Midterm Flashcards
units of porosity
%
symbols of porosity
n
define porosity
space in a sediment or rock for the storage of fluids
types of porosity in rocks and consolidated sediments
1) primary
2) secondary
- solution
- fracture
3) vesicular
define primary/original/depositional porosity
on the surface
same time that the sediments were deposited
main type in clastic aquifers (intergranular)
define secondary/induced porosity
forms in the rock after lithification or crystallization
types: solution and fracture
define solution porosity
dissolution of a carbonate (limestone/dolomite)
from acidic solutions (H2CO3) migrating through sub surface
vuggy porosity is carbonates
define fracture porosity
breaking of the rock by stress (folding & faulting) or by differential cooling (columnar jointing) of volcanic rock
define vesicular porosity
forms in basalts
as gas escapes from lava
define hydrology
study of fresh water (surface and ground) in the environment
define hydrogeology
study of the flow of water through soils, sediments, and rocks
reasons to study groundwater
1) drinking water
2) commercial and industrial uses
3) groundwater maintains stream flow
driving force of the hydraulic cycle
sun/solar radiation
hydraulic cycle
1) evaporation/transpiration
2) condensation
3) precipitation
4) storage
define saturated zone
a subsurface zone where the pores are completely filled with water
define unsaturated zone
subsurface zone where the pores will contain both air and water
define permeability
measure of the ease with which a fluid can move through a rock
define water table
the surface which separates the zone of saturation from the zone of aeration (unsaturated zone)
define capillary fringe
a subsurface zone which is completely saturated with water but if above the water table, water moves upward in this zone because of surface tension
define flowing artesian aquifer
a location in a confined aquifer where the potentiometric surface is structurally above the ground surface
define transmissivity (T)
measure of the amount of water the can be transmitted horizontally through a saturated aquifer
define T with units
transmissivity
m2/s
define b with units
aquifer thickness
m
define K with units
hydraulic conductivity
m/s
define storativity (S)
volume of water absorbed or expelled after a change in head
define S with units
storativity
unitless
define Ss with units
specific storage
1/m
define specific storativity
volume of water of a saturated formation that is stored or expelled from storage
things that affect Ss in confined aquifers
a - the compressibility of mineral matrix
B - the compressibility of pore water
if the head decreases in a confined aquifer, the effects are
pressure
mineral grains will rearrange (and H2O is expelled)
pore water will expand (and H2O is expelled)
define pw with units
density of water
kg/m3
define g with units
gravity
m/s2
define a with units
compressibility of mineral matrix
1/Pa
1/N/m2
define B with units
compressibility of water
1/Pa
1/N/m2
S=Ss*b is storativity in what kind of aquifers
confined / artesian wells
how water is expelled or absorbed in an unconfined aquifer
a
B
fluctuations of the water table (Sy - specific yield)
S=Sy+Ss*b is storativity in what kind of aquifers
unconfined, watertable wells/aquifers
define specific yield
Sy is the volume of water absorbed or expelled from storage per change in water tale elevation
Sy has a bigger influence than Ss * b (except for very fine grained sediments)
define Sy with units
specific yield
unitless
define Vw with units
volume of water drained
m3
define A with units
surface area overlying the drained aquifer
m2
define Vh with units
average decline in the head
m
define homogeneous formations
hydraulic conductivity do not vary from one location to another in the formation
when modelling, K is the same throughout the aquifer
define heterogeneous formations
hydraulic conductivity vary from one location to another in the formation
geologic features that cause heterogeneity in aquifers
1) unit thickness
2) layering
3) faces changes (changes in depositional environments)
4) solution porosity
5) fractures
spacial variability depend on
homogeneous and heterogeneous formations
directional variability depend on
isotropic and anisotropic formations
define isotropic formations
hydraulic conductivity is independent of the direction of measurement
define anisotropic formations
hydraulic conductivity vary depending on the direction of measurement
causes of anisotropic formations
1) imbrication of grains
2) clays
3) basalts
define Kb in parallel beds
weighted arithmetic average
influenced by the most permeable bed
m/s
define Kb in series beds
weighted harmonic average
m/s
porosity in unconsolidated sediments is controlled by
grain shape
grain size distribution (well vs poorly sorted)
particle arrangement (cubic vs face centered packing)
what does not affect the porosity in unconsolidated sediments
particle size
define k with units
permeability
m2
permeability (k) is affected by
pore throat diameter
sorting
fracturing
how to estimate r average for well sorted ss
use the average radius of all the grains in the sediment
how to estimate r average for poorly sorted ss
r pore = r smallest grains = D10 value
why is the porosity of gravel and silt approx the same
grain size does not affect n
what unconsolidated sediment has the greatest porosity
clay because of the platelet shape (compressibility)
define karst limestone
limestone on the surface of the earth
theory behind darcy’s law
volumetric flow rate of groundwater (Q) through an area (A) of a porous media
Darcy’s law equation
Q=q*A
define Q with units
volumetric groundwater flow
m3/s
define q with units
specific discharge
m/s
define A with units
cross sectional area
m2
2 major driving forces to groundwater flow
gravitational force
pressure force
define z with units
elevation head
m
define trident thing with units
pressure head
m
define h with units
head
m
K is dependent on
intresnsic properties of the rock
properties of the fluids
define i with units
hydraulic gradient
unitless
flow through an open tube equation
q=vn where n=100%
flow though a porous media (aquifer) equation
q=vn where n=whatever porosity %
2mm diameter of grains differentiates between
gravel and sand
field descriptions of soils include
main sediment type (GRAVEL) minor components (gravel) color moisture content consistency (fine grained) density (coarse grained) geological origin foreign materials odour presence of contaminants
the 3 layers in a jar test are
mud
silt
sand
classification of fine grained soils is based on
grain size
plasticity
define cohesive
ability of a soil to be reshaped in the presence of some moisture without crumbling
define plastic limit (PL)
the moisture content of a soil which will crumble when rolled into 1/8in d threads
define liquid limit (LL)
the moisture content of a soil required to close a distance of 0.5in along the bottom of a groove after 25 blows
what are flow nets
way to represent 2D ground water flow
assumptions with flow nets
homogeneous and isotropic
in a steady state
flow nets are drawn on what maps for unconfined aquifers
water table map
flow nets are drawn on what maps for confined aquifers
potentiometric surface
3 types of boundary conditions for flow nets
1) impermeable/no flow
2) constant head
3) water table
adequate production of a household well
0.5 gallons per min
no additional water storage capacity is needed over
5-10 gallons per min
3 things in choosing a well location
1) max yield
2) min contamination
3) max safe operation
preliminary info sources for choosing a well
1) alberta geologic survey
2) hydrogeologic maps and reports
3) alberta groundwater data base (drilling records are free but might not be reliable)
4) door to door surveys
locating water wells
geophysical surveys (\$\$$) test drilling (log cuttings) downhole geophysics (\$\$$ can use chips but lag time and cavings are problems) pump tests
considerations in choosing well locations
1) geology of area (yields)
2) contaminant sources
3) minimal distances
4) accessibility for cleaning/testing/monitoring (eg not close to buildings)
5) ground slope
4 basic types of water wells
dug (hand)
driven or jetted (hydraulically)
bored (auger wells)
drilled (rotary wells)
common casing materials
steel (strong, corrodes)
plastic (popular, new no recycled)
by alberta law, casings must
min of casing must be 10.61cm/4in
must extend more than 20cm above surface to prevent contamination
must extend more than 60cm above highest flood record in area if no water-tight cap
if non-metallic above ground, must be protected by steel casing
intake purpose
allow water but not sediment to enter well during production (but not during development)
intake types
screens
slotted/perforated casings/liners
screens material and rx type
stainless steel mostly
unconsolidated sediments
slotted/perforated casing/liners design and rx type
irregular holes consolidated formations (bed rx)
size of openings in screens
easy entry of water preventing sediments
during development: D60
2 types of annular fill
annular seal
filter/gravel pack
annular fill
must be filled with impermeable inert material
materials: cement, bentonite, grout
filter/gravel pack
gravel, clean quartz sand, glass beads
finergrained aquifers to improve yield
surface considerations under law in alberta
water cannot enter the well
securely covered by the driller and owner
pump house can only have water well and the pump
water must not pool on the pump house floor
well completion 3 steps
well development
well disinfection
yield test
well development
remove fine sediments and water introduced to aquifer
methods used to develop a well
overpumping
jetting (air or water)
surging (2 directions of flow)
developed well types
naturally developed well (remove fine material, original aquifer material)
gravel-packed (sand/gravel filter is placed in the annulus, not created from og aquifer)
well disinfection
cl 200mg/L for 12hrs
yield test
min 2hr/until water level in well returns to min 90%
determine rate and depth
safe pumping rate
extended period of time without lowering the level below pump intake
pump must not lower the h2o level below
top of aquifer
top of perforations
top of pump intakes
submissions to AEP
general stratigraphic log (geology)
well construction and design details (well license)
geophysical surveys
indirect
general processes of geophysical surveys
energy waves at or near surface
pass into subsurface
return to surface
processed and interpreted
geophysical surveys give info on
statigraphy
structural features
groundwater
man made inclusions
seismic waves
p waves (primary, want to interpret) s waves (secondary, can also penetrate subsurface) surface waves (noise)
seismic waves travel at
the speed of sound
p waves
direct (across the surface)
reflected (bounces off subsurface interface)
refracted (travels along interface, must have differing subsurface densities)
electrical conductivity/resistivity surveys
em-31/34/38
ec surveys measure ability of a subsurface formation to
transmit (conduct)
or impede (resist)
…. the movement of an electrical current thru
ability of a formation to conduct dependent on
porosity
v of water in pores
type of fluid
ground penetrating radar
microwaves
speed of light
source of GPR ground penetrating radar
transducer on a wagon or sled
method of GPR
max depth of 20m bounce conductivity time time to depth
uses of GPR
shallow disposal sites water table HC contamination stratigraphy caverns
EM surveys
electric current thru transmitter coil
close to ground
electromagnetic field
colored map
EM 31
long pole, fixed depth
salt water plumes, metals
EM 34
2 separate units, 60m deep
deep and large salt water plumes, map bedrock topography
EM 38
run at the same time as EM31
shallow depth less than 1.5m
agriculture
ERI ERT electrical resistivity surveys
groundwater exploration
resistivity is inverse of conductivity
direct current
2 electrodes INTO ground
resistivity surveys in 2 different fashions
electrical sounding
horizontal profiling
electrical sounding
variations in depth
go to diff depths with each pass
wenner or schlumberger
horizontal profiling
lateral variations
electrode spacing is not changed
recorded and contour mapped
