Download - Lec 16 Groundwater
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Questions from Last Lecture
• Are geologists licensed like doctors? Yes There is
a provincial license and only licensed geoscientistscan certified geoscience work.
• What units are m/a? (Meters per year; a=year)
• Explain the Unit Runoff concept?
• What is the base level in a graded stream?
1
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Unit Runoff / Discharge
What is the average discharge of the Ottawa River
at the Carillon Dam (~Montreal)?
– Flow = 1940 m3 sec-1
–Drainage Area = 146,300 km
2
What is the annual average UNIT discharge of the
Ottawa River?
– Unit Discharge = Flow/Drainage Area
– Unit Discharge = flow m3
a-1
/ Drainage Area (m2
)
– Unit Discharge = 0.42 m a-1
Think of unit runoff like precipitation (m/a)
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Base Flow and Channel Formation
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3. Dacy’s Law & Groundwater movement & ‘flow’
1. GROUNDWATER: definition, importance and distribution
Learning Objectives:
2. Groundwater reservoirs
5. Geologic features associated with groundwater
4. Problems with using groundwater resources
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Hydrologic cycle (reminder)
Distribution of Earth’s water
1. Oceans:
2. Freshwater:
2a.
2b.
2c.
97.2%
2.8%
glaciers:
groundwater:
Other: 0.03%
lakes:
streams:
atmosphere:
soil moisture:
2.15%
0.62%
0.009%0.005%
0.0001%
0.001%
Today’s lecture
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Definition, importance and distribution
Percentage of population reliant on groundwater for domestic use
~ 25-40% in Canada
(~9-14 million people)
~70% in Maritimes
Groundwater is a major economic resource …
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Definition, importance and distribution
Groundwater: the water that lies
beneath the ground surface.
Hydrosphere's fresh water:
1. Glaciers (~77%)
2. Groundwater (~22%)
~ 15% of total precipitation infiltrates
ground and ends up as groundwater
Groundwater is slow moving, but not static:
• recharges (rain)
• discharges (feeds rivers)
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Misconception: groundwater is found in underground lakes and rivers
(true but quite rare)
Definition, importance and distribution
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“dry” soil “saturated” soil
SolidWater
Air
Groundwater reservoirs
Groundwater → found in pores of soil and sediment + fractures of bedrock
pores
fractures
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Groundwater reservoirs
Controls on availability of groundwater
Porosity (Φ): percentage of voids or pore space (vs. total volume).
Determines how much groundwater can be stored.
• Φ = f(grain roundness, sorting, and cementation) → texture
(… rounded, … sorted = high porosity)
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Measurement of Porosity
• Volume of sand column = 1000mL
• Column contains sand plus porosity
– In dry sand the porosity is filled with air
• Replace all the air with water
• Porosity = volume water / total volume
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Groundwater reservoirs
Controls on availability of groundwater
Permeability or Conductivity: capacity of a rock or sediment to transmit afluid. Function of pore SIZES and INTERCONNECTIVITY.
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How much water can you get from
a material? “Yield” of Water
Sand ExperimentPorosity =
Water Yield =
Gravel ExperimentPorosity =
Water Yield =
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Best Groundwater reservoirs
Porosity and permeability
Granite
Quartz sandstoneShale
Fractured crystalline rock
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Best Groundwater reservoirs
Granite P o r o s i t y
Quartz sandstone
Permeability
Shale
Fractured crystalline rock
low
low
high
high
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Total Storage (% void space)
% that CAN drain by gravity
% that CANNOT
Groundwater reservoirs
Sediments
Rocks
↑ ↓
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1. Unsaturated zone
• voids filled by water and air;infiltration and percolation
• water is under suction (negative
pressure) due to capillary forces• water cannot be pumped by wells
3. Zone of saturation
• zone where all of the voids in soil,sediment and rock are completelyfilled with water
• water is under + pressure and canbe extracted by wells
Distribution of groundwater
unsaturated
zone
zone of
saturation
water table
2. Water table: upper limit of
zone of saturation; surface of
the water level in the ground
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Variations in the water table
water table
P > atmos. P; H2O under + Psat. zone
undersat. zone
Distribution of groundwater
Pressure vs. atmos. P
P < atmos. P; H2O under – P
P = atm. P
• depth varies seasonally and yearly
• slope of water table usually ~ conforms to …
unsaturated
zone
zone of
saturation
water table
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Groundwater reservoirs
What factors control infiltration?
1. Precipitation (qty / time)
2. Slope
3. Geologic Material
4. Vegetation
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Groundwater reservoirsWhere is groundwater stored?
Aquiclude: impermeable rock layer (… excludes water)
Acquitard: semi-permeable rock layer (… retards water)
Aquifer: permeable rock strata or sediment that stores and
transmits groundwater. (e.g.: sand, gravel)confining layers
(barriers to flow;
e.g.: shale)
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Groundwater reservoirs
• Unconfined aquifer: aquifer uncapped by a confining layer;
groundwater is not under pressure; top of aquifer (water table)moves freely up and down.
• Confined aquifer (syn.: artesian aquifer): aquifer sandwiched in
between two confining layers (usually inclined to allow recharge);
groundwater is under pressure and level of ‘projected’ water table(pressure or potentiometric surface) is above the top of the aquifer.
AQUIFERS
Flowing artesian well: when the water table (pressure
surface) of a confined aquifer rests ABOVE the ground level
Non-flowing artesian well: when the water table (pressuresurface) of a confined aquifer rests BELOW the ground level
Well: a hole (usually lined) drilled into the ground to penetrate
an aquifer (goal : extract water).
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Groundwater reservoirs
non-flowing
(pressure surface)
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Groundwater movement
General pattern of movement
recharge → discharge
Balance between:
1. potential energy
(high to low elevations)
2. pressure
(weight of water above)
Faster near surface
Velocity in order of cm/day
(high) (low)
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K v i
groundwater velocity
hydraulic conductivity (related to permeability) porosity
hydraulic gradient = driving force
v
K
i
Groundwater movement
Darcy’s Law – states that if permeability remains uniform, the velocity
of groundwater will increase as the slope of the water table (hydraulic
gradient) increases.
Henry Darcy (French engineer) – equation to calculate fluid velocity
through a porous media (circa 1850).
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Source: Freeze and Cherry, 1979. Groundwater. P48.
L = 50
h = 10
h = 8
510 / for sand
0.30 for sand
h (10 8) 1
L 50 25
K m s
i
-6 1.3x10 / 42 /v m s m y
K K hv i
L
Calculate groundwater flow
velocity (v ) using Darc’s Law:
Groundwater movementSample calculation #1
(w.t. of confined aquifer)
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Groundwater movement
Exchange of water between groundwater and streams depends on the
positions of the water table (w.t.) with respect to the stream bed
w.t. HIGHER than stream bed
w.t. LOWER than stream bed
Two basic types of interactions:
1. gaining streams:
gain water from the inflow
of groundwater through thestreambed
2. losing streams:
lose water to the groundwatersystem by outflow through the
streambed
G d
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Q: If contaminant spills in L. Leakalot,how long will it take to get to L. Getsitall?
3 kmLake
Getsitall
Lake
Leakalot
Elevation = 108 m (above sea level)Elevation = 133 m (above sea level)
Plan view
Sample calculation #2
Groundwater movement
G d t t
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Sample calculation #2 (continued)
5
4
10 / for sand
0.30 for sand
h (133 108)8.3x10
L 3000
K m s
i
-7 2.7x10 / 8.76 /v m s m y
K K hv i
L
Bedrock
3 km
133 m
108 m
Cross-sect ion
L Lv t
t v
3000342
8.76 /
L mt y
v m y
Groundwater movement
G d t C t i ti
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Other sources and types of
contamination include:
Groundwater Contamination
• Highway road salt
• Fertilizers
• Pesticides• Chemical and industrial
materials
• Bacteria and viruses
P bl ti i i t d ith d t
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Sewage contamination: sewage often becomes purified as it passes
through tens of meters of an aquifer composed of sand or permeablesandstone.
In extremely permeable aquifers, such as coarse gravel or
fractured bedrock, groundwater may travel long distances
without being cleaned.
Problems associated with groundwater withdrawal
Problematic issues associated with groundwater
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Problems associated with groundwater withdrawal
Before heavy pumping
After heavy pumping
• drawdown (lowering) of the w.t.• cone of depression in the w.t.
Excessive pumping of
wells can cause:
To ensure a continuous supplyof water, a well must penetrate
below the w.t.
P bl ti i i t d ith d t
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90 000 domestic wells in Eastern Ontario
Problematic issues associated with groundwater
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Problems associated with groundwater withdrawal
Treating groundwater as a nonrenewable resource –
• in many places the water available to recharge the aquifer
falls significantly short of the amount being withdrawn
•Hydrologic mass balance is out of Balance!
Subsidence: ground sinks when water is pumped from wells faster
than natural recharge processes can replace it.
Which can lead to …
Problematic issues associated with groundwater
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San Joaquin Valley, CA
1925
1955
1977
Subsidence caused by withdrawal
Problematic issues associated with groundwater
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Problems associated with groundwater withdrawal
Saltwater contamination: excessive groundwater withdrawalcauses saltwater to be drawn into wells
F t i t d ith d t
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Features associated with groundwater
Distribution of hot springs and geysers
Any observations on the geographic distribution?
Geothermal map
Feat res associated ith gro nd ater
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Features associated with groundwater
Geysers: intermittent surface
emission of hot water
• Groundwater heats,expands, changes to
steam, and erupts
• Water erupts with great
force, often rising 30-60 m
into the air
• Occur when complex
network of fractures in hot
igneous rock
Features associated with groundwater
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Features associated with groundwater
Yellowstone Natl.Park
Features associated with groundwater
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Features associated with groundwaterErosional features
Caves: large underground cavity
H2O + CO2 → H2CO3 → H+ + HCO3
-
controlled by climates and fracture networks
dissolution rates of 4-5 mm/yr
Source of CO2 ?
Karst topography: feature created when ground or surface water
dissolves rock; typically forms in limestone bedrock
Equations:
CaCO3 + CO2 + H2O⇋ Ca2+ + 2HCO3
−
(carbonic acid) (bicarbonate)
(found in
hard water)
Features associated with groundwater
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Features associated with groundwaterKarst topography (caves, sinkholes, etc.)
Features associated with groundwater
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Features associated with groundwaterKarst topography and sinkholes
Features associated with groundwater
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Features associated with groundwater
Depositional features
Groundwater is often mildly acidic →
dissolves rock (particularly limestone – CaCO3)
If CO2 is allowed to escape, reaction reverts and CaCO3 deposits
Formation of:
• stalactite …
• stalagmite …
• column/pillar …
Recall:
CaCO3 + CO2 + H2O⇋ Ca2+ + 2HCO3
−
Features associated with groundwater
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Carlsbad Caverns Natl. Park, NM
Features associated with groundwater
Formation of caves
* video
Features associated with groundwater
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Features associated with groundwater
Limestone (CaCO3) deposits of Mammoth Hot Springs (Yellowstone, Ntl. Park)
Release of CO2 (agitation, hot water T)
triggers precipitation of calcium carbonate
Travertine: CaCO3 deposited by surface or groundwater (hot springs, geysers, caves)
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Check out Geology in Action• http://www.youtube.com/watch?feature=player_detailpage&v=LWrklFuYnb0
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Summary
1. GROUNDWATER is widely distributed and a key source of
potable water for many people.
2. Groundwater is in the porosity of the rock or soil
3. Groundwater movement controlled by gradient and hydraulic
conductivity of the porous media (Darcy’s Law). 4. Groundwater can dissolve carbonate rocks leading to sink
holes and other geologic features.
5. Excess usage of groundwater is depleting the resource in many
areas or contaminating it with chemical / biological wastes.