Chapter 10 Groundwater

1
Chapter 10- Groundwater !!

• Only about 3 of the Earths water is contained
  within landmasses. The rest is in the oceans.
• Of all the freshwater, the great majority is in
  the polar ice caps and glaciers (estimates range
  from about 85 to over 90 of Earths freshwater).
• 94 of all freshwater is groundwater, and only
  about 6 of it is in rivers and lakes.
• Thus, groundwater is the largest reservoir of
  freshwater that is readily available to humans.
• Residence time average length of time that a
  surface spends in a reservoir. Residence time
  depends on amount of exchange (both inflow, and
  outflow)

2
Groundwater and the water cycle

• All water ultimately comes from the oceans. The
  hydrologic cycle (fueled by energy from the sun)
  causes water to move to other locations.
• Water that has risen to the atmosphere
  (evaporation transpiration), may fall as
  precipitation (rain, snow, ice).
• Infiltration- Process by which water from
  precipitation percolates thru the ground, and
  becomes groundwater. Some infiltration occurs
  shortly after precipitation (rain), whereas some
  has a lag time prior to melting (accumulated
  snow).
• Eventually, groundwater comes to the surface thru
  springs, may get incorporated into streams, and
  eventually returns to the ocean.

3
Features associated w/ groundwater

• 1- Some water from infiltration is retained as a
  surface film on soil particles (belt of soil
  moisture)
• 2- Some water is taken by plants.
• 3- Some water evaporates rapidly back to the
  atmosphere.
• The 3 portions of water above never become
  groundwater.
• Water that is not held by particles percolates
  down to a zone where all open spaces (pores) in
  the sediment and rock are completely filled with
  water (zone of saturation, ZS)
• Water Table the upper limit of the zone of the
  zone of saturation. The water below this level is
  referred to as groundwater. Groundwater is not
  necessarily and underground pond, so its upper
  surface is not always leveled.
• Above the water table is a moist zone where much
  of the pore spaces are filled with air, not
  completely filled with water. This is the zone of
  aeration.
• Depth of the water table Distance from the
  surface that must be passed prior to reaching the
  water table.

4
Groundwater storage

• Porosity- The percentage of pore space in a
  material. Porosity is mostly dependent on grain
  size, and degree of sorting.
• - Well sorted sediments have higher porosity
  because pore spaces are left open.
• - Unsorted sediments have low porosity because
  small particles fill much of the spaces among
  larger particles.
• Permeability- Ability of a material to let water
  pass thru it. Permeability have also depends on
  grain size and degree of sorting.
• - Materials that have large and connected pores
  have high permeability, thus allowing high flow
  velocities. This are mostly large-grained
  sediments (e.g., sand, gravel, sandstone,
  limestone, fractured bedrock).
• - Materials with that have very small pores
  have low permeability (are more impermeable), and
  alow very low flow velocities. They have very
  small grain size (e.g., silt, clay, shale).
• Aquicludes (or aquitards) are impermeable layers
  such as clay that prevent downward water movement
  thru sediments.

5
Aquifers

• Aquifers (water carriers) are the permeable
  layers where most water flow takes place. They
  are the permeable rock or sediment strata that
  transmit groundwater freely.
• Aquifers are the water-bearing layers sought
  after by well drillers.
• Impermeable layers (aquicludes or aquitards) can
  occur above or below an aquifer.
• The main factor affecting flow velocity thru
  sediments is the permeability of the materials.
  The slope of the water table, also has an effect
  a steeper slope will cause greater flow velocity.

6
Groundwater erosion and deposition

• Most groundwater contains carbonic acid (H2CO3).
  Carbonic acid forms when
• - water from rain interacts with CO2 in the
  atmosphere.
• - groundwater passes through carbon-rich,
  decaying organic materials, mostly plant
  materials.
• The reactions CO2 H2O ? H2CO3
• Carbonic acid is dissociated into Bicarbonate and
  hydrogen ions
• H2CO3 ? H HCO3-
• Therefore, goundwater is acidic (pH less than 7),
  and it dissolves rock. Since limestone underlie
  millions of square kilometers of Earths surface,
  groundwater has a great capacity of geologic
  change, as an erosional agent.
• Limestone is nearly insoluble in pure water, but
  is dissolved readily in water containing even
  small amounts of carbonic acid.
• When limestone comes in contact with groundwater,
  the carbonic acid comes in contact with the
  calcite in the limestone, dissolving it. Calcium
  and Bicarbonate ions are then formed and are
  carried in solution with the water.
• The reaction CaCO3 H ? Ca2 HCO3-

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More on erosion by groundwater

• Calcium and bicarbonate ions can precipitate at
  another location, if
• - the water evaporates, or
• - the carbon dioxide diffuses out of the
  water.
• When Ca and HCO3- precipitate, they join again to
  form CaCO3 (calcium bicarbonate)
• Usually there is always some dissolution taking
  place at a location, and precipitation taking
  place somewhere else.
• Limestone caverns are the result of erosion by
  acidic groundwater. The caves themselves are the
  result of erosion the decoration of the caves
  is the result of deposition.

8
Caves

• Cave A natural underground opening connected to
  Earths surface. Some caves are single-chambered,
  some very complex.
• - Formed when acidic groundwater dissolves
  limestone.
• - Most develop in ZS (zone of saturation), just
  below the water table.
• - As initial groundwater erosion forms a small
  cave, the limestone formation becomes more
  permeable, and the increased downhill flow of
  groundwater lowers the water table, filling the
  cave with air.
• Cavern (a non-technical term) use to refer to
  chambers within a cave.

9
Karst topography

• Named after the Krst region of Croatia, where
  these formations are very common.
• Karst topography (KT) areas that have been
  shaped by the dissolving power of limestone by
  groundwater. Generally, areas where many caves
  occur.
• - caves form near or below the water table.
• - Sinkholes (or sinks) A depression on the
  surface, formed by the collapse of a cave, or by
  the dissolution of bedrock by downward seeping
  water that is constantly recharged with CO2 from
  either the atmosphere, or from water in contact
  with organic (carbon-containing) materials.
• - Sinking streams A formerly surface stream
  that drains into a cave system, thus continuing
  underground but disappearing as a surface
  stream. Usually a dry valley is left on the
  former dry stream bed. As new caves and cave
  systems form at a location, formerly surface
  streams could potentially become sinking streams.
• - Karst springs A sinking stream that reappears
  abruptly at a certain location. Different from
  other springs, because it involves a surface
  stream that has gone underground.

Central Florida, USA. Many lakes formed as
water-filled sinkholes. (Aerial infrared
photograph)
10
More karst topography (KT)

• Common in the USA in areas underlain by
  limestone portions of KY, AL, southern IN,
  central South FL.
• Generally, dry and arid lands dont develop KT.
  Why ??
• However, ancient marks of KT can be seen in arid
  lands if former times were wetter there.
• Most KT-areas have few surface streams, so
  rainfall is quickly carried underground, and
  sinking streams are very common in those areas.
• Sinkholes range in depth from 1 m, to over 50
  meters. In some areas they represent a real
  geological hazard (mostly not covered by
  insurance companies, unless specifically
  identified previously as a risk at a specific
  location).
• - Generally, sinkholes made by collapsed caves
  are deep and steep-sided, whereas those made by
  gradual downward-seeping of water are shallow and
  have gentle slopes.

(there is insufficient groundwater).
Catastrophic small Florida sinkhole formed when
the roof of a cave collapsed.
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Whats in your houses water supply ?

• Hard water Water that contains high
  concentrations of calcium, magnesium, or iron.
  Its pH varies depending on location (geographical
  region) and the source your water company uses.
• - When groundwater is rich in calcium
  bicarbonate, hard water in households leaves
  white/whitish deposits (concretions) on pipes
  and other structures.
• - When deposits are yellowish/brownish/reddish,
  the water is rich in one of various iron
  compounds.
• Cincinnati (and in general, OH) water is hard
  (pH close to 8).
• Solutions 1- Use a water softener machine (a
  reverse/osmosis unit, or a household softener)
  2- clean fixtures regularly with acid 3- Water
  management companies treat the water by
  adding/removing materials to reach a target
  reading of pH, TDS (total dissolved solids), or
  conductivity (capability of conducting
  electricity).
• Soft water Water with few materials dissolved.
  Generally gentle on household fixtures, and
  healthier to drink.
• - Generally low pH (close to neutral or just
  above/below)
• - Parts of New York City have water with pH
  6.5-7

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Natural groundwater deposits

• Natural deposits Dripstone formations in caves.
  Each drop of water hanging from the ceiling of a
  cave looses some CO2, thus depositing some CaCO3.
  With many drops doing the same, very large
  structures may result.
• - Stalagtites hang from ceiling
• - Stalagmites mounds on the bottom, underneath
  stalagtites.
• - They may join in the middle, forming dripstone
  columns.

13
Groundwater systems (Springs, wells, aquifers)

• Groundwater residence time hundreds to
  thousands of years, eventually returning to
  surface to continue water cycle.
• Springs- Natural discharges of groundwater onto
  the Earths surface.
• - generally occur when contact occurs between an
  aquifer (permeable layers sand/gravel,
  sandstone/limestone) and an aquiclude
  (impermeable layers clay, shale).
• - In karst regions, an entire river may emerge
  (karst springs).

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More on Spring emergence

• Many karst springs in Central Florida (Orlando
  area).
• Easily recognizable by the following features
• - Karst springs common
• - Sizable streams come from lakes that have
  little or no visible surface water feeding them.
• - Disappearing streams
• - Dry valleys left by disappearing streams
• - Generally disrupted topography with disrupted
  drainage.
• Caves themselves are not visible (on
  photographs, or on topographic maps)

Several springs emerging at base of aquifer
Spring emerging from perched water table
Spring emerging from Slope side, fed by
Underground caves (KT)
Spring emerging along Fault that blocks aquifer
15
Groundwater systems (Hot Springs, wells, aquifers)

• When groundwater circulates at great depths, it
  becomes heated, and if it rises to the surface,
  it emerges as a hot spring.
• Hot springs Permanent springs whose water is 6-9
  C (10-15 F) warmer than the average temperature
  of the location.
• The source of heat is cooling igneous rocks.

Since the Western USA has most of the recent
igneous activity (vulcanism), this is the area
of the country where most hot springs are
found. Comparatively fewer are located in the
Eastern USA, and even fewer in the Central states.
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Geysers
1

• Geysers Intermittent hot springs or fountains in
  which water is ejected with great force (often
  30-60 meters). After the water ceases, steam
  comes out, usually with a loud roar.
• Occur where extensive underground caves occur
  within hot igneous rocks.
• Stages in the eruption cycle
• 1) Cool water enters underground
  caverns/fractures and is heated to near boiling.
• 2) Heated water expands, with some being forced
  towards the surface. Pressure increases greatly
  causing a jet of water. Pressure is thus reduced,
  water jet ceases, steam follows.
• 3) The empty chambers fill again, and a new
  cycle begins.

2
3
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Wells

• Wells- Holes bored into the ground, to reach a
  reservoir of groundwater. Water can be pumped to
  the surface for human use.
• To provide water, wells must reach the zone of
  saturation (aquifer). More so, it must go below
  the water table, into the water-table aquifer.
  This water table is initially the same as the
  actual water table in the area.
• Over-pumping may cause a cone of depression to
  occur, which is a lowering of the water table
  aquifer around the well. Drawdown is the
  difference between the original water table and
  the water level in the over-pumped well.
• Recharge- Water from precipitation and runoff
  when added to the zone of saturation. May be able
  to replenish lightly over-pumped wells, but there
  are limits.
• The majority wells are used to supply
  agricultural water, but also some for industries,
  and some for homes (mainly rural areas)

18
Artesian wells

• Confined aquifers- Contains water under pressure,
  because is formed from an area of recharge
  located at a higher elevation than the rest of
  the aquifer.
• Artesian wells- Naturally pressurized water in a
  well drilled into a confined aquifer.. Two
  conditions must occur
• - Water must be confined to an aquifer that is
  inclined, with one end exposed at the surface
  (zone of recharge), and
• - Aquitards both above and below the aquifer
  must be present to prevent the water from
  escaping.
• - When an artesian well is tapped, the pressure
  created by the weight of the water above will
  force the water to rise. If there were no
  friction, H2O would rise to the level of the
  water at the top of the aquifer (however, there
  is also some head loss due to friction). The
  greater the distance from the recharge area to
  the well, the greater the friction and the less
  the rise of water.
• - The level at which the water will rise is
  called Pressure surface, measured in height
  units.

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Threats to water supply

• Over-use of wells
• - Drying and dried wells
• - Land subsidence
• Natural droughts when combined of excessive use
  of wells.
• Man-made landscape modifications that reduce the
  amount of infiltration and favor the amount of
  runoff.
• Pollution in groundwater
• - Chemicals
• - ions molecules (cannot be filtered out by
  traveling through sediments)
• - can contaminate any aquifer
• - move down-slope from the source
• - Once in groundwater, they cannot be removed
  unless treatment plants are used (expensive)
• - Sewage - household septic systems
• - outdated municipal facilities
• - Salt - households
• - saltwater intrusion in coastal areas
• - Radon- Radioactive material from decay of
  uranium in rocks and sediments. Usually low
  concentration by enough to be a cancer risk

20
Threats to water supply

• Over-use of wells
• 1- Subsidence- Due to excessive withdrawal of
  water, the land has subsided almost 9 meters !!
  (San Joaquin Valley, CA)
• - The reduced pressure from the aquifer (result
  of taking water out) becomes less than the weight
  of the land mass above the aquifer. Sediments in
  the aquifer are compressed, causing an overall
  sinking (vertical shrinking) of the land.

21
Pollution in groundwater

• Common sources of pollution
• - Sewage, industrial waste, landfills, and
  agricultural chemicals (fertilizers,
  bio-enhancers, and pesticides)
• - They enter the ground above the water table
  but trickle down by infiltration. Solid materials
  suffer leaching of dissolved chemicals.
• Water table unconfined aquifers are most easily
  polluted because 1) there are no aquitards to
  prevent influx of contaminating materials, and 2)
  Contaminated materials are less frequently
  deposited in the elevated zone of recharge of
  confined aquifers.

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Sewage and groundwater

• Sediment layers have some capability of purifying
  water as it passes through them. However, the
  water must travel large enough distances/sediment
  layers for purification to occur.
• - In A above, well 1 is receiving contaminated
  groundwater due to a faulty septic tank in a
  house located upstream of the source of
  groundwater. The water has not raveled through
  sufficient cavernous sandstone to be purified.
• In B above, well 1 is receiving clean water
  because the water from the faulty septic tank has
  been purified reasonably well by having traveled
  through enough distance/sediment layers.

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Pollution of groundwater resources
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Protecting our water resources 1.6 gpf is the
way to go !!

• Surface waters
• - Wise, well planned stream modifications (for
  energy, irrigation, drinking water)
• - Treatment plants for used water prior to
  returning to streams.
• - Minimize disposal of materials with the water
  (dont use in-sink-aerators dont throw oils and
  other materials down the sink)
• - Use water wisely (1.6 gpf toilets versus
  common 6 gpf units)
• - Proper re-habilitation of heated effluents
  from nuclear and other plants.
• Groundwaters
• - Proper functioning of septic systems.
• - Stringent regulations on households and
  industries for disposal of materials.
• - Proper containing of liquid industrial waste
  (lagoons), and required treatment plants for
  industrial wastes.
• - Proper containment of solid industrial waste
  (appropriate liners) to minimize leaching.
• - Proper containment and placement of landfills.