Hydrogeology
Water table, aquifers and
aquitards, potentiometric surface
A look at the
water table, aquifers and aquitards, potentiometric surface, cone of
depression, confined and unconfined aquifers
I) Water Table:
A)
Background:
- A
misunderstood concept among the general public:
-
People (well owners...) often refer to underground "lakes" and
"rivers"
-
As geologists: most of us have a better
understanding of the water table
B) 3 zones of water distribution in
the subsurface:
-
Zones are defined by differences between fluid
pressure and atmospheric pressure
See Figure 4.17 from Fetter, p. 108
1) Saturated (phreatic)
zone:
- Fluid
pressure > overlying atmospheric pressure
- Caused by
the weight of the overlying water
- As top of
saturated zone is approached:
-
Fluid pressure decreases
- At the
water table: fluid pressure = atmospheric pressure
Def: Water Table: The undulating surface at which pore water pressure = atmospheric
pressure
-
Map of the water table = a potentiometric surface
map
2) Capillary fringe:
- A transition zone
- Still has abundant hygroscopic water (water in pores)
- Can be very thick
ex: up to 40' thick
near the Nevada test site
- This zone is an important consideration with contaminant flow:
Contaminants may
be soluble in pore water even if pores aren't saturated
3) Unsaturated (vadose)
zone:
-
Hygroscopic pressure is less than atmospheric pressure
- Capillary
water is minimal
- Water is "bound"
water: contained in clay mineral structure or in organic compounds
II) Rules of the
water table:
See textbook, p. 109
A) In the absence of ground-water flow, the water table will be flat
B) A sloping water table indicates that groundwater is flowing
C) Groundwater discharge zones are in topographic low spots
D) The water table has the same general shape as the surface topography
E) Groundwater generally flows from topographical high spots toward
topographical low spots
Note: last two rules primarily apply to humid regions
III) Aquifers:
A) Definitions
Confining layer: A geologic layer with little
intrinsic permeability (ki < 10-2 cm2)
- Does not transmit significant amounts of water
- Below the water table, all units contain groundwater
- Rates of water storage and transmittance are relative
- A problem: comparisons are relative; one person's aquifer (silty sand
in an otherwise clay-rich area) may be another person's aquitard (silty sand in
a gravelly area)
- Brings us to some more definite terms:
Aquifuge (confining layer): is essentially
impermeable
Aquitard (leaky confining layer): can transmit small
amounts of water
B)
Types of aquifers:
1) Unconfined aquifers
(water table aquifers)
See Figure 4.19 from Fetter, p. 111
- Are close to the land surface
- Have continuous permeable layers from land surface to the base of the
aquifer
- Recharge is by seepage from land surface OR by baseflow (lateral
groundwater movement)
2) Confined aquifers:
- Are overlain by a confining layer
- Amount to a non-renewable resource
- Water may be 100,000's or millions of years old
See Figure 4.20 From Fetter, p. 112
a) Formation
of confining aquifers:
-
Form in several different geologic settings:
i)
alternating units deposited on a regional dip
ii)
facies changes
iii)
upwarp created by intrusions
b) Methods
of recharge:
-
2 possible types of recharge:
i)
Outcrop area:
-
May be far away
ii)
Slow leakage from overlying leaky confining layer
-
Recharge is very slow
c) Artesian
wells
-
A special case in some confined aquifers
See Figure 4.21 from Fetter, p. 113
-
Water in confined aquifers is under pressure
- Creates a
potentiometric surface that lies above the upper confining layer
Def:
Potentiometric surface: the height to which water will rise in a well
-
Artesian aquifer: pressure in a confined aquifer (represented by potentiometric
surface) is higher than the bed surface, water in a well bore will rise above
the bed
-
Flowing
Artesian well: Potentiometric surface is higher than the land surface
d) Pumping
in confined aquifers:
-
Pumping lowers the aquifer surface in a cone
of depression
See Figure 4.24 from Fetter, p. 117
-
Cone of depression represents a pressure boundary
-
Position above the upper confining bed is not really related to water levels
(potentiometric surface) in upper bed
3) Perched aquifers:
- Are
unusual
- Are small
-
Occur when a confining layer prevents groundwater from percolating through the
unsaturated zone
See Figure 4.22 from Fetter, p. 113
IV) Potentiometric surface maps
- Are two-dimensional representations of a three-dimensional
surface (the water table)
- Are similar to contour maps
- Are useful for identifying
groundwater divides:
Definition:
Groundwater divide: a “high” on the contour map that restricts groundwater
flow:
A drop of water on the divide will split, go in
either (both?) directions
Separates groundwater basins, is
important when considering contaminant flow.
See Figure 4.23 from Fetter, p. 115
- A minor difference between contour
and topographic maps:
Potentiometric surface
lines can divide or converge
A)
Constructing a potentiometric surface map:
- Has many similarities to constructing a
contour map:
- Use a topo map as a
base map
-
Topo map influences interpolations when contouring, since groundwater mimics
topography in unconfined aquifers
-
In confined aquifers: groundwater potentiometric surface does not necessarily
mimic topography
- Groundwater v's uphill
at gaining streams
- Groundwater v's
downhill at loosing streams
B)
Measurements must meet certain criteria to be included on a potentiometric
surface map:
1) must be made within a
short time interval
2) must be made from the
same aquifer
3) must be referenced to
a common datum (normally sea level)
4) water must be in
static state (not responding to pumping
Note:
the topographic map influences interpolations when contouring, since
groundwater mimics topography in
unconfined aquifers
Hydrogeology Lecture
#8
Transmissivity,
storativity, compressibility, leakage, homogeneity, isotropy
Aquifer Characteristics:
I) Aquifer characteristics:
- We have talked about porosity, effective porosity,
intrinsic permeability, hydraulic conductivity
-
Now: will get into transmissivity, storativity,
specific storage
A)
Transmissivity:
- Another aquifer property, moves beyond the concept
of Darcy's hydraulic conductivity
Def: Transmissivity: A
measure of the amount of water that can be transmitted horizontally through a
unit width by the full saturated thickness of the aquifer under a hydraulic
gradient of 1.
- Think of this as a "
window frame" in the aquifer
- How much water can pass
through the window frame?
- Note: assumes horizontal
groundwater movement
- This isn't always true
- Formula:
T = Kb
where: T = transmissivity,
units = L2/T
common
units: ft2/d, m2/d
K = hydraulic conductivity, units = L/T
common
units: ft/d, m/d
b = saturated thickness of the aquifer, units
=L
common units: ft, m
- This adds a second dimension that was missing when we talked about
hydraulic conductivity
- Transmissivity can be
summed for multilayer aquifers:
T = Si=1n Ti (sum from i=1 to n of Ti)
B) Storativity:
- The next aquifer property
- Becomes a factor when the
aquifer looses or gains water
-
Each aquifer has a different ability to expel or
absorb water
Def: Storativity (S): The volume of water that a permeable unit will
absorb or expel from storage per unit
surface area per unit change in head
- Units: storativity is a
dimensionless quantity
- Storativity is due to
porosity in the aquifer
- Storativity is different for confined and unconfined aquifers: will
discuss this below
- Aquifers also have elastic properties:
- This leads us
to the concept of specific storage:
