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Welcome
to GLG101C Introduction to Geology
Fall 2004
Professor James Tyburczy |
| Department
of Geological Sciences |
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Chapter
13 Groundwater |
Chapter 13
Groundwater and the Hydrological Cycle
Check
out the Water Resources web site of the US
Geological Survey
Water budget
of the earth - Oceans (96%), Glaciers and ice caps (3%), Groundwater (1%), Rivers
and lakes (0.01%), Atmosphere (0.001 %), Biosphere (0.0001%) [Earth's deep interior
- perhaps as much as in the oceans??]
Hydrologic Cycle
- Evaporation, transpiration, precipitation, infiltration, runoff
- Powered by
solar energy
- There is
more precipitation over the oceans than on land, so this is the mechanism
by which water is returned from oceans to the land surface)
- Precipitation
on land - either infiltrates to ground water, runs off to streams, evaporates,
or is transpired by plants
- Infiltration
- about 15 - 20 % of precipitation on land infiltrates into the groundwater
system
Hydrology and
Climate
Warm air -
can hold more water than can cold air
Air cools as
it goes over mountains - so moisture condenses and precipiates
The air (now
very dry) then continues on, forming a rain shadow (that is, an area of very
low rainfall on the lee - or downwind - side of mountain ranges)
Examples -
Sierra Nevada, Cascade Mountains - very dry deserts to the east of these mountain
ranges
Why study groundwater
- Water is
vital to life
- Groundwater
represents the largest easily reachable source of fresh water
Groundwater
- water in the ground occupying the pore space between the grains of the sediments
or rock
- Porosity
- fraction of open space between mineral grains in the subsurface
- Permeability
- how easily water can be pulled from a rock (or how easily ground water can
flow through a rock)
- Porosity
versus permeability
- Sandstones
generally have high porosity and high permeability
- Shales
generally have medium to high porosity, but generally have low permeability
- Crystalline
rocks (igneous, metamorphic rocks) - low porosity and low permeability
- But -
highly fractured rocks of any type can have high porosity and high permeability
if there are enough fractures
Water in the
ground
- Unsaturated
zone (zone of aeration) - pores filled with air and some water
- Saturated
zone - pores filled completely with water
- Water table
(groundwater table) - is the top of the Saturated Zone
- Aquifer -
a highly permeable layer or zone of rock or sediment that stores water and
from which water can be pulled
Water Table,
aquifers, and the flow of groundwater
- The topography
of the water table mimics the topography of the surface, but it is muted
- Groundwater
flows from places where the water table elevation is high to places where
the water table elevation is lower - it flows downhill under the force of
gravity
- In humid
areas the water table may intersect streams, resulting in 'effluent streams'
that gain water by groundwater flow
- In arid areas
(like the Phoenix area) the water table generally lies below stream beds,
resulting in 'influent streams' that lose water to the water table
- Flow rates
of groundwater in aquifers are a few centimeters per day, a rate of 15 cm/day
is pretty fast
- Recharge
of aquifers - occurs from regional rainfall onto outcrops of permeable sediments
and rocks (also in Phoenix - from canals and from over irrigation)
- Discharge
of aquifers - water pours out on to surface
- Effluent
streams - groundwater flows in to streams
- Springs
- natural flow of groundwater when surface of Earth intersects water table
- Impermeable
layers ('aquicludes') - can cause perched water table
- Pumping of
groundwater - drawdown and cone of depression -
can interfere with nearby wells. Production wells (such as for city water
supplies) can pump thousands of gallons per minute.
Water use in
the U.S.
- Need a
few liters per day to live
- Household
use today - about 1200 liters per person per day (300 gallons/person/day)
- Total
water use - 6000 liters per person per day (1500 gallons/person/day) - mostly
for agricultural use
General Problem
- 'Mining' of Groundwater - extracting groundwater faster than nature replenishes
it.
This problem
occurs in Arizona, Southern California, Central U.S. (Texas to Colorado ) Ogallala
Aquifer (and elsewhere)
Problems associated
with excessive groundwater withdrawal
- Loss of
the resource (groundwater mining) and need for deeper (and more expensive)
wells
- Land subsidence
and ground cracking (fissures). When land subsidence occurs it is generally
not possible to restore permeability to the aquifer by adding water later
- Salt
water intrusion - coastal areas - Long Island, NY and Salinas, CA, and elsewhere.
Salt water is more dense than fresh water.
How old
is groundwater
Varies
with depth and rate of recharge in any given area
May
be just a few years old, or can range up to thousands of years old (deepest
aquifers)
Oldest ages
indicate that replenishing ground water resources may be a difficult task
(on a human time scale) if they become excessively depleted
how to avoid
depletion of groundwater resources?
Careful conservation
and water management (as State of Arizona is trying to do now)
Artificial
recharge - put excess water back into underground aquifers for future use.
Is
being tried in many places here in Arizona and worldwide
Forms of grounwater
pollution
Point
source versus non-point source pollution
Dissolved
salts, including nitrates from fertilizers - Na+, Ca+2, Mg+2, K+, NO3- nitrate,
HCO3- bicarbonate, SO4-2 sulfate, Cl- chloride. Phoenix water - 1/2 gram per
liter of dissolved salts
Bacteria
(for example, e. coli from human waste)
Heavy metals
(lead, mercury, etc....)
Hydrocarbons
Light hydrocarbons
(float on water, i.e. on top of water table) such as gasoline, organic solvents
Heavy hydrocarbons
(sink to bottom of water and can lodge in bedrock) such as DNAPLs [Dense non-aqueous
phase liquids] like TCE - can be very hard to clean up
Acids
- mine waste, industrial waste
Radioactive
wastes and nuclides
Groundwater
Cleanup
Can be very
slow - years or longer in worst cases
Best idea -
Prevention of groundwater pollution
- Liners
for solid waste dumps
- Recycle
so there is less solid waste in dumps
- Replace
leaking tanks
Natural filtering
action of soil and rock (physical and biological)
- In low
population areas, 30 - 40 meters of soil can act to clean up water from
a single household septic tank
Pump and treat
- pump out contaminated groundwater, clean up pollutants, use the water directly
or pump it back down into the aquifer
Add chemical
or physical retardants to groundwater, such as
- Zeolites
- to absorb pollutants
- Oxidants
- react to neutralize certain kinds of pollutants
Biological
methods - custom tailored bacteria that eat pollutants - very new and uncertain
but lots of potential
Geological Effects
of Groundwater
Hot springs,
geysers - surface water percolates down, is heated by shallow magma body,
rises back to surface as hot spring or geyser
- Geothermal
energy - used in a few places, non polluting
Groundwater
erosion and deposition
- Dissolves
rock (particularly limestone) -> caverns, sink holes. Karst topography
- Carries
cement (calcite, silica)
- Causes
petrified wood, geodes - replacement of organic materials with minerals
Phoenix Area
Water Supply
1/3 to 1/2
of its water from groundwater
1/3 to 1/2
from surface water (dams on the Gila, Salt, Agua Fria, and Verde Rivers)
The rest
from the Colorado River (Central Arizona Project [CAP] Canal)
Tucson area
is much more dependent on groundwater
Water table
is 50 - 500 feet beneath the surface in different areas of the Phoenix Basin
From the
1930's to 1990's water table dropped up to several hundred feet in places,
resulting in ground subsidence and ground fissures in a number of areas in
and around Phoenix.
Recharge
in the Phoenix area - from rainfall, but also from canals and from excess
irrigation water
Artificial
recharge is being tried in a number of areas in Phoenix using CAP water not
currently needed for water supplies
'Safe Yield'
- by 2025 the state will require that recharge in to groundwater equals discharge
out of our groundwater supply.
Like most
big cities, Phoenix has a number of superfund cleanup sites related to
groundwater pollution - some of them are DNAPLs and so are hard to clean up.
©2004, James A. Tyburczy, Department of Geology, Arizona State University
If you have any questions or concerns regarding this page, please address
them to jim.tyburczy@asu.edu.
Be specific in your description of the problem!
Last update 11/18/2004 |