is called
the water table
– Water, contained in an unconfined aquifer, is free
to move laterally in response to differences in the
water table elevations
Aquifer Types
• Unconfined - storage LARGE depends on specific yield
• Confined - storage SMALL depends on compressibilities
Confined and unconfined aquifer
Perched aquifer:
Groundwater ponded above an impermeable layer within a larger
unconfined system.
Ground Water
and Surface Water
• These are almost always connected
– If a stream contributes water to the aquifer
it’s called a “losing stream”
– If a stream receives water from the aquifer
it’s called a “gaining stream”
– Same stream can be both at different
places or at different times
Aquifer properties
1.
2.
3.
4.
5.
6.
Porosity
Specific retention
Specific yield
Permeability
Transmissibility
Storage coefficient
Porosity
• Percentage of open spaces in rock and
sediment that can hold water.
• This determines the amount of water that a rock
can contain.
• In sediments or sedimentary rocks the porosity
depends on grain size, the shapes of the grains,
and the degree of sorting, and the degree of
cementation.
Well-rounded coarse-grained sediments usually have higher porosity than finegrained sediments
Poorly sorted sediments usually
have lower porosity because the
fine-grained fragments tend to fill in
the open space.
Material
well-sorted sand or gravel
sand and gravel, mixed
glacial till
silt
clay
Porosity
(%)
25-50
20-35
10-20
35-50
33-60
Specific retention, Sr
• Sr the ratio of the volume of water a
geomaterial can retain against gravity
drainage to the total volume of the
geomaterial.
• Known as ability to retain water
• Sr= Wr/V, where
– Wr is a water retention volume and
– V is total volume
Specific yield, Sy
• Sy the ratio of the volume of water that
drains from a saturated geomaterial
flowing to the attraction of gravity to the
total volume of the geomaterial
• Sy= Wy/V, where
– Wy= volume of water discharge
– V= total volume
n= Sy + Sr
Specific Yield
Material
coarse gravel
medium gravel
fine gravel
gravelly sand
coarse sand
medium sand
fine sand
silt
sandy clay
clay
Specific Yield
(%)
Max
Min Mean
25
12
22
26
13
23
30
21
25
35
20
25
35
20
27
32
15
26
28
10
21
19
4
18
12
3
7
5
0
2
Permeability
• A measure of the degree to which the pore spaces are
interconnected, and the size of the interconnections.
• Low porosity usually results in low permeability, but high
porosity does not necessarily imply high permeability. It
is possible to have a highly porous rock with little or no
interconnections between pores.
• A good example of a rock with high porosity and low
permeability is a vesicular volcanic rock, where the
bubbles that once contained gas give the rock a high
porosity, but since these holes are not connected to one
another the rock has low permeability.
• Unit m/day or m/year
Storage coefficient
• Storativity(S) or Storage coefficient
• •The volume of water that a permeable
unit will absorb or expel from storage per
unit surface area per unit change in head.
Specific Storage
• Specific storage (Ss) or Elastic storage
coefficient
• •The amount of water per unit volume of a
saturated formation that is stored or
expelled from storage owing to
compressibility of the mineral skeleton and
the pore water unit change in head.
Specific Storage
•
•
•
•
Ss = ρwg (α + nβ), where
ρw= density of water
g =the acceleration of gravity
α = compressibility of aquifer skeleton
(1/(M/LT2))
• β = compressibility of water (1/(M/LT2))
• n = porosity (L3/L3)
Specifics
Unconfined
Sy = n - Sr
Confined
S = b.Ss
n porosity
Sy specific yield (gravity
drainage)
Sr specific retention (like
field capacity)
b thickness
Ss specific storage
Ss = g.(a + n. b )
g specific weight
a matrix compressibility
b water compressibility
Transmissivity
• Amount of water that can be transmitted
horizontally through a unit width by the full
saturated thickness of the aquifer under a
hydraulic gradient of 1.
T= K B
• T: transmissivity(or m2/d)
K: hydraulic conductivity (L/T)
B: saturated thickness of the aquifer (L or m)
Groundwater Flow rate-Darcy Law
• Simple relationship that states that flow
velocity is directly proportional to:
– hydraulic gradient: slope of the WT.
– hydraulic conductivity (K): parameter
describing the permeability of the aquifer (also
depends on the density and viscosity of the
fluid).
• Typical rates are on the order of 1-10
cm/day for most aquifers.
Pollution of Groundwater
• Need a sense of ground water flow
– Warm up responses to the velocity of
groundwater flow is dependent on:
• porosity and permeability
• permeability and hydraulic gradient
• porosity and hydraulic gradient
• pressure gradient
28%
61%
7%
4%
Applications
•
•
•
•
•
•
•
•
•
Recharge surficial and deep aquifers
Hazardous waste and landfill leachate
Swale Design
Percolation or Retention Pond Design
Yield of an Aquifer
Green Roof Design and Operation
Seepage Through Reservoirs
Exfiltration Design and Operation
Yearly Volume Budgets
Darcy’s Law
• Darcy’s law provides an accurate description of
the flow of ground water in almost all
hydrogeologic environments.
• Flow rate determined by Head loss dh = h1 - h2
Darcy’s Law
• Henri Darcy established empirically that the flux
of water through a permeable formation is
proportional to the distance between top and
bottom of the soil column.
• The constant of proportionality is called the
hydraulic conductivity (K).
• V = Q/A, V a – ∆h, and V a 1/∆L
V = – K (∆h/∆L)
and since
Q = VA (A = total area)
Q = – KA (dh/dL)
Hydraulic Conductivity
• K represents a measure of the ability for flow
through porous media:
• Gravels -
0.1 to 1 cm/sec
• Sands -
10-2 to 10-3 cm/sec
• Silts -
10-4 to 10-5 cm/sec
• Clays -
10-7 to 10-9 cm/sec
Conditions
• Darcy’s Law holds for:
1.
2.
3.
4.
Saturated flow and unsaturated flow
Steady-state and transient flow
Flow in aquifers and aquitards
Flow in homogeneous and
heterogeneous systems
5. Flow in isotropic or anisotropic media
6. Flow in rocks and granular media
Example of Darcy’s Law
• A confined aquifer has a source of recharge.
• K for the aquifer is 50 m/day, and n is 0.2.
• The piezometric head in two wells 1000 m
apart is 55 m and 50 m respectively, from a
common datum.
• The average thickness of the aquifer is 30
m, and the average width of aquifer is 5 km.
Compute:
• a) the rate of flow through the aquifer
• (b) the average time of travel from the head of the
aquifer to a point 4 km downstream
• *assume no dispersion or diffusion
The solution
• Cross-Sectional area=
30(5)(1000) = 15 x 104 m2
• Hydraulic gradient =
(55-50)/1000 = 5 x 10-3
• Rate of Flow for K = 50 m/day
Q = (50 m/day) (75 x 101 m2)
= 37,500 m3/day
• Darcy Velocity:
V = Q/A = (37,500m3/day) / (15
x 104 m2) = 0.25m/day
• Seepage Velocity:
Vs = V/n = (0.25) / (0.2) =
1.25 m/day (about 4.1 ft/day)
• Time to travel 4 km downstream:
T = 4(1000m) / (1.25m/day) =
3200 days or 8.77 years
• This example shows that water moves
very slowly underground.
Limitations of the Darcian
Approach
1. For Reynold’s Number, Re, > 10 or where the flow
is turbulent, as in the immediate vicinity of pumped
wells.
2. Where water flows through extremely fine-grained
materials (colloidal clay)
Darcy’s Law:
Example 2
• A channel runs almost parallel to a river, and they
are 2000 m apart.
• The water level in the river is at an elevation of 120
ft and 110m in the channel.
• A pervious formation averaging 30 m thick and with
K of 0.25 m/hr joins them.
• Determine the rate of seepage or flow from the
river to the channel.
Confined Aquifer
Confining Layer
Aquifer
30 ft
m
Example 2
• Consider a 1-m length of river (and channel).
Q = KA [(h1 – h2) / L]
• Where:
A = (30 x 1) = 30 m2
K = (0.25 m/hr) (24 hr/day) = 6 m/day
• Therefore,
Q = [6 (30) (120 – 110)] / 2000
= 0.9 m3/day/ft length = 0.9 m2/day
Thank You
End of Class
Good Luck for Final...