Revised 8 / 06 (Monroe 6th ed.)
Including...
Introduction
Origin of Groundwater
Occurrence and Movement of Groundwater
Porosity
Permeability
Water Table
Vadose Zone
Groundwater and Stream Flow
Aquifers and wells
Groundwater Contamination
Sukalot: a group activity
...and the Gods wept: the Love Canal
Prospecting for Groundwater
Geological Role of Groundwater
Give a general description of what groundwater is
DEFINE: confined vs. unconfined aquifers
DIGRESS: Floodplain aquifers vs. bedrock aquifers
Groundwater has always held a special and mystical place in man's thoughts
Early workers had trouble figuring out where groundwater came from
Aristotle (384 - 322 B.C.)
Groundwater precipitated out of caverns in the earth
Because of "the cold which it encounters there"
Rainfall insufficient to account for rivers and springs
Seneca (3 B.C. - 65 A.D.)
Rainfall penetrates "only a few feet into the earth"
Perrault and Mariott - mid-17th century Frenchmen
Demonstrated the relationship between rainfall and spring flow
Groundwater and wells have been the basis of life forever
Wells have always been important to the development of an area
Walled cities needed a good INTERNAL source of water to withstand sieges
Michener's "The Source" (Psalm of the Hoopoe Bird)
Historic and recent urbanization have resulted in increased development of groundwater resources
Kanats - Underground water systems used in Persia (Iran)
Rome - did lead poisoning from water pipes cause the Empire to fall?
Wells - some were quite deep
Orvieto, Italy - 200' deep
Had 2 spiral staircases used by donkeys to bring water to the surface
Some Chinese wells were >4900' deep! - How did they dig these?
Water availability still represents the ultimate demographic control
However, several developments have allowed increased agriculture and urbanization in areas which are fundamentally arid
The ability to economically retrieve deep groundwater
As opposed to near-surface "floodplain" water
The ability to move it long distances
This is a fundamentally poor practice in the long term
And will probably result in severe social, economical and political unrest in the N-T-D-Future
Nearly all comes from surface precipitation which percolates into the ground
Also from connate waters - those included with the original rock
Both marine sediments and intrusives
Global groundwater supply is estimated at 8.5 million cubic kilometers
Estimates of the water budget in the U.S. indicates that:
Precipitation averages 30" per year
Evapotranspiration returns 21" to the atmosphere
Runoff returns 9" to the sea
So, where does groundwater come from?
Obviously, major groundwater systems develop very slowly
And, they are in a very delicate balance relative to use vs. recharge
DEFINE: Recharge area - where water is added to the aquifer
In many areas, groundwater can be considered a non-renewable resource
This is especially true of deep aquifers
Also arid environments (Example: So. California, Phoenix-Tucson)
Several factors relate to the occurrence and movement of groundwater
The percentage of the total volume of rock which is occupied by void space
Factors which determine porosity (Monroe; Fig. 16-1, pg. 499)
Commonly tied to the type of rock which contains the water (Monroe; Table 16-1, pg. 499)
Degree of sorting - probably most important
Packing arrangement
The porosity of the individual clasts
Amount of cement already filling up the void spaces
Grain size does not directly affect porosity
Some shales can actually have up to 90% open space!
However, extremely fine-grain rocks usually make poor aquifers
Due to surface tension which holds the water in the rock
Igneous and metamorphic rocks
Generally very dense crystalline rocks
Porosity usually controlled by fractures and faults
Lithology and surface weathering become very important here
DIGRESS TO: Chemical weathering and Bowen's Reaction Series
Granite vs. Gabbro vs. Schist
The measure of a rocks capability to transmit liquid through it's pore spaces
Generally a very slow process!
Book says average is "a few centimeters per day"
This is obviously only an average, and can fluctuate greatly
The size of the pore spaces is more important than the amount of void space!
As I said above, extremely fine-grain rocks usually make poor aquifers
Due to surface tension which holds the water in the rock
Porosity alone is useless as far as aquifers are concerned
Without interconnected pore spaces, the water (or oil, or gas) won't flow
DIGRESS TO: Wetsuits, thermos bottles, and thermopane windows
The upper surface of the groundwater (Monroe; Fig. 16-2, pg. 500)
The level of water in adjacent wells
All openings in the rock below are saturated with water
But the water table doesn't extend down into the ground forever
Increased pressure reduces the pore spaces in the rock
Not necessarily completely level
Roughly parallels the ground surface (Monroe; Fig. 16-3, pg. 501)
Rises with the hills & sinks with the valleys
Intersects the surface in springs, streams and lakes
Above the water table
Completely dry to partially wet (but not saturated!)
Also called the Zone of Aeration
Water moves downward through this zone to the water table
Basically consists of 3 separate zones
Zone of soil moisture
Portion most familiar to us - it's at the top
Responds to local moisture conditions and precipitation
Where the roots live
Therefore, this water is mostly trapped, and lost by evapotranspiration
Intermediate Zone
Below the zone of soil moisture - usually dry
Water percolates through the Intermediate Zone to the water table
The capillary fringe
Extends a short distance above the water table
Thread-like extensions of water which migrate upward by capillary action
Like colored water up a celery stalk
Groundwater and surface water are part of the same system
Effluent stream (Monroe; Fig. 16-3, pg. 501)
Derives its water from the water table
Common to temperate climates
Associated with relatively stable water tables
Directly reflects the water table
Streams flow when the water table intersects the surface
Dry when the water table drops
EXAMPLE: Deer Creek bridge
Influent stream
Adds water to the groundwater supply
Common in arid regions
The water is usually from more humid areas upstream which are destined to flow down into a desert
EXAMPLE: the Colorado River and the Nile
Associated with fluctuating water tables
Aquifer: a sub-surface layer of rock which, because of its porosity and permeability, will hold and transmit water
Must satisfy both requirements or it's not an aquifer
Book says "yield to a well," but not necessarily
Aquifers also supply springs and rivers
Aquiclude - can't hold or can't transmit water (fails one or both)
Two basic types of aquifers
Unconfined aquifer
Water level stands at the water table
The water level will drop as a result of pumping
A "Cone of Depression" will form around the well (Monroe; Fig. 16-5, pg. 503)
How long the cone stays there is proportional to the rate which water can move through the aquifer to locally recharge an individual well
Can affect nearby wells
Over-pumping of several adjacent wells can cause the cones to intersect
Can result in a regional lowering of the water table
Confined aquifer (Monroe; Fig. 16-6, pg. 504)
A permeable horizon between two impermeable rock layers
REVIEW: Recharge area - where water is added to the aquifer
Can result in artesian wells
Free flowing wells associated with confined aquifers
Artesian-pressure surface
The level to which water will rise in a confined aquifer
Basically, water "seeks its own level"
However, friction within the aquifer interferes with this
The artesian-pressure surface will decline in relative altitude away from the recharge area
Flowing vs. non-flowing artesian wells
Depends on whether the pressure surface intersects the surface or not
Too many wells into an artesian system can reduce the pressure
Change flowing wells to non-flowing wells
Water wells
Drilled or dug to tap water from aquifers
Over-pumping can be a real problem in areas of slow recharge
Almost like mining - non-renewable resource in many (most?) areas
Southwest - relate the problems in Tucson (Dave Christopherson)
Southern California - relate some of the water haggles
Mono Lake a good example
Now California wants to divert water from up here
I say give them the water, even if it means some additional dams
That's better than them all moving up here
Remember-water is the ultimate demographic control, and population centers will adjust to reflect availability
Coastal areas - encroachment of sea water is a special problem
EXAMPLE: Fountain Valley, California (Monroe; Fig. 16-14, pg. 514)
Land subsidence due to Over-pumping
(Monroe; Figs. 16-15 to 16-18, pgs. 515 to 516)
Due to compaction of the sediments after the water is removed
Also results from over-production of oil and/or gas
EXAMPLE: Gulf Coast island
This can lead to a permanent reduction in porosity due to compaction
Springs (Monroe; Fig. 16-4, pg. 502)
Places where water flows or seeps onto the surface
Occur where the water table intersects the current erosional level of the ground
Actually, effluent streams are just springs with a lot of water!
Can be caused by many different situations
Aquifers can become contaminated easily and in many ways
Axiom: the closer to the surface, the easier to contaminate
Many in the more developed areas have already been polluted beyond rational use
There are lots of ways an aquifer can become contaminated
Unfortunately, many aquifers were contaminated in the past without an understanding of, or regard for, the long-term consequences
Direct pumping of pollutants underground
"Out of sight, out of mind"
Sanitary landfills in recharge areas (Monroe; Fig. 16-19, pg. 517)
Percolation of water through the dump and into an aquifer
Not-so-sanitary landfills
Hazardous waste dumps
Poor underground mining practices
Relate the problems in the Tri-State lead/zinc district
Nuclear waste disposal
Possibly the most significant long-term problem
Relate the current findings at Hanford
The radioactive waste is reaching the Columbia much sooner than expected
The Columbia River basalt may be more permeable than originally thought
Search on for a "permanent" storage facility
Yucca Mountain, Nevada
In areas of poor recharge, restricted permeability, etc., contaminants can persist far longer than we have!
Scenario:
Sukalot, a city of 37,003 on the plains of eastern Colorado, gets its municipal water from several producing wells drilled into the Fullawata Sandstone. Local medical personnel (and mothers) report increases in stillbirths, as well as increased birth defects and genetic disorders, and demand a study to determine the cause. Recent testing indicates alarming amounts of chemical pesticide contamination (300X greater than EPA maximums), as well as significant increases in toxic metals, possibly related to gold and copper mining in the Rockies. We are at a public hearing to examine the problem, and determine a solution.
The cast:
Water Quality Control Board representative - reports the contamination problems
Medical representative - relates medical findings
EPA representative - your basic bureaucrat demanding a fix (at any cost)
City Engineer - the voice of reality concerning what can and cannot be done
The Mayor - worried about his city (and re-election in 5 months)
City legal council - worried about his city's liability (and the mayor's re-election)
Mayor's Public Relations Dog - worried about his mayor's image (and re-election)
Farmer's representative - supports use of pesticides
Miner's representative - supports mining (past, present, and future)
Pissed off citizens, some with deformed kids, etc.
Procedure:
Assign roles to students. Break for 30 minutes so everyone can consider their position and "get into the part." Begin meeting and fully explore the situation.
Click here for an additional activity related to contamination at the Love Canal
Groundwater is a minable resource
Also, in many cases, a non-renewable resource
Permanent lowering of the water table in many developed areas indicates that normal amounts of rainfall are not sufficient to replenish the aquifers
Demand for groundwater is steadily increasing
Especially in the developing countries - "many of which are in arid regions"
Why are the developing countries in arid regions?
They're currently undeveloped because they don't have any water
Methods used to located groundwater vary
Water witching (dowsing)
Has anybody here ever used one?
More "scientific" methods
Geologic mapping
Geophysical surveys
Basically trying to define favorable units and/or structures
Important to the formation of many sedimentary rocks
As a cementing agent
Transports natural cementing agents into unconsolidated sediments
ie. calcite, silica, and iron oxide
Precipitates out into the void spaces and binds individual grains together
Often results in a loss of porosity and permeability
Geysers (Monroe; Fig. 16-22, pg. 520)
Percolating groundwater is heated by a near-surface magma source
Increased pressure keeps the water at the bottom from boiling
Continued heating raises temperature of water closer to surface
Some of the water escapes
Reduces the pressure at depth
The hot water flashes to steam and the geyser erupts
Mineral deposits commonly surround the geysers
These are leached from subsurface rocks which the super-heated water passes through
2 kinds of hot-water deposits occur
Siliceous sinter - composed of silica
Travertine - composed of calcium carbonate (Monroe; Fig. 16-24, pg. 522)
Hot Springs
More wide spread than geysers (Monroe; Fig. 16-20 and 21, pgs. 519 and 520)
Like geysers, they obtain their heat from buried magma
Or they can be Juvenile Water (recently freed by cooling magma)
Geothermal energy
Often these areas can be tapped as a source of geothermal energy
Make electricity
Limestone areas - (starts on Monroe; page 505)
Underground caverns
Stalactites, Stalagmites, Columns
Karst topography (Monroe; Fig. 16-7, pg. 505)
Surface features common to areas underlain by limestone
Several major karst areas in the U.S.
Distinctive features include: (Monroe; Fig. 16-9, pg. 507)
A lack of surface drainages
Water sinks into underground caverns and waterways
Re-appears farther away
Often as a river emerging from a giant spring
Large surface depressions
Can be kilometers across
Probably caused by subsurface solution cavities
Sinkholes (Monroe; Fig. 16-8, pg. 506)
Smaller depressions
Definitely the result of solution of the underlying limestone
Some extend down into caverns
Can act as natural wells if they intersect the water table
Sinkholes still forming in many karst areas
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