Lec 04 – Chapter 7 - Groundwater & Karst topography
(dissolution of rocks, creation of features)
First, an intro (or review) of some important basics
I.
Porosity / Permeability
A. “phys properties that control volume & mvmnt of GW are:
i. porosity = % void space in rock
ii. permeability = hydraulic conductivity of rock
B. porosity:
defined:
Ф = volume void space
total rock volume (solid plus voids)
important concepts:
Ф dependent upon grain size and packing in unconsolidated sediments
- Highest in clays just after deposition…as much as 80%, mostly filled with
water
- clays greatly compact as water is expelled..creates major building
problems in places like Gulf Coast (buildings accelerate compaction,
compaction changes the substrate, changing substrate causes
structures to tilt, fail (like Leaning Tower of Pisa)
C. permeability – typically measured by fluid flow velocity, like 10 cm/day
perm is NOT synonymous with porosity!! In fact, limestones are
famous for having vuggy porosity and very poor permeability.
D. Hydraulic conductivity – Darcy’s Law relates discharge (volume per unit
time) to cross-sectional area of flow (A), the permeability of material (K), and
the hydraulic head difference (Δh) between the 2 points of msrmt (Δl)
Q = -K Δh = q
A
Δl
(negative sign helps keep track of the direction of flow)
E. permeability of sediments – some good rules of thumb
- perm dependent upon pore space size and connectedness – so coarse
gravels have much higher perm than clay
- perm decreases as sorting decreases
F. permeability of rocks
depends upon both primary and secondary void space
primary perm often is high in sandstone, conglonerate
perm in limestone usually secondary (created AFTER deposition)
ign, m-m, and chemically sedimentary rx have low primary perm
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II. water table – primarily a “visualization” tool…top of saturated zone
water in subsurface moves downward in the “unsaturated” (“vadose”)
zone
when it hits the saturated zone, water can no longer move vertically, so it
begins to move horizontally
H2O in
vadose zone
H2O in saturated zone
Slope of water table = Δh = hydraulic gradient
Δl
III. Aquifers – can be both
 unconfined (open to atmosphere) and
 confined (with impermeable confining layers above and below)
in the case of confined aquifer, the equivalent surface to the water table is known as
the potentiometric surface)
in either case, these are represented by free water levels found in wells that
penetrate these aquifers.
IV. Thermal springs and geysers
Found in areas with high geothermal gradient
If pressure is high enough and water hot enough, hot water can flash
into steam as pressure is released as water gets close to surface.
V. Subsidence of ground due to fluid withdrawal
In subsurface, fluid in pores between grains serve to help support the
overburden load.
Wt of overburden
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If fluid is withdrawn, grains compact down to grain-to-grain contact, so overall
volume and height of the sedimentary beds is reduced, and ground level drops.
Some examples from Mexico City:
 Entire CITY subsided 13 feet from 1891 to 1959 due to GW withdrawal!
 Opera house alone subsided 10 feet, so now protion of the first floor is below
ground level.
Example from New Orleans:
 Portions of NO have subsided to 13 feet BELOW sea level.
 Result: parts of Miss. R. are located 16 feet ABOVE the city:
VI. Karst topography (important geomorphic features in limestone country)
A. Introduction
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GW effect on topography can be significant in limestone regions
Solution landforms are unique
Created by acidic GW eating away limestone, dolomite, marble, evaporates
“extraordinary landforms” grouped under the blanket term “karst”:
 Closed depressions
 Collapse features
 Caves
 Diverted underground drainage
The term comes from German, Slavic, Italian “krs” or “kras” – “barren ground” or
“bleak, waterless place” – after the type locale at N. end of Adriatic Sea, between
Italy and Yugoslavia
Topography of this region:
 Large caves
 Irregular topo with closed depressions
 Interrupted stream valleys
 Hollows that drain into subsurface (Fig 7-12)
Optimum conditions for karst development are humid conditions and high PCO2
which can come from near plant roots
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B. controls on Karst topography
 lithology and structure
- in terms of chemistry, the closer to CaCO3, the more likely to dissolve and
create karst
- less likely to form karst in dolomites
- in carbonates, primary porosity tends to decrease with time, but
secondary porosity tends to increase with time
- secondary porosity develops with GW flow along bedding planes &
fractures
- permeability becomes enhanced, allowing even more GW to percolate
through, accelerating karst development
most important factor in karst generation = joints, because they ensure
widespread circulation of GW in the host rock
so what is best for karst is:
1. pure calcite
2. intersecting joint sets
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solution process
- complex process involving both dissolved CO2 and H2O - “carbonate
equilibrium”
- essentially, the more CO2 available, the more H2CO3 can be made, which
DISSOLVES CaCO3 (fig 7-13)
- soil water, from near plant roots, is very high in CO2 (green plants create
O2 from CO2 “photosynthetic rxn”), so there is a lot of CO2 around plant
roots as the plants draw it in to make oxygen
GW hydrology – thought has evolved into 2 schools
1. GW percolates downward through Vadose zone to a clearly defined
water table, then moves laterally
2. GW is a series of interconnected cavities, but without a defined WT
Why the controversy?
Because typically a Water Table aquifer has elevations that are pretty similar,
not fluctuating vertically
Suggested solution – consider the system as a water table system, but very
irregular
C. karst landforms
 sinkholes (Fig 7-14, plate 5B)
o form one of two ways:
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downward sol’n of LS by water (solution sinkhole)

collapse of a roof of a solution cavity collapse sinkhole)
Either way, you get a circular / oval sinkhole at the surface.
You see them:
 bowl-shaped
 funnel-shaped
 cylinder-shaped
they usually form in large numbers, over an area (40 per km 2)
usually random spatially, though sometimes may line up along a joint
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Let’s look at specifics….
 Solution sinkholes – Fig 7-15 – solution begins above ground, water works
downward, dissolving from the top down
 Typically bowl-shaped, on low slopes (i.e., nearly flat)
 More frequent in massive, fractured limestone, rather than porous LS (I
wonder why… good exam question)
 Vegetation speed up the sol’n process
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Collapse sinkholes – Fig 7-16, 7-17 – solution begins underground, creates
a cavity into which a roof “collapses”
Stoping (internal dissolution, typically from the top down) believed to be
important failure mechanism
Latest thinking suggests that drought, with lowering of WT, can hasten collapse
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Sinkhole ponds & karst lakes – Fig 7-18 & 7-19
These fill with water because:
o Central drain or sinkhole becomes plugged
o WT rises and “intersects” the sinkhole
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Solution chimneys, vertical shafts – usu formed along bedding planes
Solution along fractures – fig 7-21 , Fig – 7-27 – solution enhanced along
joint planes, leads to etching and widening – this produces “cutters” – Fig 722 – widened joints that get filled with soil
Sometimes, Ls can look like Swiss Cheese! Fig 7-23
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Transition from fluvial to karst drainage
o One of the unique aspects of karst drainage is that surface drainage is
disrupted, or actually disappears! This is due to surface water
dropping to lower elevations through various solution features, rather
than the typical stream channel
o Surface streams can end abruptly at sink- and swallow-holes (Fig 7-24)
o Streams can start abruptly when they emerge from a karst “spring”
o Often see “fluviokarst”, where you have both (NW NJ?)
river
shale (normal river channel)
“disappearing stream”
(water drops to WT through fractures)
limestone
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Caves (yeah baby) – what we want to try to see…..
Defined as “elongate cavities in limestone”
Produced by both sol’n and by mechanical / chemical erosion of flowing water
The form along “paths of greatest GW solution” and discharge “where subsurface
flow is great enough to remove dissolved LS and keep undersaturated water in
contact with cave walls”…so what do we mean by “undersaturated”? important
concept!!
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Cave formation “requires a network…along which water can flow from recharge to
discharge areas (i.e., from high head to low head) Fig 7-27
Conduits for flow grow at different rates, controlling the size of the tunnel
Parameters such as fracture width, GW flow rate, temp, and partial pressure of CO 2
are all important in determining cave growth
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Cave features
o Size and shape described by diameter, length, spatial pattern (Fig 7-26)
o Can have a single conduit or a “maze” (easy to get lost)
o Components include:
 Entrances
 Termination
 Passages
 Rooms
o Entrances are found under all sorts of circumstances (“most caves have no
natural entrances and are undetected from the surface”
o Terminations – where the cave finally ends (“even the longest of caves (up to
30 miles of passages) eventually ends”
 Number of diff ways to develop terminations – ceiling collapse, fill with
debris, or the conduits get smaller and smaller
 NOTE: general dip profile of a cave follows overall dip of the Water
Table (Fig 7-27)
o Passages / Rooms –
 Passages defined as Length > width (or height) –
 A passage looks like a hallway
 Rooms are the opposite (?)
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Cave origins – a number of theories
o “strong disagreements” – like when are there NOT strong disagreements in
geology?
o “issue remains in doubt”
o but the principal theories all focus upon the relationship of GW flow to the WT
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the 3 diff theories (Fig 7-27):
1. caves form above WT through solution by vadose water
2. caves form beneath WT by deep circulation of phreatic water
3. caves form AT the WT or in shallow phreatic zone, often in response ot rise and
fall of WT
theory 1. form above WT
o early part 20th century
o subsurface streams flowing down toward WT
o mechanisms include solution by vadose (unsat zone) water, underground
stream abrasion, and water percolating down the dip of bedding
theory 2. form below WT via deep circulation of phreatic (sat zone) water
o early 20th century
o inference, due to presence of recent travertine deposits, that caves originally
formed below WT, then were raised relative to WT, then received travertine
deposits (which are interpreted to be deposited subaerially)
theory 3. form due to WT fluctuation, near the top of WT
o “stacked” horizontal caves in FOLDED limestone made people wonder if
caves formed in shallow phreatic zone, between high and low WT positions
o some cave elevations may be related to elevations of nearby river terraces
Cave deposits: usu formed of travertine, which is precipitated out of GW solution
o stalactite – drip down from ceiling
o stalagmite – build up from floor
o column – they connect
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