Lec 16 Groundwater

8/19/2019 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)

2


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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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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)



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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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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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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


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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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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• 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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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


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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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


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


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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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



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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 …



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San Joaquin Valley, CA

1925

1955

1977

Subsidence caused by withdrawal



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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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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



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Yellowstone Natl.Park



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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)



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Features associated with groundwaterKarst topography (caves, sinkholes, etc.)



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Features associated with groundwaterKarst topography and sinkholes



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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

−



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Carlsbad Caverns Natl. Park, NM


Formation of caves

* video



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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.

Lec 16 Groundwater

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