Chapter 19
A Hidden Reserve: Groundwater
LECTURE OUTLINE
earth
Portrait of a Planet
Third Edition
©2008 W. W. Norton & Company, Inc.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
A Hidden Reserve: Groundwater
Prepared by
Ronald Parker
Earlham College Department of Geosciences
Richmond, Indiana
Groundwater
 Significant amounts of water reside underground. It is…
 A major component of the hydrologic cycle.
 A major source of water for a thirsty world.
 Largely hidden from view.
Thus, groundwater is poorly understood by many people.
It is a precious resource that is susceptible to contamination.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater
 Groundwater is an important resource.
 Drinking water for people and livestock.
 Irrigation.
 Industry.
 It has been used
for millennia.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
The Hydrologic Cycle
 Groundwater is a component of the hydrologic cycle.
 Hydrologic cycle processes.
 Evaporation.
 Transpiration.
 Precipitation.
 Infiltration.
 Runoff.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
The Underground Reservoir
 Some precipitation enters the subsurface via infiltration.
 Soil properties and vegetation govern infiltration rate.
 Infiltrated water adds to soil moisture and groundwater.
 Soil moisture wets the soil.
 Some is wicked up by roots, some is evaporated.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
The Underground Reservoir
 Some infiltrated water percolates to a deeper level.
 It is added to water that fills subsurface void spaces.
 This is groundwater.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Porosity
 Groundwater resides in subsurface pore spaces.
 Pores are open spaces within any sediment or rock.
 The total volume of open space is termed porosity.
 Geologic materials exhibit a wide range of porosities.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Porosity
 Two categories of porosity: primary and secondary.
 Primary porosity – Originally formed with the material.
Voids in sediment.
Vesicles in basalt.
Open reef framework.
 Primary porosity may decrease.
With burial compaction.
With cementation.
 Crystalline rocks have
very little primary porosity.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Porosity
 Two categories of porosity.
 Secondary porosity –
Developed after rock
formation.
Fracturing.
Faulting.
Dissolution.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Permeability
 The ease of water flow due to pore interconnectedness.
 High-permeability material allows water to flow readily.
 Water flows slowly through low-permeability material.
 Many large and straight flow paths enhance permeability.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Aquifers and Aquitards
 Aquifer – Sediment or rock that transmits water easily.
 Aquitard – Sediment or rock that hinders water flow.
 Aquifers and aquitards are commonly interlayered.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Aquifers and Aquitards
 Unconfined – An aquifer that intersects the surface.
 In contact with the atmosphere.
 Easily contaminated.
 Confined – An aquifer beneath an aquitard.
 Isolated from the surface.
 Less susceptible to pollution.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Aquifers and Aquitards
 Example: Mahomet aquifer.
 Permeable sands in linear glacial meltwater channels.
 Channels carved into underlying bedrock.
 Water-bearing sands supply prodigious volumes of water.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Aquifers and Aquitards
 Example: Dakota sandstone aquifer.
 Cretaceous fluvial sandstones interlayered with shales.
 Deformation of the Black Hills uplifted the western end.
 Recharge in the Black Hills fills a gigantic aquifer system.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Aquifers and Aquitards
 Example: Phoenix aquifer.
 Basin and Range faulting created basins next to uplifts.
 Coarse basin sediments are recharged along the uplifts.
 Basins store large volumes of water used by cities.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
The Water Table
 The water table is a subsurface boundary.
 Above the water table, pores are mostly filled with air.
This is called the vadose (or unsaturated) zone.
 Below the water table, pores are filled with water.
This is called the phreatic (or saturated) zone.
 The capillary fringe separates the two zones.
Formed of moisture wicked upward above the water table.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
The Water Table
 The depth to the water table is variable.
 In humid settings, the water table is closer to the surface.
 In arid settings, it may be 10s to 100s of meters down.
 Perennial surface water exposes the water table.
 Streams.
 Lakes and ponds.
 Wetlands.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
The Water Table
 The water table is the top of the zone of saturation.
 Water table position changes with rainfall.
 During seasonally rainy periods, the water table rises.
 During prolonged droughts, the water table falls.
 Ponds dry up if the water table falls below the bottom.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Water Table Topography
 The water table is not flat; it is a sloping surface.
 The water table is a subdued replica of the topography.
 The water table is high where the land is high.
 The water table is low where the land is low.
 Water flows from higher elevations to lower elevations.
 Topography is useful for estimating groundwater flow.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Perched Water Tables
 Discontinuous aquitards may exist in the subsurface.
 These arrest downward infiltration to the water table.
 These aquitards form perched water tables.
 Overlie unsaturated material.
 Represent a “false” water table.
 More easily dewatered.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Flow
 Groundwater flows slowly under the influence of gravity.
 Flow in the unsaturated zone is straight downward.
 In the saturated zone, flow is more complicated.
 Governed by gravity and pressure.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Flow
 Hydraulic head, potential energy driving flow, is due to…
 Elevation above sea level.
 Pressure exerted by weight of overlying water.
 In the illustration below, the hydraulic head at p1 > p2.
 They have the same elevation.
 The weight of water over p1 is greater than that over p2.
 A piezometer is used to measure hydraulic head.
 An open-ended pipe.
 Installed below the water table.
 Water level is the hydraulic head.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Flow
 Flow is determined by measuring hydraulic head.
 Flow always moves from high to low hydraulic head.
 Thus water table highs flow to water table lows.
 Flow paths, however, are not straight lines.
 Flow follows a curved, concave-up path.
 Water can flow upwards moving to lower hydraulic head.
Upward flow
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Flow
 Groundwater infiltrates through recharge areas.
 Flow is directed downward.
 Commonly found in topographic uplands.
 Groundwater exits the subsurface from discharge areas.
 Flow is directed upward.
 Usually observed in topographic lows.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Flow
 Groundwater flow occurs on a variety of scales.
 Local – Shallow flow over short times and distances.
 Intermediate – Flow of moderate depth, time, and distance.
 Regional – Deep, long-distance, long-duration flow.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Flow Rates
 Groundwater movement is slow relative to surface water.
 It must percolate through pore openings.
 Flow is further slowed by friction and electrostatic forces.
 Typical rates of flow.
 Ocean currents
3 km / hour
 Steep river channel 30 km / hour
 Groundwater
0.00002 km / hour
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Flow Rates
 Groundwater flow rate is governed by several factors.
 Permeability of the porous material.
High permeability increases the rate of groundwater flow.
Low permeability decreases it.
 The hydraulic gradient – Spatial change in hydraulic head.
The head change over a
horizontal distance, and...
The driving force for
groundwater flow.
Steeper – Faster flow.
Less steep – Slower.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Flow
 Darcy’s Law – An equation that predicts the volume of
water passing through an area of an aquifer in a given
time.
Q = K (h1- h2/j) A
where,
Q = Discharge volume (m3/day).
K = Hydraulic conductivity (m/day).
h1- h2/j = Hydraulic gradient (m/m or dimensionless).
A = Cross-sectional area perpendicular to flow (m2).
 May be paraphrased…“Discharge volume is proportional
to the hydraulic gradient times the permeability.”
 Hydraulic conductivity is material permeability for water.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Flow
 Geologic conditions vary in the subsurface.
 Darcy’s law predicts flow increase with an increase in…
 Hydraulic conductivity, or
 Hydraulic gradient.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Tapping Groundwater
 Human use requires that groundwater be captured.
 Wells – Holes excavated or drilled to obtain water.
 Springs – Natural groundwater outlets.
 There are many types of wells and springs.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Tapping Groundwater
 Wells are holes drilled or dug into the saturated zone.
 Water is recovered by lifting or pumping.
 Water flows from the aquifer into the well.
 Drawdown occurs if removal exceeds flow to the well.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Tapping Groundwater
 With drawdown, the water table near the well drops.
 Water table decline forms a cone of depression.
 A downward-pointed cone-shaped surface.
 Steepest near the well; flattens with distance.
 The cone may expand outward with continued pumping.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Tapping Groundwater
 Drawdown from multiple wells in an area is additive.
 Cones of depression often interfere.
 A small well creates a small cone.
 A large well creates a large cone.
 One may dewater the other.
 Competing uses often conflict.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Tapping Groundwater
 Artesian wells tap confined, tilted aquifers.
 Upland recharge pressurizes the aquifer.
 Water rises in artesian wells to the potentiometric surface.
Analogue of the water table for a confined aquifer.
Determined by hydraulic head elevation in recharge area.
 A well casing below this surface will flow without pumping.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Tapping Groundwater
 Water distribution systems mimic artesian aquifers.
 Water is pumped to an elevated storage tank.
 This elevation creates an artificial potentiometric surface.
 Pressure drives water through the
distribution system.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Tapping Groundwater
 Springs are locations of natural groundwater discharge.
 Springs are marked by…
Hydrophilic vegetation.
Perennial wetlands.
Saturated soils.
Nonfreezing ground.
Streamflow.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Tapping Groundwater
 Springs are locations of natural groundwater discharge.
 Springs are important resources for humans.
 Yield fresh, clear, clean water.
 No need for drilling.
 Spring flow is often steady.
 Springs form in many ways.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Tapping Groundwater
 Springs result from varied geologic features.
 Where the water table intersects the land surface.
 At the contact between high- and low-permeability layers.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Tapping Groundwater
 Springs result from varied geologic features.
 Where water-bearing fractures intersect the surface.
 Where a fault juxtaposes permeability contrasts.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Tapping Groundwater
 Springs result from varied geologic features.
 Leakage of a confined aquifer along a joint or fracture.
 Exposure of a perching layer at the surface.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Tapping Groundwater
 Oases in the Sahara develop
from spring flow.
 Water from recharge areas flows
to oasis discharge points.
 These locations have been
culturally important for millenia.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Hot Springs and Geysers
 Hot springs are groundwater discharges of hot water.
 Temperatures range from 30o to 104oC.
 The waters are usually rich in dissolved minerals.
 They develop in two settings.
 Where deep groundwater surfaces along faults.
 In geothermal regions.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Hot Springs and Geysers
 Where deep groundwater surfaces along faults.
 Deep groundwater is warm.
 The source of heat is the geothermal gradient.
 In geothermal regions.
 High geothermal
gradients linked to
shallow magma.
 Circulation returns
heated groundwater
to the surface.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Hot Springs and Geysers
 Distinctive geological features.
 Hot springs turn volcanic ash into a viscous slurry.
 Boiling springs create bubbling mudpots.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Hot Springs and Geysers
 Distinctive geological features.
 Hot springs lose dissolved minerals upon cooling.
 These minerals crystallize as deposits of travertine.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Hot Springs and Geysers
 Distinctive geological features.
 Geothermal springs may produce brightly colored pools.
 Colors are due to bacterial metabolism of sulfur minerals.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Hot Springs and Geysers
 Boiling water and steam erupts cyclically from geysers.
 Water is heated to the boiling point in a vertical spring.
 Pressure exerted by the water column prevents boiling.
 Pressure drops as bubbles form and all the water boils.
 The cycle repeats after the emptied chamber is refilled.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Hot Springs and Geysers
 Hot springs are prized as health resorts and spas.
 Yellowstone National Park, Wyoming.
 Hot Springs, Arkansas.
 Salton Sea, California.
 Rotorua, New Zealand.
 Iceland.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Problems
 Groundwater is an important natural resource.
 It accounts for 95% of all the fresh water on Earth.
 It supplies a substantial portion of drinking water needs.
 Groundwater is threatened by…
Mismangement.
Overuse.
Pollution.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Depletion
 Severe water table decline can alter surface water flow.
 By capturing flow, wells may dewater streams and lakes.
 Especially problematic in
arid and semi-arid regions.
San Pedro River,
SE Arizona.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Depletion
 Cones of depression are capable of reversing flow.
 An expanding cone may capture pollutants.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Depletion
 Beneath coastal land, fresh water “floats” on salt water.
 Pumping causes the fresh/salt boundary to rise.
 Eventually, salt water may enter the pumping well.
 Salt water intrusion renders the water unpotable.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Depletion
 Lowering the water table by diverting surface water.
 Channelization has diverted Everglades surface flow.
 This has led to drying of large parts of the Everglades.
 Lack of freshwater flow has resulted in salt-water intrusion.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Depletion
 Water in pore space acts to hold grains apart.
 When ground water is removed…
 Sediment grains compress, pores collapse.
 The land surface cracks and sinks.
 Subsidence is mostly irreversible.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Depletion
 Dramatic examples of subsidence are well-known.
 The Leaning Tower of Pisa, Italy.
 Venice, Italy.
 The San Joaquin Valley, California.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Depletion
 Proactive measures can prevent withdrawal subsidence.
 Structures are designed to recharge surface water.
 Recharge basins return water to the groundwater system.
 This serves to reduce the effects of excess withdrawal.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Chemistry
 Groundwater reacts as it flows through an aquifer.
 Reactions are like chemical weathering.
 Dissolution/precipitation.
 Oxidation/reduction.
 Hydrolysis.
 Hydration.
 Dissolved ion concentrations depend on T, P, and pH.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Chemistry
 Ion concentrations generally increase with time.
 Longer residence times result in more dissolved material.
 Deep regional flow systems are typified by brines.
 Minerals may crystallize out of groundwater.
 This is a mechanism that leads to sediment cementation.
 Common cements are calcite, quartz, and iron oxides.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Quality
 Groundwater is often of high quality.
 The filtering effect of porous media removes particulates.
 Clay minerals can absorb certain dissolved ions.
 Natural groundwater may contain unwanted substances.
 Hardness.
 Dissolved iron, manganese, and hydrogen sulfide gas.
 Dissolved arsenic.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Quality
 Hardness – Dissolution of carbonates adds Ca2+ and Mg2+.
In excess, these cations…
Reduce the effectiveness of soap.
Mineralize in plumbing to create flow-clogging scale.
Hardness is mitigated by ion exchange (water softening).
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Quality
 Iron and manganese – Derived from rock weathering.
These cations occur in reducing (O2-poor) groundwater.
Cause taste/odor problems and stain plumbing and laundry.
 Hydrogen sulfide (H2S) – Rotten-egg gas.
Contribute to taste and odor problems.
Commonly associated with high Fe and Mn.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Quality
 Natural processes may render groundwater unpotable.
 Arsenic – Naturally occurring poison.
Derived from weathering of…
Pyrite-bearing sediments and metamorphic equivalents.
Silica-rich volcanic ash deposits.
EPA Maximum Contaminant Limit (MCL) is 10 ug/L (ppb).
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Contamination
 Human activities add pollutants to groundwater flow.
 Dissolved and pure organic and inorganic compounds.
 Dissolved metals.
 Pathogenic microbes.
 Groundwater transports pollutants away from a source.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Contamination
 There are many sources of groundwater contamination.
 Pollution is often not recognized until damage occurs.
 Groundwater cleanup is slow, expensive, and limited.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Contamination
 Sanitary wastes.
 Failing septic systems.
 Animal feedlot runoff.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Contamination
 Agricultural wastes.
 Fertilizers.
 Pesticides.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Contamination
 Toxic chemicals.
 Industrial wastes.
 Paints and thinners.
 Degreasers and solvents.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Sources of Contamination
 Toxic chemicals.
 Petroleum storage.
Underground storage tanks (USTs).
Gas stations.
Petroleum terminals.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Sources of Contamination
 Toxic chemicals.
 Landfill leachate.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Sources of Contamination
 Toxic chemicals.
 Mining wastes
Mine overburden and spoil.
Mill processing tailings.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Contamination
 Two broad categories of contaminant introduction.
 Point source – A contaminant plume emanates from 1 spot.
Concentrations are highest near the source.
Concentrations decrease with distance away.
 Nonpoint source – Pollution introduced over a broad area.
Concentrations diffused across a region.
More difficult to identify and address.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Contamination
 Groundwater contaminant plumes change over time.
 They grow in length with groundwater flow.
 They grow in width by diffusion and dispersion.
 Large plumes pollute large areas and affect many people.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Contamination
 Contaminant characterization is needed for cleanup.
 Monitoring wells are installed to assess flow behavior.
 Chemical testing quantifies the amount of contaminants.
 Remedial strategies are designed to reduce health risks.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Contamination
 Contaminant remediation is expensive.
 Most remedial strategies include removing the source.
 Pump and treat.
 Volatilize and vaporize.
 Steam clean.
 Bioremediation utilizes bacteria to clean groundwater.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Contamination
 Contamination is best prevented by managing landuses.
 Landfills are now constructed with clay and plastic liners.
 Underground storage tanks are now double-lined.
 Best-management practices (BMPs) reduce contamination.
 Land-use management is especially important in
wellhead protection areas (WHPAs).
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Groundwater Excess
 Rising water tables.
 Excess groundwater may initiate slope failures.
Groundwater adds weight to the material.
Increase in pore pressure decreases material strength.
 Basement flooding.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Caves
 Caves develop when groundwater dissolves limestone.
 Groundwater is weakly acidic.
 CO2 reacts with water to form carbonic acid.
H2O + CO2 = H2CO3
 CO2 is added to rainwater as it falls through air.
 CO2 is also added to water infiltrating soil organic matter.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Caves
 Cave networks develop when proper conditions exist.
 Limestone bedrock.
 Abundant fresh water.
 Caves grow as joints are enhanced by solution.
 Cave geometry reflects
the joint pattern.
 Stratal changes
influence dissolution.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Caves and Karst
 Speleothems are formed from precipitation of dripstone.
 Develop when groundwater entering a cave degasses CO2.
 CaCO3 is precipitated from this water on interior surfaces.
 Over time, CaCO3 coatings grow into spectacular forms.
Stalactites – Hang down.
Stalagmites – Point up.
Columns.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Caves and Karst
 Limestone dissolution creates unique karst landscapes.
 Karst landforms bear evidence of dissolution.
 Disappearing streams.
 Natural bridges.
 Caves.
 Speleothems.
 Sinkholes.
 Springs.
 Karst creates irregular terrain.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Karst Landforms
 Karst landforms develop in a series of stages.
 Establishment of the water table in limestone.
Requires draining the seawater that formed the limestone.
Exposure above sea level in a humid region.
 Development of a cave network via dissolution.
Dissolution maximized near the water table.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Karst Landforms
 Karst landforms develop in a series of stages.
 The water table drops in response to stream incision.
 This initiates a new layer of cave formation.
 Groundwater moving down through the first cave…
Degasses CO2.
Precipitates dripstone and flowstone speleothems.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Karst Landforms
 Karst landforms develop in a series of stages.
 Continued dissolution results in eventual roof collapse.
 This process creates sinkholes and troughs.
 Remnant limestone forms ridges, hills, and natural bridges.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Karst Landforms
 Sinkholes result from roof collapse.
 Sinkholes decorate large regions of karst landscapes.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Karst Landforms
 Tower karst is prominent in China and Thailand.
 Natural bridges are formed when collapse is incomplete.
 Disappearing streams fall into underground caves, only
to emerge as a spring many kilometers away.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Cave Life
 A highly specialized ecosystem has developed in caves.
 Bacteria metabolizing sulfur minerals.
 Bats.
 Fish.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater
Caves and Karst
Landscapes
This concludes the
Chapter 19
A Hidden Reserve: Groundwater
LECTURE OUTLINE
earth
Portrait of a Planet
Third Edition
©2008 W. W. Norton & Company, Inc.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 19: A Hidden Reserve: Groundwater