essential independent
variables that guide the
development of the karst
landscape = 'coordinate features'
CHEMICAL DRIVING FORCE
PRECIPITATION
chemical reactions drive
towards equilibrium
(saturation and cessation
of dissolution)WITHOUT
PRECIPITATION
PHYSICAL DRIVING FORCE
PRECIPITATION
removes saturated solutions
and replaces with fresh
solutions
RELIEF
HYDROGEOLOGIC SETTING
TECTONIC SETTING
THIICKNESS OF SOLUABLE ROCK
places
limits on the
extent of
karst
development
e.g. western
Pennsylvania only
10-12m
E.G. Adriatic coast 9000m
STRATIGRAPHIC
AND
LITHOLOGICAL
SETTING
Karst hydrologic cycle
Karst aquifer and
associated groundwater
basins assumed to be
embedded in larger
basins containing
non-karstic rocks
(ALLOGENIC)
Runoff originates from
allogenic streams
together with internal
runoff (Qi = IN
DIAGRAM )
Overland flow
disappears into
sinkholes ->
internal runoff
(Qa )
Karst aquifer assumed to
have a single output = base
level stream with flow QB from
springs
Porosity and permeability OF
AQUIFERS - ranges
Primary porosity
fracture
porosity
conduit
porosity
INFLUENCE ON
Water infiltrating
percolates downwards
under gravity until it
reaches level where all
pores are water-filled.
Phreatic
zone
water-saturated
zone below the
water table
Vadose
zone
unsaturated
zone above the
water table
Karstic aquifers
contain integrated
systems of pipe-like
conduits that act as
drains for the highly
localised transport
of watery
Properties of carbonate
aquifers vary widely
KARST RECHARGE
OCCURS IN = Catchments of
mixed geology
solution attributable
both:
1) precipitation (autogenic
recharge)
AND 2) water that flows on to the karst
from elsewhere (allogenic
recharge)
Denudation varies as geochemical
evolution of the water different
BECAUSE
1) soil and
boil vary from
point to point
recharge is
organised differently
AUTOGENIC CASE =
diffuse and relatively even
ALLOGENIC CASE = ocused + high
volume + solution restricted to line of
watercourse
Theoretical Cave
Development Models
in Carbonates
VADOSE THEORY
Caves formed by
action of underground
streams flowing at or
above the water table
solution takes place most actively at
the top of the saturated zone
BECAUSE aggressiveness is lost
once water descends through sinkholes
and enters the water table
DEEP PHREATIC THEORY
origin = Davis's work on
Peneplains
KARST = A distinctive topography in which the
landscape is largely shaped by the dissolving
action of water on carbonate bedrock
results in
surface and
subsurface features
sinkholes
vertical
shafts
disappearing streams
springs
underground
drainage systems
caves
Caocite Dissolution
1) In the presence of water, calcite
dissociates to yield a positively charged
calcium cation and a negatively charged
carbonate anion
CaCO3 -><- Ca++ + CO3 2-
Dissolution of the limestone or
dolomite is most intensive where the
water first contacts the rock surface
2) Carbon dioxide gas
dissolves in water to
produce carbonic acid
CO2 + H20 -><- H2CO3
Hydration of
aqueous CO2 to
form neutral
carbonic acid
3) The carbonic acid
dissociates into a
hydrogen cation (protom)
and a bicarbonate ion.
H2CO3 -><- H+ + HCO3
Ionization of the carbonic acid to
form a bicarbonate ion and a proton
4) Free protons H+ combine
with carbonate anions
HCO3 -
HCO3 - = which are
more soluble in water
than carbonate
H+ + CO3 2- -> <- HCO3
Reaction of the proton with a carbonate
ion on the crystal surface to form a
second bicarbonate ion
5) Continuing production of bicarbonate
releases calcium ions Ca2+ into the
boundary layer of the water
Kinematics
1. Desorption of the second
bicarbonate ion and its diffusion
across the boundary layer into the
bulk solution
2. Release of the calcium
ion from the crystal
3. Diffusion of the
calcium ion across the
boundary layer into the
bulk solution
The ions are carried away in
solution
Both limestone and marble =
composed of the mineral
calcite= chemically is calcium
carbonate: CaCO3
MARBLE = metamorphosed limestone
Dolomite is a double carbonate of
calcium and magnesium:
CaMg(CO3 )2 = less soluble than
calcite = karst on dolomite = less
well developed than on limestone or
marble
HOWEVER reactions are fully reversible = when
precipitates are exposed to un-saturated ground water =
re-dissolve
changing
water
table
levels
subsidence
Distinctive
landforms
Saturated and
undersaturated solutions
SATURATED = dynamic equilibrium
with no more solution: the solution
either
supply of CO2 going
into solution being
exhausted
where it is balanced
by outgassing of
CO2
can lead to
groundwater becomes
SUPERSATURATED
with dissolved minerals
further dissolution = not
possible, =carbonate salts of
calcium and magnesium may
precipitate from the water
features E.G.
stalactites.
UNSATURATED =
there is net dissolution
of the mineral
SOMETIMES refferred to as
AGGRESSIVE
Aggressive dissolution = flow is
focussed in pre-existing openings in the
rock OR in the zone of water-table
fluctuation where ground water is in
contact with the atmosphere
IN = humid regions.
wet regions
have much
higher
denudations
rates than drier
regions
EXAMPLE FLORIDA
LANDFORM
Distinctive landforms due
to high rock solubility,
which causes secondary
porosity and subsidence.
FORMATION
the carbon
dioxide
cascade
1. As rain falls
through the
atmosphere, it
picks up CO2
which dissolves in
the droplets
2. Once rain
hits the
ground, it
percolates
through the
soil and picks
up more CO2
3. a weak solution of
carbonic acid is formed
EQUATION = H2O + CO2 -> <-H2CO3
Hydration
4. infiltrating water naturally
exploits any cracks or crevices in
the rock.
5.. Over long periods, with a
continuous supply of CO2
-enriched water, carbonate
bedrock begins to dissolve.
CHEMISTRY = Cavities
and voids develop a=
limestone or dolomite
dissolved into component
ions of calcium (Ca++),
magnesium (Mg++) and
bicarbonate (HCO3 - ).
6. Openings in the
bedrock increase in
size
7. underground
drainage system
begins to develop, =
allows more water to
pass = accelerating
the formation of karst
8. leads to the
development of
subsurface caves
Environmental variables affecting karst formation
PRECIPITATION=
DOMINANT
wet regions have
much higher
denudations rates than
drier regions
TEMPERATURE
ORGANIC MATTER
TURBULENCE
CHEMISTRY
Salts
Calcite and dolomite are
ionic salts. In pure water
they dissociate into their
constituent ions
1) CaCO3 -><- Ca++ + CO
2) CaMg(CO3 )2 -> <- Ca++ + Mg++ + 2CO
Ph
Calcite: pH ~ 8.4
CO2 in air
dissolved in cloud
droplets Falls as
lower pH
precipitation with
very slow
dissolution
Soils
Water, and Humic
Acid; pH ~ 4-7
Precipitation soaks
though O and A
horizons and adds
plant acids
CaCO3 (s) + 2H+ (aq) -><- Ca++ (aq) + CO2 (g) + H2O
Temperature
Cold water contains
more CO2
CO2 + H2O -><- H2CO3 -><- H+ + HCO3 -
Pressure
deeper H2O ,
more CO2
absorbed -> more
‘aggressive’
water
Mixing
can result in
undersaturated H2O
Speed of flow
runoff can move a long
distance in a few days
runoff = time taken for a
solution to approach
saturation)
Solubility:
calcite in pure water =
very small (less than
that of quartz)
Add CO2 to water = carbonic
acid = calcite begins to dissolve.
Evolution
: Karst aquifers form by flowing
water containing carbon dioxide
(CO 2) which dissolves carbonate
rocks.