Download Lecture 13 Groundwater

Lecture 13 Groundwater
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Where does subsurface water come from?
The water table
Storage and movement of groundwater
Springs
groundwater pollution
Formation of caves and karst topography
Subsidence
 Where does subsurface water come from?
 When rain falls, some of the water runs off, some
evaporates, and the remainder soaks into the
ground -- infiltration -- as part of the hydrologic
cycle.
 Thus, like surface water, subsurface water has
been supplied primarily from precipitation.
 Subsurface water resides and moves in pore
spaces.
 Balance of water in the hydrologic cycle.
 How is the water that generates a spring like this
stored underground? How does it move?
 Storage and movement of groundwater
 Porosity
Porosity is the amount of pore space in a
soil or rock as a percentage of the total
volume. Porosity determines the amount of
groundwater that can be stored. (Table 15.1 lists
porosities of soil and rock.)
Porosity in various
kinds of rocks.
(W.W. Norton)
 Permeability
Permeability measures the ability of soil or rock to
allow the passage of fluids, e.g. the permeability
of sands and gravels is much higher than that of
clay.
 Impermeable layers that hinder water movement
are called aquitards (such as clay layers).
Permeable rock layers that transmit groundwater
freely are called aquifers (such as layers of sands
and gravels).
Impermeable
and permeable
materials.
(W.W.Norton)
Aquifer and aquitard. (W.W. Norton)
 Darcy's law:
provides a basic governing equation for flow through a
porous medium. It states that the rate of flow per unit area is
proportional to the hydraulic gradient.
Q=kiA
Q=volumetric flow rate (or discharge, volume per unit time)
k=hydraulic conductivity (coefficients of permeability)
i=hydraulic gradient
A=cross-sectional area
(Table 5.2 lists hydraulic conductivities, P.331)
 The water table
 Part of the water that soaks into the ground is held by
plants near the surface. Water that is not held in this zone
of soil moisture can go downward until it reaches a zone
of saturation where all the open pore spaces are filled with
water. Water with the zone of saturation is called
groundwater. The upper surface of the groundwater is
known as water table.
 When a well is drilled a short distance into the saturated
zone, the level of water in the well coincides with the
water table.
 The water table is rarely level; its
shape is usually a subdued replica of the
surface topography.
 The most important reason for this is that
groundwater moves very slowly, thus
frequent enough rain falls keep water piled
up even in the high areas.
 The water table is the upper surface of the zone of
saturation. (Hamblin and Christiansen)
 Distribution of subsurface water. The shape of the water table
is usually a subdued replica of the surface topography. During
periods of drought, the water table falls, reducing streamflow
and drying up some wells. (Tarbuck and Lutgents)
 Springs
Springs are mysterious to many people: they flow
constantly freely from the ground in seemingly
inexhaustible supply but with no obvious source.
 Thousand Springs along the Snake River canyon,
Idaho. The springs on the canyon wall are fed by
underground water through the high porous and
permeable basaltic bedrock. (Hamblin and
Christiansen)
 Springs form
(1) whenever the water table intersects Earth's
surface so a natural outflow of groundwater results;
(2) when an aquitard blocks the downward
movement of subsurface water and forces it to
move laterally.
 Spring may form when an aquitard blocks the
downward movement of subsurface water and
forces it to move laterally. (Tarbuck and Lutgens)
 Groundwater pollution
Groundwater pollution is a serious matter in areas where
groundwater is used as water supply.
 Septic tanks
Septic tanks are widely used by households in some area
that lack full sewer networks. Sewage water is discharged
into the soil to be filtered out. The drainage of septic tanks
should be placed at sufficient distance from water wells in
shallow aquifers.
 Septic tanks. Sewage water passes through septic tanks and is
discharged into the soil to be filtered out. A) contaminated
water moves rapidly through the cavernous limestone and has
traveled more than 100 meters to reach Well 1. B) Water is
purified in a relatively short distance through permeable
sandstone.(Tarbuck and Lutgents)
Sanitary landfills
 Materials leached from landfills may find their way into the
groundwater, contaminating water supply.
 Sanitary landfills …
Fine-grained soils such as clay provide a more
desirable landfill site than coarser-grained soils.
1) Finer soils have a lower permeability.
2) Finer soils are able to exchange heavy toxic
cations in the wastes (such as lead, zinc,
chromium, and mercury) with Na, Ca, and Mg
cations.
Saltwater encroachment
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A) Fresh water floats on the denser salt water and forms a lens-shaped body
that may extend considerable distance below sea level. B) When excessive
pumping lowers the water table, the base of the freshwater zone will rise by 33
times that amount. This may result saltwater contamination of wells (called
saltwater encroachment). (Tarbuck and Lutgens)
 Formation of caves and karst topography
Groundwater dissolves rock. Limestone and
dolomite are quite soluble in carbonic acid,
which forms because rainwater dissolves
CO2 from the air and from decaying plants.
The most spectacular results of
groundwater's erosion are limestone caverns.
 A soda straw “forest” in Carlsbad Caverns, New
Mexico. When a water drop reaches air in the cave,
some of the dissolved carbon dioxide escapes
from the drop and calcite precipitates.
 Karst topography (after Kras Plateau in Slovenia)
is another result of the dissolving power of groundwater in
a limestone or dolomite region.
 Karst areas have irregular terrain sinkholes.
 Sinkholes can form gradually as the limestone below the
soil is dissolved and the surface is gradually lowered. They
can also form abruptly when the roof of a cavern collapses.
 Karst areas in the U.S. appear in limestone regions in SW
Illinois, southern Indiana, central Tennessee, Kentucky,
Alabama, Florida, and Texas.
 (Left) Infrared image shows an area of karst topography in
central Florida. The numerous lakes occupy sinkholes.
(Right) This small sinkhole formed suddenly in 1991 when
the roof of a cavern collapsed, destroying this home in
Frostproof, Florida.
 Subsidence
 When groundwater is withdrawn faster than
natural recharge, significant subsidence can occur,
damaging construction, water supply lines, sewers,
and roads.
 This is particularly pronounced in the areas of
unconsolidated sediments. As the water is
withdrawn and the water pressure drops, the
sediments undergo additional compaction, causing
the ground to subside.
 The marks on this utility pole (left) indicate the level of the
surrounding land in preceding years. Between 1925 and
1975 this part of the San Joaquin Valley (right) subsided
almost 9 meters because of the withdrawal of groundwate
and the resulting compaction of sediments.
 Subsidence of buildings in Mexico City resulted from
compaction after groundwater was pumped from
unconsolidated sediment beneath the city. Subsidence has
caused this building to tilt and sink more than 2 m.