Lesson 3: Properties of Aquifers Flashcards
the capacity to do work.
energy
equal to the product of the net force exerted and the distance through which the force moves, SI unit?
work (w), joule (J) = kg.m2 / s2, N.m
is equal to the product of mass of the body and its acceleration, SI unit?
force (F), N = kg.m / s2
is equal to the mass multiplied to the
gravitational force (~9.8 m/s2) exerted to it by the earth, SI unit?
weight (w), kg.m / s2 = N
the ______ of a fluid or solid is its mass per unit volume, SI unit?
density (p), kg / m3
The _______ of a substance is its weight per unit
volume, SI unit?
specific weight (y), N / m3
is the force applied to a unit area
perpendicular to the direction of the force, SI unit?
pressure (P), N / m2
In hydrogeology, how is pressure measured?
relative to atmospheric pressure
how does atmospheric pressure vary?
varies with changing weather patterns
Standard atmospheric pressure
1.013 x 10^5 Pa.
what type of fluid is water
Newtonian
what do u mean by a newtonian fluid
means that its resistance to relative motion is proportional to the dynamic viscosity
Standard unit of dynamic viscosity
N.s/ m2
another convenient unit for dynamic viscosity
poise (P), g/s.cm
property of water where If pressure is applied, the same mass of fluid will be contained within a smaller volume.
a compressible fluid (compressibility)
describe the relationship between change in density and the change in pressure
change in density is (directly) proportional to the change in pressure.
The proportionality constant for compressibility is known as ?
the bulk modulus
The SI unit of bulk modulus
N / m2
assemblages of individual grains that were deposited by water, wind, ice and gravity.
sediments
openings between the sediment grains
pore spaces
key to the study of hydrogeology
pore spaces, voids, cracks
is the percentage of the rock or soil that is void of
material.
porosity
porosity equation
n = 100(Vv/V)
Vv > is the volume of void space in a unit volume of earth material
V > is the unit volume of earth material, including both voids and solids
laboratory procedure to yield a value of the effective porosity
emersion method
describe how to determine effective porosity by emersion method
- Laboratory porosity is determined by taking a sample of known volume (V)
- The sample is dried in an oven at 105oC until it reaches a constant weight.
- The dried sample is then submerged in a known volume of water until it is saturated
- The volume of the voids is equal to the original water volume less the volume in the chamber after the saturated sample is removed.
describe effective porosity in hard rocks vs sediments
effective porosity is often measured thru emersion method for hard rocks, but for most unconsolidated sediments effective porosity is just the total porosity
Peyton et al. (1986) concluded in one of his studies that the effective porosity of a sediment is a function of ?
effective porosity of a sediment is a function of:
- the size of the molecules that are being transported relative
- to the size of the passageways that connect the pores.
passageways typically smaller than pores
pore throats
what happens to the effective porosity if the molecule being transported has a greater diameter than some of the pore throats
the effective porosity would be limited with respect to the molecule
total porosity equation
Porosity (n)= 100 [1-(pb/pd)]
where:
pb > is the bulk density of the aquifer material; and
pd > is the particle density of the aquifer material
2 packing extremes of well rounded well sorted sediments
cubic and rhombohedral
Cubic packing of spheres has a porosity of?
47.65%
Rhombohedral packing of spheres has a porosity of?
25.95%
T or F: the diameter of the spheres influences the
porosity.
False: The diameter of the spheres DOES NOT influence the porosity.
If a sediment contains a mixture of grain sizes, how will the porosity change?
the porosity will be lowered.
aside from sorting what other 2 factors can affect porosity?
- the shape of grains and the
- fabric/orientation of the particles.
two factors for grain shape/form
roundness and spherecity
the range of sphericity
prismoidal, sub-prismoidal, spherical, sub-discoidal, discoidal (4.5-0.5)
the range of roundedness
very angular, angular, sub-angular, sub-rounded, rounded, well-rounded (0.5-5.5)
sediments are classified mainly on the basis of?
size (diameter of individual grains)
another term for rocks with: silt/clay-sized particles, sand-sized particles, and gravel-sized particles
lutites, arenites, rudites (respectively)
Name classification systems in use for sediment sizes and used by whom?
- udden-wentworth scale (modified wentworth) by American Geologists
- Phi (Φ) scale
- grain size scale used by soil scientists of US dep of Agri
- grain size scale used by engineers (ASTM Standards)
full ASTM
American Society for Testing and Materials
how is dry strength tested?
- dried ball of cohesive sediment, ~1cm diameter
- it is squeezed between thumb and forefinger
- if it crumbles when dry, no dry strength.
- ratings of low, medium and high depend on effort to break the dried ball into pieces
how to measure toughness?
- take moist sample of cohesive sediment
- roll between palms to make a thread of sediment about 3mm thick
- low pressure to do this means low toughness, medium chuchu
The grain-size distribution of a sediment may be conveniently plotted on a _______
semilogarithmic paper
what size mesh with what opening size separates the sand fraction from the fines in the engineering classification system?
200-mesh screen, with an opening of 0.075 mm,
The gradation of the fines (<0.075 mm) is determined by ?
a hydrometer test/analysis
what rate is the hydrometer test/analysis based on, what is that dependent on? (?)
based on the rate that the sediment settles in water, dependent on density of sediments(?)
describe hydrometer test
1m long cylindrical tube (plastic or glass but mostly plastic), 1 liter of water with flocculant
after hydrometer test and sieve test, what do we do?
plot values on grain-size distribution curve to:
1. plot in ternary diagram, and
2. get uniformity coefficient (Cu)
a measure of how well or poorly sorted the grains are.
The uniformity coefficient (Cu) of a sediment
uniformity coefficient is the ratio of the grain size that is _______ finer by weight (____) to the grain size that is _____ finer by weight (_____)
60%;
d60;
10%;
d10
what do values of Cu mean?
- Cu < 4 = well sorted.
- Cu > 6 = poorly sorted
what does the effective grain size in the grain size distribution curve?
d10, the 10% line on the grain-size curve
porosity on sediment size
inversely. the smaller size, the higher porosity
porosity on unconsolidated vs consolidated
higher porosity on unconsolidated
porosity on degree of weathering
directly. the more weathered, the higher porosity
porosity on electrical resistivity
inversely. more porous, less electrical resistivity (bc more water)
2 process that reduce the porosity of sedimentary rocks
diagenesis lol
1. Compaction reduces pore volume by rearranging the grains and reshaping them.
2. The deposition of cementing materials
cementing materials
calcite, dolomite or silica
2 process that increase the porosity of sedimentary rocks
- The dissolution of material that is dissolved by the pore fluid will increase porosity
- fracturing
4 types of geologic porosities
- primary porosity
- secondary porosity
- fracture porosity
- vuggy porosity
The main or original porosity system in a rock or unconfined alluvial deposit.
primary porosity
A subsequent or separate porosity system in a rock, often enhancing overall porosity of a rock.
secondary porosity
2 causes of secondary porosity
- chemical leaching of minerals
- generation of a fracture system
T or F: Secondary can replace the primary porosity, but cannot coexist with it
FALSE. Secondary can replace the primary porosity OR coexist with it
secondary type of porosity in rocks that otherwise would not be reservoirs for hydrocarbons
fracture porosity
why are rock types like intrusions or metasediments without fractures not normally considered a reservoir for hydrocarbons
due to their primary porosity being destroyed (for example due to depth of burial)
This is secondary porosity generated by dissolution of large features (such as macrofossils) in carbonate rocks leaving large holes, vugs, or even caves.
vuggy porosity
rock examples that most often have vuggy porosity
limestone and dolostone
are well-known and widespread examples of sedimentary rocks of chemical or biochemical origin.
Limestones and dolomites
how do limestones and dolomites become very porous?
- these rocks were originally part of an aqueous solution
- precipitation is also reversible, so it redissolves
- as more water moves thru bedding planes, they are preferentially dissolved and enlarge
=very porous
example of limestone formation that has an opening large enough to permit thousands of tourists a day to pass through them
Carlsbad Cavern, New Mexico
aside from limestone and dolostone, what are other cavernous stones?
gypsum and salt
range of porosity in clastic rocks
3 to 30%, highly variable
porosity values for limestones and dolomites
less than 1 to 30%
Plutonic and metamorphic rocks are typically thought to have a _______ porosity as they are formed of ________
very low;
interlocking crystals
in the deep granite test hole in northern Illinois, porosity was measured to be what percent at 1600m?
1.42% at 1600m depth: few fractures, possibly primary
same borehole as great as 2.15%, with fractures
2 process that increase overall rock porosity
Weathering and fracturing
Weathered plutonic and metamorphic rocks can have porosities in the range of ?
where are they found?
30% to 60%,
along fault zones, more fractured = more porous
texture formed when lava cooling rapidly at the surface will trap degassing products resulting in holes in the rock
vesicular texture
Porosity of basalt generally ranges from ?
1% to 12%
Pumice can have a porosity ?
as high as 87%
although the vesicles are not well connected. (low permeability)
tuff has a porosity ranging from
14% to 40%
Recent unconsolidated volcanic ash may have a porosity of ?
50%
Weathering of volcanic deposits can increase the porosity to?
excess of 60%
the Dumoy aquifer contains what deposits?
volcanic deposits like ash and pumice
It is ratio of the volume of water that drains from saturated rock owing to the attraction of gravity to the total volume of the rock
Specific Yield (Sy)
Water molecules cling to surfaces because of ?
surface tension of water.
If gravity exerts a stress on a film of water surrounding a mineral grain, some of the film will ______. The remaining film will be _____, with a ______ surface tension so that the stress of gravity will be______ by the surface tension
pull away and drip downward;
thinner;
greater;
exactly balanced
the moisture clinging to the soil particles because of surface tension.
Pendular water
the ratio of volume of water a rock can retain against gravity drainage to the total volume of rock
Specific Retention (Sr) of a rock or soil (storativity)
what can be computed by adding the volume of the specific yield and specific retention.
The porosity of a rock/material
n = Sy + Sr
It is the ability of a rock to transmit water
Hydraulic Conductivity
constitute the most significant hydrologic properties
Hydraulic Conductivity (permeability) & porosity
example of rock that exhibit porosity but lack
interconnected voids
vesicular basalt
examples sediments and rocks that have porosity, but the pores are so small that water flows through the rock with difficulty.
Clay and shale
a French Engineer, made the first systematic study of the movement of water through a porous medium, when?
Henry Darcy, mid-1800s
what did Henry Darcy discover?
- water flow through a bed of a
= proportional to the difference in the height of the water between the two ends of the filter beds and
= inversely proportional to the length of the flow path
= proportional to a coefficient, K, which is dependent upon the nature of the porous medium
=proportional to cross sectional area of pipe
thin vertical pipe used to measure rate of flow
piezometer
full Darcy’s law
Q =-KA [(hA - hB)/L]
=-KA (dh/dl)
or q = -K (dh/dl)
where q = Q/A
represents the change in head between 2 points
dh
represents distance between two points in darcys experiment
dl
represents the hydraulic gradient
dh/dl
why does darcys law have a negative sign
bc it indicates that the flow is in the direction of decreasing hydraulic head
what does q represent in darcy’s law
specific discharge
dimensions of specific discharge
L/T
describe the relationship between discharge and specific weight
Discharge is directly proportional to the specific weight, y, of the fluid
The units that are typically used to express discharge in streams or rivers include ? (3)
m³/s, ft³/s, and/or acre-feet per day
is the force exerted by gravity on a unit volume
of fluid
Specific weight, y
describe the relationship between discharge and dynamic viscosity
Discharge is also inversely proportional to the dynamic viscosity of the fluid, μ
a measure of the resistance of the fluid to the shearing that is necessary for the fluid flow.
dynamic viscosity of the fluid
a function of the size of the openings through which the fluid moves.
Intrinsic Permeability
Intrinsic Permeability unit
darcy
represent some of the most prolific producers of groundwater.
Unconsolidated coarse-grained sediments
what is intrinsic permeability a function of?
size of openings thru which fluid moves
relationship of sediment grain size to surface area the water contracts
The smaller the size of the sediment grains, the larger the surface area the water contacts.
For well-sorted sediments, what is the relationship of intrinsic permeability and grain size
For well-sorted sediments, the intrinsic permeability is proportional to the grain size of the sediment
4 factors relating intrinsic permeability to grain size For sand-sized alluvial deposits
- median grain size increases = larger pore openings = increase permeability
- increase standard deviation of particle size = more poorly sorted = decrease permeability for median diameter
- greater decrease for coarser than finer for number 2
- Unimodal (one dominant size) samples have a greater permeability than bimodal (two dominant size) samples
Clastic sedimentary rocks have primary permeability characteristics similar to those of ?
unconsolidated sediments.
what can reduce the size of the throats that connect adjacent pores for Clastic sedimentary rocks
diagenesis
how can diagenesis can reduce the size of the throats for clastic sedimentary rocks
cementation and compaction
these rock types typically have a low primary permeability
Crystalline rocks
these rocks can have a high porosity and if the openings are large and well-connected, then they may also become highly permeable
Volcanic rocks
3 ways Secondary permeability can develop in rocks
through fracturing, dissolution, and weathering.
The hydraulic conductivity of earth materials can be measured in the laboratory thru the use of ?
permeameters
It is the undulating surface at which pore water pressure is equal to atmospheric pressure.
Water Table
In the absence of groundwater flow, what happens to the water table
flat
A sloping water table indicates ?
the groundwater is flowing
Groundwater discharge zones are in ?
topographical low spots
The position of the water table often follows the ?
general shape of the topography.
Groundwater generally flows away from _____________ toward __________.
away from topographical high spots —-> toward topographical lows.
It is a geologic unit that can store and transmit water at rates fast enough to supply reasonable amount to wells.
Aquifer
Intrinsic permeability of aquifers would range about ?
10^-2 darcy upward.
7 common rock units known to be aquifers.
Unconsolidated sand and gravels, sandstone, limestone, dolomites, basalt flows, fractured plutonic and metamorphic rocks
geologic unit having little or no intrinsic permeability
Confining Layer
intrinsic permeability values of confining layers
less than 10^-2 darcy
3 classifications of confining layers
- aquitard
- aquiclude
- aquifuge
an absolutely impermeable unit that will not transmit any water
Aquifuge
is a layer of low permeability that can store groundwater and also transmit it slowly from one aquifer to another
Aquitard
Aquitard is aka?
leaky confining layer
is a rock unit or geologic formation through which inappreciable quantity of water moves or is transmitted.
Aquiclude
Aquifers that can be close to the land surface, with continuous layers of materials of high intrinsic permeability extending from the land surface to the base of the aquifer.
water-table aquifer or unconfined aquifer
aquifers that are overlain by a confining layers.
Confined aquifers
another name for confined aquifers
artesian aquifers
the surface representative of the level to which water will rise in a well cased to the aquifer (for unconfined)
Potentiometric surface for a confined aquifer (water table niya)
water table of confined aquifer
potentiometric surface
If the potentiometric surface of an aquifer is above the land surface, what may occur?
a flowing artesian well may occur.
is a layer of saturated soil which forms above the main water table, some not enough to sustain a populations
Perched aquifer
3 ways confined aquifers can form
- created by alternating aquifers and confining units deposited on a regional dip
- created by deposition of alternating layers of permeable sand and gravel and impermeable silts and clays deposited in intermontane basins
- created by upwarping of beds by intrusion
things u need to create potentiometric surface maps in GIS
-drilling data
-coordinates
-depth to water table for more than 10 samples
local recharge zones
lakes
is the measure of the amount of water that can be transmitted horizontally through a unit width by the saturated thickness of the aquifer under a hydraulic gradient of 1.
transmissivity
transmissitivity equation
the transmissivity is the product of the hydraulic conductivity and the saturated thickness of the aquifer:
T=bK
how to calculate transmissivity for multilayer aquifer
total transmissivity is the sum of the transmissivity of each of the layers
the volume of water that a permeable unit will absorb or expel from storage per unit surface area per unit change in head
Storage Coefficient
aka storage coefficient
Storativity
aquifer characteristic referring to when In then saturated zone, the head creates pressure, affecting
the arrangement of mineral grains and the density of the water in the voids. If the pressure increases, the mineral skeleton will expand; if it drops, the mineral skeleton will contract.
Elasticity
is the amount of water per unit volume of a saturated formation that is stored or expelled from storage owing to the compressibility of the mineral skeleton and the pore water per unit change in head.
Specific Storage
aka specific storage
Elastic Storage Coeffcient
At a given plane in a saturated aquifer, a downward stress is placed on the aquifer skeleton by the weight of the overlying rock and water. This called ?
total stress
There is an upward stress on the plane caused by the fluid pressure. The upward stress will, in part, counteract the total stress, so the resulting stress that is actually borne by the aquifer, called the _________, is less than the total stress
effective stress
8 aquifer characteristics
- transmissivity
- storage coefficient (storativity)
- elasticity
- specific storage (elastic storage coefficient)
- compressibility
- effective stress/total stress
- homogeneity
- isotropy
total stress equation
Ot = Oe + P
Oe =effective stress
P =pressure
one that has the same properties at all locations
homogeneous unit
In __________ formations, hydraulic properties change spatially. One example would be a change in thickness.
heterogeneous
are units that have the same intrinsic permeability in all directions.
Isotropic
if the geometry of the voids is not uniform then there may be a direction in which intrinsic permeability is greater
Anisotropic
factor q in darcys law
specific discharge
another term for specific discharge
Darcinian velocity
T or F: Darcinian velocity is not a true velocity as the cross-sectional area, A, is partially blocked with soil material.
True
is a measure of the quantity of any fluid flow over unit time.
Discharge
how is discharge measured as?
volume or mass
how to plot the gradient of potentiometric surface
using interpolation method
