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Hydrology Final > Lecture notes > Flashcards

Lecture notes Flashcards

1
Q

Aquifer

A

A geological unit that can store enough water and transmit it at a rate fast enough to be significant.

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2
Q

Challenges with aquifer overuse

A
  • pumping costs go up with overuse
  • Land subsidence
  • less water
  • Takes a long time to recover
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3
Q

Major types of aquifers

A
  • unconsolidated/semiconsolidated sand and gravel
  • sandstones
  • carbonate rock
  • sand stone and carbonate interbedded
  • igneoud and metamorphic rock
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4
Q

Compaction and Diagenisis

A
  • decreases porosity.
  • Infilling of the pores with “cement” (material that precipitates out) calcite/dolomite, silica, hematite
  • A sediment with a high porosity loses a lot of original pores.
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5
Q

Primary porosity vs secondary porosity

A

Primary porosity- porosity when lithified, a function of sorting/angular vs. round/how it was lithified
second porosity-fractures/joints/faults. After lithification

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6
Q

Porosity in igneous and metamorphic rocks

A

primary porosity-gas bubbles (pumice, pyroclastics, ash have higher porosity)
secondary porosity-exfoliation

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7
Q

specific yield

A

S_y volume of H20 relative to the total volume of the rock that is released from pore spaces of an initially saturated rock under gravity drainage.

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8
Q

Why does Sy go down as we get to coarser grains?

A
  • We don’t know Cu
  • Not a nice well sorted material
  • Smaller grains between larger ones. (Think about not just dominant grain, but a mixture as well)
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9
Q

Fluid compression

A

Bequals -dv/v/p units 1/Pa
This equation can be used to answer the following:
Take a square from bottom surface. What will the v change be when moving it. What is the pressure at the bottom?

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10
Q

viscosity

A

How easily fluid flows. How thick fluid is. H bonds between molecules are important for viscosity.

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11
Q

viscosity of water

A

Water is relatively viscous because of it’s H bonds.

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12
Q

How does water flow relative to grains it flows through?

A

Flows closer closer to the rocks because of attraction of water to the rocks.

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13
Q

Permeameter

A

measures the hydraulic conductivity

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14
Q

How to define the water table

A

at the water table to pressure head is zero.

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15
Q

How does density change throughout a lake

A

Everything the same density. There is not a net head.

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16
Q

Hydrostatic

A

No flow. Head is the same everywhere.

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17
Q

For a system total head is a function of

A

elevation and pressure in a system

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18
Q

Gradient

A

difference between 2 points/distance between them

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19
Q

Absence of groundwater flow if

A

flate water table

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20
Q

Groundwater flows is

A

There is a sloping water table

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21
Q

Groundwater discharge zones are generally located in…

A

topographically low areas.

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22
Q

In humid areas what does the water table look like?

A

It has about the same shape as surface topography.

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23
Q

Groundwater generally flows away from

A

topographically high spots and towards topographic lows.

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24
Q

Confining layer around the aquifer

A

low permeability keeps water in aquifer.

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25
Q

Aquifuge

A

Will not transmit water

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26
Q

Aquitard

A

very low permiability

Subset-Aquicade-leaky confining layer.

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27
Q

unconfined aquifer

A

top is the water table

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28
Q

confined aquifer

A

artisian surrounded by confining layers. Only way it will be recharged is the places where there is no cap above it.

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29
Q

Polentiometric surface

A

the level to which the water will rise

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30
Q

perched aquifer

A

a wedge of material with an impenetrable layer beneath it. Usually can’t supply a whole city just a few houses

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31
Q

Piexometer

A

Can measure the pressure in a system. A device used to measure liquid pressure in a system by measuring the height to which a column of liquid rises against gravity or a device which measures the pressure.

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32
Q

Cone of depression

A

lowering of pot. surface from overuse.

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33
Q

What is the problem with maps made showing ground level and aquifer levels?

A

They are only true at the time made.

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34
Q

isotropic

A

same in all directions (example sandstone)

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35
Q

Aspects of fabric of aquifer

A
  • homogeneous or heterogeneous
  • Change in thickness
  • Layers have different properties
  • Facies changes
  • Change in material properties
  • Isotropic (same in all directions)
  • Anisotropic (not the same in all directions micas)
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36
Q

specific retention

A

the ratio of the volume of water a rock can retain against gravity drainage to the total volume of the rock.

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37
Q

Darcy’s law

A

Q equals -KA (dh/dl)

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38
Q

Viscosity

A

How easily fluid flows. How thick fluid is. H bonds between molecules are important for viscosity.

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39
Q

As temp goes up hydraulic conductivity goes _________.

A

up.

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40
Q

Hazen method

A

Measure for your rock what exactly is the hydraulic conductivity.

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41
Q

Permeameter

A

Can be constant head or falling. The issue is being able to carefully sample it.

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42
Q

Falling head

A

nicer for cohesive sediments. Uses less water. Want to make sure that water doesn’t have any bubbles

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43
Q

How do we define the water table

A

At the water table the pressure head is zero

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44
Q

Total head is represented by the sum of

A

The elevation head and the pressure head.

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45
Q

What is the net head in a lake?

A

Everything is the same density. There is not a net head. The total head is the same everywhere.

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46
Q

Hydrostatic

A

No flow.

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47
Q

For a system total head is a function of_______________ in a system

A

elevation and pressure

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48
Q

Gradient

A

The difference between 2 points/distance between them

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49
Q

Absense of groundwater flow if the water table is __________.

A

Flat

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50
Q

If there is a sloping water table what happens to the groundwater?

A

It flows.

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51
Q

What happens to the water table when it is humid?

A

The water table takes on about the same shape as the surface topography.

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52
Q

How does groundwater generally flow?

A

Away from topographically high spots and towards topographic lows.

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53
Q

Confining layers around an aquifer have

A

low permeability. (Keeps water in the aquifer)

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54
Q

Aquifuge

A

Will not transmit water

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55
Q

Aquitard

A

very low permiability

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56
Q

Aquicade

A

A subset of an aquitard. A leaky confining layer.

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57
Q

Unconfined aquifer

A

the top of the aquifer is the water table

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58
Q

Forces on groundwater that keep it moving

A 
Gravity (pulls water down)
External Pressure (atmospheric pressure/overlying material.)
Molecular (water adheres to surfaces/capillary)
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59
Q

Resistive forces on groundwater

A

friction
viscosity
stresses

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60
Q

Velocity of groundwater

A

Relatively slow. About 30m/year on average

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61
Q

Bernouilli equation

A

E-tm =1/2 v^2 +gx +p/density

When this equals a constant it tells you that you have conservation of energy principle for flowing liquids

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62
Q

Energy in groundwater

A

mechanical, thermal, chemical.

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63
Q

piexometer

A

Measures energy of the fluid in the system.

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64
Q

The major variable in an aquifer causing the density of water to vary

A

salinity.

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65
Q

point water head

A

Actual water level in a well or piezometer.

h=z+h_point

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66
Q

Freshwater head

A

Height of a column of freshwater in a well just sufficient to balance the pressure in an aquifer at that point.
h=z+h_fresh

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67
Q

Water flows from ________ pressure to _________ pressure.

A

Higher to lower

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68
Q

If hp is the same for both sides, how will water flow?

A

It will flow depending on elevation.

69
Q

Under what conditions can we use Darcy’s law successfully

A

For porous media (large # of very small fluid pathways)

Relatively low velocities (laminar flows)

70
Q

When will Darcy’s law not work

A 
Karst or fractured rock (largely calcite or dolomite)
Igneous rock with a lot of large cracks.
When Reynold's number is too high.
Pumping wells
Steep hydraulic gradients.
71
Q

Reynold’s number

A

Used to determine if the flow is laminar or turbulent.

72
Q

In a pipe when do you transition from laminar to turbulent flow?

A

When R > 2000

73
Q

For porous medium when do you transition from laminar to turbulent flow?

A

When R

74
Q

Effective porosity

A

Pores available for fluid flow

n_e

75
Q

seepage velocity

A

sort of an average linear rate that the water moves.

76
Q

isotropic medium

A

physical properties are equal in all directions.

77
Q

In a homogenous and isotropic medium

A

Fluid moves in 1 direction.

Change in v due to p variation.

78
Q

Gradient of the hydraulic head can be measured with.

A

wells or piezometers from at least 2 locations.

79
Q

With an array of piezometers in the same aquifer you get

A

a good idea of variation.
Take measurements at different depths
Find the location of the water table
Find flow direction.

80
Q

Contouring lines with equal hydraulic head.

A

Equipotential surface, the hydrualic head is the same.

81
Q

Water flows from

A

highs to lows (relative to hydraulic head)

82
Q

What happens to water flow if the gradient is zero?

A

No flow of water

83
Q

The gradient of the head is _________ to equipotential surface is homogenous and isotropic medium.

A

orthogonal

84
Q

direction of the flow is ___________ to the gradient

A

opposite

85
Q

Flow is determined by

A

K (hydraulic conductivity) and permeability distribution

86
Q

What happens if a medium is anisotropic?

A

We get a different flow.

87
Q

Flow net

A

a graphical representation of flow lines and equipotential lines for a given aquifer.

88
Q

Slug test

A

Way of measuring in situ values for k

89
Q

Limitations of slug test

A

Only give k immediately around a well.

90
Q

Steady flow tests

A

more accurated
require a totally confined aquifer
require at least 2 locations.

91
Q

pumping tests

A

used in situ assesment of transmissivity and storage and k.

92
Q

flow nets

A

intersection of equipotential lines and flow lines. Flow patterns controlled by the shape of the basin, the water table, and how hydraulic conductivity varies horizontally and vertically.

93
Q

recharge area

A

where water is crossing the water table downwards, usually occurs at topographic highs.

94
Q

discharge areas

A

where water is crossing the water table upwards into the unsaturated zone on land or the base of some surface body water.
(Often occurs at topographic lows)

95
Q

natural basin yield

A

average rate of discharge in the absence of anthropogenic disturbances (not talking about pumping)

96
Q

The line separating recharge from discharge

A

the midline

97
Q

hydraulic head at zones of recharge

A

low

98
Q

local flow

A

a relatively fast flowing system. Whatever goes down is likely to come up.

99
Q

Shallow flow patterns

A

temp more or less the same as the surface

*doesn’t give the earth much of a chance to clean off that water.

100
Q

stagnation points in a flow field

A

local velocity is zero

101
Q

Flat topography and flat water table (how water moves there)

A

A lot of stagnation and not a lot of water movement.

Evapotranspiration can however cause movement

102
Q

Where do water paths tend to converge?

A

Areas of higher conductivity.

103
Q

Equipotential surfaces bend around/refract around…

A

the area of highest conductivity

104
Q

carbon as an isotope

A
  • can come is 12 (usually), also 13 and 14.
  • temp can control which isotope is preferentially taken up
  • By looking at variation you get an idea of what is going on.
105
Q

How is carbon 14 formed?

A

cosmogenic. High energy particles fly off from the sun interact with nitrogen and produces carbon 14.

106
Q

anthropogenic radioactive elements

A

nuclear bombs, cfcs, etc.

107
Q

Juvenile water

A

magmatic water that has not cycled.

108
Q

magmatic water

A

juvenile water and water from subduction

109
Q

meteoric water

A

water from precipitation, includes water from lakes and rivers. Does not include the water deep in the earth.

110
Q

connate water

A

Water that was deposited by geological means. Simultaneously with the surrounding rock and held with out flow.

111
Q

Karst topography, type of rocks

A

limestone, dolomite, gypsum

112
Q

Wetlands are important in terms of

A
  • supplying water downstream
  • trapping flood water
  • recharge groundwater
  • remove pollution
  • plants and animals (high diversity)
  • nutrients
  • recreation as well
113
Q

wetlands

A

areas generally inundated by surface or groundwater. Enough to keep a healthy vegetation cover.

114
Q

Types of wetlands

A

marsh, swamp, bog, pean, fen, lake.

115
Q

Marsh

A

Continuously inundated with water.
Soft stemmed vegetation (not trees)
Water mostly from surface water (some ground water)

116
Q

swamp

A

dominated by woody plants (trees).

A lot of organic soils

117
Q

Bog

A 
spongey peat deposits
acidic water
most water comes from precipitation
not as nutrient rich
contains peat
118
Q

peat

A

immature coal, a lot of decomposed plant material.

119
Q

Fen

A

like a bog, but fed by groundwater, and not as acidic.

120
Q

What around a lake can control the water’s path?

A

plants

121
Q

How do you measure discharge and recharge from the bottom of the lake?

A

seepage meters.

For larger lakes you use larger seepage meters.

122
Q

saltwater intrustion

A

the movement of saline water into an aquifer from which freshwater is extracted. Usually due to human activity.

123
Q

How much salt is too much for groundwater?

A 
200-300 mg/l means you can taste it.
7000mg/l large farm animals
3000 mg/l small farm animals
250-350 mg/l town drinking water
less than 450 for agriculture
124
Q

Where is salty groundwater a concern?

A
  • Coastal basins (where aquifers are close to the oceans)
  • Sedimentary basins where deep brines migrate up, and there are salt deposits of ancient seas.
  • Aquifers near septic tanks and landfills
  • Irrigation and salt deposition
125
Q

How are earth’s layers towards the sea?

A

They are tilted towards the sea because of dynamic topography

126
Q

Passive mechanism for seawater intrustion

A

net result when you lower the water table- saline groundwater moves landward which increases saltwater encroachment.
This is a natural and slow process and will eventually rebound.

127
Q

Active mechanism for seawater intrustion

A

If the natural hydraulic gradient is reversed (human activity) pumping water out changes head gradient, and makes saltwater move towards freshwater.

128
Q

Groundwater is not pure water, what else does it contain?

A
  • Natural contaminants (ex-arsenic or uranium)
  • Human contaminants
  • Can be affected by air pollution (ex-mercury from coal burning plants)
129
Q

The first example of water treatment

A

Choloera epidemic in London in 1854. People getting sick were all taking water from the same well. This was because the aquifer was sampling a cesspit.

130
Q

1980s clean water act

A

Sets limits to how much “stuff” you can have in the water and still drink it.
Brought up the questions of what “stuff” can we test successfully?
And what risk does the water have?-Is it worth the risk?

131
Q

Nitrates

A

come from agriculture and pesticides. Are a main concern from all states for groundwater contamination.

132
Q

Methane

A

CH4. A lot of methane that often requires monitoring. Often a lot in land fills, can cause an explosion over time

133
Q

Water quality in the vadose zone

A

Liquid and waper. Liquid goes down as it is largely controlled by gravity. Vapor is much harder to track and predict.

134
Q

Contaminants below the water table

A

All liquid, and easier to track than that above the water thable.

135
Q

What are the different mechanisms for moving groundwater?

A

*Diffusion-the process by which solute moves from areas of high concentration to lower concentration.

136
Q

solute

A

material that has been dissolved into water.

137
Q

Frick’s law

A 
steady state F equals -D (dc/dx)
where F is the mass flux of the solute
d is the diffusion coefficient
c is the solute concentration
dc/dx is the concentration gradient
Tell us how fast stuff is going to diffuse out.
138
Q

Porous media diffusion

A

Diffusion will be slow because fluid moves around grains and other similar things with relatively long pathways.

139
Q

Are diffusion rates temperature dependent?

A

Yes

140
Q

Does ground water need to move for diffusion?

A

No.

Even if hydraulic head is zero you can get diffusion

141
Q

What happens to diffusion when permeability is low?

A

Diffusion may be faster than the water flow.

142
Q

Advective transport

A

Going with the flow of water

143
Q

Which is greater the density of salt water or freshwater?

A

Salt water

144
Q

How do you get a diffuse boundary?

A

Tidal pumping.

145
Q

Ghyben-Herzberg relationship

A

Assumes static conditions. Depth to seawater below sealevel is proportional to the hydraulic head times (density of freshwater/(density of saltwater-density of freshwater))

146
Q

Dupuit-Ghyben-Herzberg relationship

A

Try to more realistically describe the boundary between fresh and saltwater.

147
Q

Estuaries and wetlands

A

near oceans. Fresh groundwater seepage controls a lot of what is going on.

148
Q

Submarine Groundwater discharge

A

Used to be believed to be very small compared to streams. Then 226-Ra was found. The new conclusion is that this discharge is about the same as rivers in oceans.

149
Q

Remediation-what do you do if you have a saltwater intrusion?

A
  • Solution not instant and takes a long time
    • You can inject H20
    • You can build a barrier
    • You can reduce the saltwater
    • Move current wells
150
Q

As sealevel rises what will happen to groundwater?

A

We will get saline groundwater moving toward the center of land. If sea level goes down saline solution will move closer to the Mid Ocean Ridge.

151
Q

Advection

A

Transport of substances with the same velocity as the fluids.
*Material carried at the speed of groundwater.

152
Q

Amount of solute transported by advection?

A

concentration times average linear velocity

153
Q

plug flow

A

All pore fluids are replaced by the solute material at the front.

154
Q

Mechanical dispersion

A

The mixing between waters having different concentrations as fluid moves along a flow path.

155
Q

How does pores affect how material moves?

A

pore size, friction within pores, direct paths vs long paths.

156
Q

Longitudinal dispersion

A

Issues along the direction of the flow path. Different mechanisms result in different speeds

  • Randomness in different paths
  • Size of pores
  • How velocity varies with distance from the median.
157
Q

Tranverse dispersion

A

Normal to the path

158
Q

D*

A

effective/molecular diffusion coefficient

159
Q

diffusion

A

relatively slow by itself without any aids.

160
Q

When dispersion is added to diffusion

A

You get a much further spread of the material in a shorter amount of time.

161
Q

What happens if you inject material and it moves with groundwater?

A

material spreads out with time.

Acts as a function of fluid flow and the nature of the porous medium.

162
Q

Factors controlling dispersion of solutes

A
  • How well solute mixes with water
  • Density of solute
  • Solubility
  • microbial action
  • radioactive
  • sorption
  • conservative/reactive solutes
163
Q

sorption

A

the degree to which a solute bonds to the surface of the medium (absorbs), or desorbs (substance is released from the surface within an aquifer)

164
Q

Is Cl conservative or reactive?

A

Conservative in the oceans, because there are not many significant reactions. If there is a lot of clay it can be reactive and filter out the chlorine.

165
Q

Is bicarbonate conservative or reactive?

A

Very reactive

166
Q

Migration of solute depends on

A

advection, dispersion, material properties, and properties that lead to retardation.

167
Q

superfund sites

A

In 1980 it was realized many sites were badly polluted and effecting health. US put in an act that areas were so polluted it was beyond local authorities to clean up.

168
Q

Kankakee arch

A

Between Michigan and Illinois river basin

169
Q

The more water flow through the _______ part of the basin the less likely it is to be drinkable.

A

deep

Hydrology Final (5 decks)

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