GROUNDWATER

1
GROUNDWATER Freeze and Cherry Groundwater and
Fetter Applied Hydrogeology are the old and
new testament of groundwater hydrology Dingman
has a good groundwater section (Chapter 8), which
you will be responsible for by the end of the
term
2
Properties of Aquifers Aquifer - Geologic unit
that can store enough water and transmit it at a
rate fast enough to be significant recall Poros
ity is -can be measured as 1 minus the ratio
of the bulk density to the particle density of
the material
n (Va Vw)/V Vv/V
3
Effective porosity is the fraction of the
porosity that is available for transporting water
(excludes fraction of pores too small to hold
water, or those that are not inter-connected -
can be measured in the lab directly by saturating
a dried sample of known volume and measuring
water uptake in a sealed chamber over time - for
unconsolidated coarse-grained sediments there is
no significant difference
4
Uniformity coefficient - measure of
sorting On a grain-size vs finer plot d60
is the grain size diameter that corresponds to
60 finer by weight d10 is the grain size
diameter that corresponds to 10 finer by
weight (i.e. d60 is coarser than d10) - Cu less
than 4 is well sorted, more than 6 is poorly
sorted
5
Porosity of Sedimentary Rocks - sedimentary
rocks usually have lower porosity than
unconsolidated sediment because of compaction,
and infilling of cementing material (e.g.
calcite, dolomite, silica), although dissolution
can reverse the latter effect Ground water can
be associated with both Primary Porosity -
porosity between grains
6
Secondary pores (fractures) can be enlarged
through dissolution by the ground water flow -
sedimentary rock may have primary porosity from
deposition and secondary porosity from fractures
along bedding planes - secondary porosity also
possible in cohesive sediments through
wetting/drying, tectonic activity, etc. -
limestones, dolomites, gypsum can all have
deposition reversed - when in groundwater zone
dissolution can occur - flow starts initially
through limited pore spaces, fractures, and
bedding planes, and porosity enlarges over time
7
Porosity of Plutonic and Metamorphic Rocks -
primary porosity extremely low, but often not
zero - porosity increased over time by weathering
and fracturing - fracturing increases porosity of
crystalline rocks 2 to 5 - chemical and physical
weathering increases with porosity - highly
weathered plutonic and metamorphic rocks can have
posities between 30 to 60 - sheet-like
structures of weathering minerals such as micas
can have very high porosities Porosity of
Volcanic Rocks - lava cools rapidly at surface,
traps degassing products - holes in rock
(vesicular) may or may not be interconnected -
cracks form during cooling - volcanic rocks vary
in porosity but can be very high - basalt has
lower gas content with porosity between 1 and
12 - pumice (very high gas content) can have
porosity approaching 90 (but effective porosity
if not this high) - weathering of volcanic
deposits will also increase porosity
8
Specific Storage (Ss) Storativity or
Storage Coefficient (S) where b is the
saturated thickness of the aquifer Specific
Yield (Sy) - volume of water that drains from a
saturated rock by gravity to volume of rock - in
an unconfined aquifer, SSy
9

• Specific Retention (Sr)
• volume of water held behind by capillary forces
  to volume of rock
• (this water is also referred to as pendular
  water)
• - essentially identical to the concept of field
  capacity
• - specific yield can be determined in the lab
  using soil column methods
• - soil in column is saturated from below and
  allowed to drain without evaporation going on
• - allowed to drain for months before equilibrium
  is reached
• - volume of water drained to the volume of column
  is Sy
• - above difficult to do with rock
• - can also be measured in the field with pump
  tests (discussed later)

10
Hydraulic Conductivity of saturated media and
Darcys Law - ability of the rock to transmit
and hold water are most important hydrologic
properties - as pointed out only effective
porosity important with regards to groundwater
flow (e.g. vesivcular basalt - lack of
interconnectivity, Clays and shales - pores too
small)
11
Darcys experiment Reality For educational
purposes only
12
Darcys experiment Henry Darcy in 1856 was
playing around with movement of water through
sand filtration columns for the City of Dijon,
France. Darcy found that the flow of water
through a bed of a given nature is -
proportional to the difference in the height of
the two ends, - inversely proportional to the
length of the flow path - proportional to the
x-sectional area of the pipe - flow is further
related to a coefficient dependant on the nature
of the media is Darcys Law for saturated
flow through a pipe, Where ha and hb are heads
at two ends of pipe L is length pf flow
path K is hydraulic conductivity A is the
cross-sectional area of the pipe Q is discharge
13
or negative sign is for flow in direction of
decreasing head dh/dl is the hydraulic gradient
recall Darcys Law for unsaturated flow from text
is - we are not substituting Vx here, Q is
discharge (remove A from above equation and we
will have a velocity of flow)(see below). - the
term d(zp/(w) is now combined into one term dh
14
Darcys Law
Ohms Law (rearranged) Where i current K
electrical conductivity K 1/ ? where ?
resistivity V voltage L distance A area
15
Form of Darcys law we will use most often
calculates specific discharge v is not a
true velocity v is also known as the Darcian
Velocity K is also known as Darcys
proportionality constant or coefficient of
permeability - whereas in unsaturated flow K is
a function of soil moisture, soil and fluid
properties, in saturated flow it a function of
soil and fluid properties only
16
- discharge is proportional to the specific
weight ( of the fluid - Q 1/ (dynamic viscosity
of the fluid - resistance of fluid to shearing) -
Qd2 (square of the diameter of pores) Darcys
law can be re-expressed as where C is a
shape factor (because x-sectional area of a pore
is also related to its shape)
17
- intrinsic permeability (Ki) is representative
of the character of the porous medium alone -
basically a function of the size of the openings
through which the fluid moves - larger the square
of the pore diameter, the lower the resistance to
flow - Ki is essentially the openess of the
flow path (in e.g. cm2) - can also
expressed in units called darcys 1
darcy9.87x10-9 cm2
No fluid properties
18
(No Transcript)
19
By convention, if we always refer to K as the
hydraulic conductivity, we can refer to Ki the
permeability (Note Freeze and Cherry use k for
permeability which is much more common) -
during the formation of clastic sedimentary rock,
cementation and compaction can restrict throats
between pores - therefore, even though porosity
may be reduced only slightly, permeability can be
greatly reduced - crystalline rocks have low
permeability, although volcanics can have high
permeability if connectiviy is good - secondary
permeability (like secondary porosity), through
fracturing, weathering
20
Estimation of K Hazen Method - approximation -
for sandy sediments with d10 is betweem 0.1 and
3.0 mm - developed on the basis of sand filtering
for drinking water - durable empirical
equation K is in cm/s d10 is in cm C is
unitless coefficient which ranges from 40 in v.
fine sand to 150 in coarse sand
21
Koseny-Carmen Equation -for more non-uniform
soils - explicitly incorporates fluid properties
and porosity where n is porosity
22
Fair-Hatch Equation - uses first two terms in
the Koseny-Carmen equation and replaces the third
with where m is a packing factor (usually
5) C is the shape factor (6 for spheres, 7.7.
for angular) P is of material held between
adjacent seives dm in this case is taken as the
geometric mean of the rated sizes of adjacent
seives
23
Permeameters - tube filled with sample - can be
actual tube used to collect sample so minimal
disturbance 2-types of permeameter 1.
Constant-head - used for non-cohesive sediments
(sands) - best for samples with Kgt0.01cm/min -
rearranging Darcys law we can obtain where V
is the volume of water discharging in time t L is
the length of sample A is the x-sectional areah
is the hydraulic head K is the hydraulic
conductivity - dh/dl should mimic field -h should
never be greater than 0.5L (dont want v to get
too high. Why?)
24
(No Transcript)
25
2. Falling-head permeameter - used for cohesive
seds with lower K - initial water level in a
falling head tube is measured as h0 - after a
period t (several hours), level is measured again
as h (See Fetter for derivation of these
equations) where At is x-sectional area of
falling head tube Ac is the x-sectional area of
the sample tube For both constant-head and
falling-head - make sure sample is fully
saturated - use dearied water if
possible Question How would we arrive at Ki
given a value for K above?
26
The Water Table Question How is the water table
defined? The following generalizations are
valid 1. In the absence of flow the water table
will be flat 2. A sloping water table indicates
flow 3. Ground-water discharge occurs in low
zones 4. The water table has the same general
shape as the surface topography (but less relief
change) 5. Ground water generally flows from
topographic highs to lows
27
Aquifers - near the Earths surface there are
few materials that are absolutely impermeable -Ki
for aquifers is generally gt10-2 darcy Confining
layer - layer having low or no peremeability -
whether a layer is considered confining or not
will depend on main aquifer material - usually
confining layers have some permeability, just
very low types of confining layers Aquiclude -
layer of low permeability that can store and
transmit groundwater slowly between aquifers (now
more commonly referred to as leaky confining
layer) Aquifuge - absolutely impermeable and
contains no water
28
Unconfined aquifer (water table aquifer) - layer
where highly permeable material extends to the
water table - recharge can be from any
direction Confined aquifer (artesian aquifer) -
aquifers overlain by confining layer - recharge
from recharge areas where strata tips up, or
leakage through confining layer - artesian wells
are drilled into confined aquifers - water from
an artesian well can rise above surface -
potentiometric surface is the level to which
water will rise in a cased well Perched Aquifer
- aquifer in the vadose zone because of a lens of
impermeable material - common in glacial outwash
(clay from ponds), volcanic deposits (weathered
ash deposits with low K sandwiched between high K
basalt)
29
(No Transcript)
30
Water Table and Potentiometric Surface
Maps Question What is the difference between
the water table and the potentiometric
surface? - surfaces measured in wells open only
to the aquifers of interest - measurements should
be made within a brief period of time - each well
needs to be tied into a common datum (e.g. sea
level) - datum should be same for surface
topography (esp. ponds, springs, etc.) -
measurements in pumping wells should be made
with no pumping and after enough time for
rebound - map should include location of lakes
and streams - potentiometric levels are not
influenced by surface topography - very important
to know whether your well is measuring confined
or unconfined aquifer - also important if
measuring potentiometric surface to know you are
in same confined aquifer
31
Extrapolating Well Measurements - often we want
to map the direction of water flow in a confined
or unconfined aquifer, but do not have enough
wells - graphical solution for 3 or 4-points 1.
Map scale drawing showing location of wells 2.
Note water level at each well 3. Measure map
distance and elevation change between each
well 4. Find map distance for unit change in head
between each pair 5. Mark even increments of
head change along each line 6. Make lines of
equal head
32
Piezometer and Piezometer Nests - basic device
for measuring head is a tube or pipe through
which the water level can be measured - open to
water flow at bottom and open to atmosphere at
top - intake usually slotted pipe or commercially
available well point - must allow for intake of
water, but not clastic material - water levels
measured by pressure transducers or with manual
soundings - nests are groups of peizometers at
one location, but with well points going to
different depths to obtain vertical hydraulic
gradient
33
Steady-State flow vs Transient flow Steady-state
flow - when at any point in a flow field,
magnitude and direction of flow are constant with
time - flow velocity may vary from point to point
in the field, but the pattern is constant through
time Transient flow (unsteady flow, nonsteady
flow)
34
Compressibility and Effective Stress - at any
point in an aquifer, the weight of overlying
material applies downward stress on the aquifer
material. This is Total stress - upward stress on
the aquifer material caused by fluid pressure
counteracts total stress to a degree - difference
is the effective stress (i.e. the stress actually
applied to the aquifer skeleton) and of
course In confined aquifers P can change with
little change in saturated thickness of aquifer.
In these cases sT will remain essentially
constant so that se is what changes Question
What will happen to the effective stress acting
on the aquifer skeleton if we pump water out of a
confined aquifer?
35
Compressibility of water is constant at 4.4x10-10
Pa-1 Need a estimation of compressibility of the
solid matrix -assume that the matrix acts as a
elastic body - subject the matrix to a change in
effective stress and it will deform Aquifer
compressibility is defined as where a is
aquifer compressibility db is the change in
aquifer thickness b is the original aquifer
thickness
36
There are 2 ways an aquifer can compress 1. By
compression of individual rock grains and
crystals 2. The first is negligible to
non-existent in most cases If necessary,
compressibility can be determined in the lab
using loading cells - aquifer compressibility
will also depend on loading history of aquifer -
compressibility of water close to
least-compressible rock - must be realized that
stress field at depth is 3-D, but changes in the
horizontal stress field can be considered
negligable - i.e. large changes in stress only
occur in the vertical direction Summary of
compaction when fluid pressure (head) is reduced
in a confined aquifer, the following will
occur 1. Effective stress will increase 2.
Aquifer material now bears an additional portion
of the overburden load 3. Aquifer compacts a bit
releasing water from storage 4. Reduction in
water pressure also causes water to expand
slightly releasing additional water - if
compaction is propagated to the surface, land
subsidence can occur
37
(No Transcript)
38
(No Transcript)
39
(No Transcript)
40
(No Transcript)
41
Near Las Vegas
Earth Fissures
42
Approximate maximum subsidence amounts as of 1997
for selected locations in the Southwest
Arizona Arizona Nevada Nevada California California Texas Texas
Eloy 15 ft Las Vegas 6 ft Lancaster 6 ft El Paso 1 ft
West of Phoenix 18 ft New Mexico New Mexico Southwest of Mendota 29 ft Houston 9 ft
Tucson lt1 ft Albuquerque lt1 ft Davis 4 ft    
    Mimbres Basin 2 ft Santa Clara Valley 12ft    
        Ventura 2 ft    
43
(No Transcript)
44
Now we can use interferometric processing of
Synthetic Aperture Radar (SAR) data.
45
Homogeneity and Isotropy Homogeneous units -
values of Storativity and K similar throughout
for sandstones similar grain size, porosity,
cemetation, thickness for plutonics and
metamorphics similar fracturing, strike and dip,
etc. - definition is usually arbitrary, but one
common one is that the distribution of K must be
monomodal Heterogeneous units - hydraulic
properties change spatially - can be changes in
thickness, bedding of different hydraulic
properties, etc. - interlayered clay and sand
deposits can create extreme layered
heterogeneity - limestones often heterogeneous
because solution pathways form along bedding
planes discontinuous heterogeneity - occurs at
faults or large-scale stratigraphic features
46
(No Transcript)
47
trending heterogeneity - usually in units where
deltas, alluvial fans, etc have formed - -
trending heterogeneity in large sedimentary
formations can cover 2 to 3 orders of magnitude
in Ki If a porous medium has equal intrinsic
permeability in all directions it is said to be
isotropic If the pattern of voids allows for a
path of least resistance (i.e. direction in which
Ki is higher) the unit is said to be
anisotropic - fractured rocks, basalts often
highly anisotropic - sedimentary rocks may have
many homogenous units - directions of maximum
and minimum anisotropy are the principal
directions of anisotropy If an xyz coordinate
system is setup along the principal
directions KxKyKz is an isotropic
situation KxKyKz is an anisotropic
situation KxKyKz is a transversley isotropic
situation (common in layered deposits