Lecture notes Flashcards
Aquifer
A geological unit that can store enough water and transmit it at a rate fast enough to be significant.
Challenges with aquifer overuse
- pumping costs go up with overuse
- Land subsidence
- less water
- Takes a long time to recover
Major types of aquifers
- unconsolidated/semiconsolidated sand and gravel
- sandstones
- carbonate rock
- sand stone and carbonate interbedded
- igneoud and metamorphic rock
Compaction and Diagenisis
- 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.
Primary porosity vs secondary porosity
Primary porosity- porosity when lithified, a function of sorting/angular vs. round/how it was lithified
second porosity-fractures/joints/faults. After lithification
Porosity in igneous and metamorphic rocks
primary porosity-gas bubbles (pumice, pyroclastics, ash have higher porosity)
secondary porosity-exfoliation
specific yield
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.
Why does Sy go down as we get to coarser grains?
- 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)
Fluid compression
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?
viscosity
How easily fluid flows. How thick fluid is. H bonds between molecules are important for viscosity.
viscosity of water
Water is relatively viscous because of it’s H bonds.
How does water flow relative to grains it flows through?
Flows closer closer to the rocks because of attraction of water to the rocks.
Permeameter
measures the hydraulic conductivity
How to define the water table
at the water table to pressure head is zero.
How does density change throughout a lake
Everything the same density. There is not a net head.
Hydrostatic
No flow. Head is the same everywhere.
For a system total head is a function of
elevation and pressure in a system
Gradient
difference between 2 points/distance between them
Absence of groundwater flow if
flate water table
Groundwater flows is
There is a sloping water table
Groundwater discharge zones are generally located in…
topographically low areas.
In humid areas what does the water table look like?
It has about the same shape as surface topography.
Groundwater generally flows away from
topographically high spots and towards topographic lows.
Confining layer around the aquifer
low permeability keeps water in aquifer.
Aquifuge
Will not transmit water
Aquitard
very low permiability
Subset-Aquicade-leaky confining layer.
unconfined aquifer
top is the water table
confined aquifer
artisian surrounded by confining layers. Only way it will be recharged is the places where there is no cap above it.
Polentiometric surface
the level to which the water will rise
perched aquifer
a wedge of material with an impenetrable layer beneath it. Usually can’t supply a whole city just a few houses
Piexometer
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.
Cone of depression
lowering of pot. surface from overuse.
What is the problem with maps made showing ground level and aquifer levels?
They are only true at the time made.
isotropic
same in all directions (example sandstone)
Aspects of fabric of aquifer
- 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)
specific retention
the ratio of the volume of water a rock can retain against gravity drainage to the total volume of the rock.
Darcy’s law
Q equals -KA (dh/dl)
Viscosity
How easily fluid flows. How thick fluid is. H bonds between molecules are important for viscosity.
As temp goes up hydraulic conductivity goes _________.
up.
Hazen method
Measure for your rock what exactly is the hydraulic conductivity.
Permeameter
Can be constant head or falling. The issue is being able to carefully sample it.
Falling head
nicer for cohesive sediments. Uses less water. Want to make sure that water doesn’t have any bubbles
How do we define the water table
At the water table the pressure head is zero
Total head is represented by the sum of
The elevation head and the pressure head.
What is the net head in a lake?
Everything is the same density. There is not a net head. The total head is the same everywhere.
Hydrostatic
No flow.
For a system total head is a function of_______________ in a system
elevation and pressure
Gradient
The difference between 2 points/distance between them
Absense of groundwater flow if the water table is __________.
Flat
If there is a sloping water table what happens to the groundwater?
It flows.
What happens to the water table when it is humid?
The water table takes on about the same shape as the surface topography.
How does groundwater generally flow?
Away from topographically high spots and towards topographic lows.
Confining layers around an aquifer have
low permeability. (Keeps water in the aquifer)
Aquifuge
Will not transmit water
Aquitard
very low permiability
Aquicade
A subset of an aquitard. A leaky confining layer.
Unconfined aquifer
the top of the aquifer is the water table
Forces on groundwater that keep it moving
Gravity (pulls water down) External Pressure (atmospheric pressure/overlying material.) Molecular (water adheres to surfaces/capillary)
Resistive forces on groundwater
friction
viscosity
stresses
Velocity of groundwater
Relatively slow. About 30m/year on average
Bernouilli equation
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
Energy in groundwater
mechanical, thermal, chemical.
piexometer
Measures energy of the fluid in the system.
The major variable in an aquifer causing the density of water to vary
salinity.
point water head
Actual water level in a well or piezometer.
h=z+h_point
Freshwater head
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
Water flows from ________ pressure to _________ pressure.
Higher to lower
If hp is the same for both sides, how will water flow?
It will flow depending on elevation.
Under what conditions can we use Darcy’s law successfully
For porous media (large # of very small fluid pathways)
Relatively low velocities (laminar flows)
When will Darcy’s law not work
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.
Reynold’s number
Used to determine if the flow is laminar or turbulent.
In a pipe when do you transition from laminar to turbulent flow?
When R > 2000
For porous medium when do you transition from laminar to turbulent flow?
When R
Effective porosity
Pores available for fluid flow
n_e
seepage velocity
sort of an average linear rate that the water moves.
isotropic medium
physical properties are equal in all directions.
In a homogenous and isotropic medium
Fluid moves in 1 direction.
Change in v due to p variation.
Gradient of the hydraulic head can be measured with.
wells or piezometers from at least 2 locations.
With an array of piezometers in the same aquifer you get
a good idea of variation.
Take measurements at different depths
Find the location of the water table
Find flow direction.
Contouring lines with equal hydraulic head.
Equipotential surface, the hydrualic head is the same.
Water flows from
highs to lows (relative to hydraulic head)
What happens to water flow if the gradient is zero?
No flow of water
The gradient of the head is _________ to equipotential surface is homogenous and isotropic medium.
orthogonal
direction of the flow is ___________ to the gradient
opposite
Flow is determined by
K (hydraulic conductivity) and permeability distribution
What happens if a medium is anisotropic?
We get a different flow.
Flow net
a graphical representation of flow lines and equipotential lines for a given aquifer.
Slug test
Way of measuring in situ values for k
Limitations of slug test
Only give k immediately around a well.
Steady flow tests
more accurated
require a totally confined aquifer
require at least 2 locations.
pumping tests
used in situ assesment of transmissivity and storage and k.
flow nets
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.
recharge area
where water is crossing the water table downwards, usually occurs at topographic highs.
discharge areas
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)
natural basin yield
average rate of discharge in the absence of anthropogenic disturbances (not talking about pumping)
The line separating recharge from discharge
the midline
hydraulic head at zones of recharge
low
local flow
a relatively fast flowing system. Whatever goes down is likely to come up.
Shallow flow patterns
temp more or less the same as the surface
*doesn’t give the earth much of a chance to clean off that water.
stagnation points in a flow field
local velocity is zero
Flat topography and flat water table (how water moves there)
A lot of stagnation and not a lot of water movement.
Evapotranspiration can however cause movement
Where do water paths tend to converge?
Areas of higher conductivity.
Equipotential surfaces bend around/refract around…
the area of highest conductivity
carbon as an isotope
- 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.
How is carbon 14 formed?
cosmogenic. High energy particles fly off from the sun interact with nitrogen and produces carbon 14.
anthropogenic radioactive elements
nuclear bombs, cfcs, etc.
Juvenile water
magmatic water that has not cycled.
magmatic water
juvenile water and water from subduction
meteoric water
water from precipitation, includes water from lakes and rivers. Does not include the water deep in the earth.
connate water
Water that was deposited by geological means. Simultaneously with the surrounding rock and held with out flow.
Karst topography, type of rocks
limestone, dolomite, gypsum
Wetlands are important in terms of
- supplying water downstream
- trapping flood water
- recharge groundwater
- remove pollution
- plants and animals (high diversity)
- nutrients
- recreation as well
wetlands
areas generally inundated by surface or groundwater. Enough to keep a healthy vegetation cover.
Types of wetlands
marsh, swamp, bog, pean, fen, lake.
Marsh
Continuously inundated with water.
Soft stemmed vegetation (not trees)
Water mostly from surface water (some ground water)
swamp
dominated by woody plants (trees).
A lot of organic soils
Bog
spongey peat deposits acidic water most water comes from precipitation not as nutrient rich contains peat
peat
immature coal, a lot of decomposed plant material.
Fen
like a bog, but fed by groundwater, and not as acidic.
What around a lake can control the water’s path?
plants
How do you measure discharge and recharge from the bottom of the lake?
seepage meters.
For larger lakes you use larger seepage meters.
saltwater intrustion
the movement of saline water into an aquifer from which freshwater is extracted. Usually due to human activity.
How much salt is too much for groundwater?
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
Where is salty groundwater a concern?
- 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
How are earth’s layers towards the sea?
They are tilted towards the sea because of dynamic topography
Passive mechanism for seawater intrustion
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.
Active mechanism for seawater intrustion
If the natural hydraulic gradient is reversed (human activity) pumping water out changes head gradient, and makes saltwater move towards freshwater.
Groundwater is not pure water, what else does it contain?
- Natural contaminants (ex-arsenic or uranium)
- Human contaminants
- Can be affected by air pollution (ex-mercury from coal burning plants)
The first example of water treatment
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.
1980s clean water act
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?
Nitrates
come from agriculture and pesticides. Are a main concern from all states for groundwater contamination.
Methane
CH4. A lot of methane that often requires monitoring. Often a lot in land fills, can cause an explosion over time
Water quality in the vadose zone
Liquid and waper. Liquid goes down as it is largely controlled by gravity. Vapor is much harder to track and predict.
Contaminants below the water table
All liquid, and easier to track than that above the water thable.
What are the different mechanisms for moving groundwater?
*Diffusion-the process by which solute moves from areas of high concentration to lower concentration.
solute
material that has been dissolved into water.
Frick’s law
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.
Porous media diffusion
Diffusion will be slow because fluid moves around grains and other similar things with relatively long pathways.
Are diffusion rates temperature dependent?
Yes
Does ground water need to move for diffusion?
No.
Even if hydraulic head is zero you can get diffusion
What happens to diffusion when permeability is low?
Diffusion may be faster than the water flow.
Advective transport
Going with the flow of water
Which is greater the density of salt water or freshwater?
Salt water
How do you get a diffuse boundary?
Tidal pumping.
Ghyben-Herzberg relationship
Assumes static conditions. Depth to seawater below sealevel is proportional to the hydraulic head times (density of freshwater/(density of saltwater-density of freshwater))
Dupuit-Ghyben-Herzberg relationship
Try to more realistically describe the boundary between fresh and saltwater.
Estuaries and wetlands
near oceans. Fresh groundwater seepage controls a lot of what is going on.
Submarine Groundwater discharge
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.
Remediation-what do you do if you have a saltwater intrusion?
- 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
As sealevel rises what will happen to groundwater?
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.
Advection
Transport of substances with the same velocity as the fluids.
*Material carried at the speed of groundwater.
Amount of solute transported by advection?
concentration times average linear velocity
plug flow
All pore fluids are replaced by the solute material at the front.
Mechanical dispersion
The mixing between waters having different concentrations as fluid moves along a flow path.
How does pores affect how material moves?
pore size, friction within pores, direct paths vs long paths.
Longitudinal dispersion
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.
Tranverse dispersion
Normal to the path
D*
effective/molecular diffusion coefficient
diffusion
relatively slow by itself without any aids.
When dispersion is added to diffusion
You get a much further spread of the material in a shorter amount of time.
What happens if you inject material and it moves with groundwater?
material spreads out with time.
Acts as a function of fluid flow and the nature of the porous medium.
Factors controlling dispersion of solutes
- How well solute mixes with water
- Density of solute
- Solubility
- microbial action
- radioactive
- sorption
- conservative/reactive solutes
sorption
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)
Is Cl conservative or reactive?
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.
Is bicarbonate conservative or reactive?
Very reactive
Migration of solute depends on
advection, dispersion, material properties, and properties that lead to retardation.
superfund sites
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.
Kankakee arch
Between Michigan and Illinois river basin
The more water flow through the _______ part of the basin the less likely it is to be drinkable.
deep
