CHAPTER 15 - GROUNDWATER
Overview
Groundwater fills pore space, cracks and crevices in rocks beneath the ground
surface. It is the largest source of freshwater on earth, if ice is excluded.
Groundwater represents about 15% of the water that fell as precipitation.
Porosity and permeability reflect the ability of a rock to hold and move ground
water. Downward percolation of ground water continues until porosity ends.
Rock openings are filled with water in the saturated zone, the top of which is
the water table. Above the water table is an unsaturated zone (= vadose
zone). Lenses of impermeable rock may produce perched water tables above
the main water table.
Groundwater moves following Darcy's Law (explained in detail in Box 15.1).
Slope of the water table and permeability influence velocity of groundwater.
Aquifers carry water and alternate in the earth's crust with aquitards that
retard groundwater flow. Aquifers may be either confined or unconfined
(Figure 15.6). Wells must be drilled into a saturated zone in order to produce
water, and drawdown may produce a cone of depression. Confined aquifers
may produce artesian conditions. Springs allow groundwater access to the
earth's surface. Groundwater interacts with surface water in gaining and losing
streams. Polluted groundwater is a serious problem. Pollutants caused by
human activity include pesticides, herbicides, heavy metals, bacteria, viruses,
parasites, toxic compounds, acid mine drainage, high level and low level
radioactive waste, gasoline and salt water intrusion. Some pollutants are
naturally occurring. Some purification can be expected by groundwater flow,
but polluted ground water is difficult to clean-up. Overpumping of aquifers
may cause subsidence and threatens groundwater supplies. Artificial recharge
may be required to combat falling water tables.
Caves, sinkholes, and karst topography result from the solutional effects of
groundwater (Figure 15.23). Mineral precipitation by groundwater, primarily in
the form of calcite but including other minerals, is responsible for stalactites,
stalagmites, flowstone, petrified wood, concretions, geodes and alkali soils.
Hot springs and geysers, with their associated deposits reflect the activity of
hot groundwater, which may be tapped as geothermal energy.
Learning Objectives
1. Ground water represents percolation of about 15% of the precipitation
falling to the earth, accounts for 0.61% of the world's water, and is second
only to glaciers as a source of freshwater.
2. Porosity is the percentage of a rock's volume comprised of openings and it
measures the rock's ability to hold water. Most rocks hold some water in either
pores or joints, but porosity is highly variable. Permeability measures the
capacity of a rock to transmit fluids. Many porous rocks are permeable, but
shale has high porosity and low permeability.
3. Water percolates into the Earth as far as porosity exists and saturates the
lower portions of the porous intervals (saturated zone). The upper surface of
the saturated zone is the water table. Above the water table, porosity is filled
by air and water, forming an unsaturated zone (= vadose zone). Water is
drawn by capillary action from the saturated zone into the vadose zone.
Perched water tables result from local variations in permeability, such as shale
lenses in sandstones.
4. Groundwater movement follows Darcy's Law (velocity = permeability x
hydraulic gradient; explained in Box 15.1). Water table slope influences
groundwater velocity: steeper slopes allow faster movement.
5. Aquifers are porous and permeable rock bodies through which groundwater
moves easily. Aquitards have low permeability and/or porosity that prevent
groundwater movement. Unconfined aquifers have water tables and exhibit
rapid groundwater movement. Confined aquifers lack water tables and exhibit
slow groundwater movement.
6. Wells must penetrate a saturated zone to produce groundwater. Water
tables fluctuate with season. If the water table falls below the bottom of a
well, it is dry. Drawdown from pumping wells produces local lowering of the
water table. Recharge raises the water table. A cone of depression may form
around a well because of drawdown. In artesian wells, water rises above its
confined aquifer because the groundwater is under pressure due to elevation
differences.
7. Springs form where the water table intersects the surface, or structures
bring water to the surface. The surface of gaining streams is the water table,
while losing streams lie above the water table.
8. Rain can leach surface contaminants and move them into groundwater.
Human activity produces potential pollution from pesticides, herbicides,
fertilizers, heavy metals and toxic compounds, bacteria, viruses and parasites
from animal, plant and human waste, acid mine drainage, and radioactive
waste (both low level and high level). Gasoline may float on the water table.
Some pollutants are naturally occurring. Some filtration and purification can be
expected through groundwater flow, if it is slow. Heavily pumped wells near
coasts can be contaminated by saltwater intrusion.
9. Dropping water tables create problems with supply and subsidence, through
compaction. Artificial recharge may offset these problems.
10. Natural groundwater is slightly acidic because of dissolved carbon dioxide
from the atmosphere or soil gases. Its contact with calcite in limestone causes
solution forming caves, sinkholes and karst topography. Calcium and
bicarbonate in solution can be precipitated as calcite in the form of stalactites,
stalagmites, columns and flowstone.
11. Groundwater may also form petrified wood, concretions, geodes (bodies in
Figure 15.26 are amygdules), cement sedimentary rocks, and develop alkali
soils.
12. Hot springs have groundwater warmer than the human body. Heating of
the water is by either proximity to a magma chamber, or through the
geothermal gradient. Geysers erupt periodically because of constrictions in
conduits to the surface allow the temperature of the groundwater to rise to
vapor, which then condenses as the eruption proceeds. Hot groundwater
produces deposits of sinter (silica) or travertine (calcite). Geothermal energy is
derived from hot groundwater through the production of electricity from
natural steam. It is also utilized for heating, paper manufacturing, ore
processing, food preparation and other non-electric uses
Boxes
15.1 - IN GREATER DEPTH - DARCY'S LAW AND FLUID POTENTIAL
About one hundred fifty years ago, Henry Darcy related the effects of hydraulic
head and hydraulic gradient to the direction and velocity of groundwater
movement. Hydraulic head is the combined effect of the difference in elevation
and pressure on groundwater. Fluid potential (or just potential) is the
acceleration of gravity times the hydraulic head. The hydraulic gradient is the
difference in head divided by the distance of flow. Groundwater moves from
high head to low head, in the direction of the slope of the water table. Velocity
of groundwater flow is the hydraulic gradient times the permeability, also
known as Darcy's Law. Groundwater flow can be mapped by equipotential
lines. Flow lines are always perpendicular to equipotential lines as groundwater
moves from high potential or head to low potential or head.
15.2 - ENVIRONMENTAL GEOLOGY – HOW DO WE FIND GROUNDWATER RESOURCES?
This discussion contrasts the practice of water witching or dowsing, in which a
dowser sites a potential well using a forked stick or metal rod, and the
scientific method of groundwater exploration using the principles of
hydrogeology. Most rocks hold some water and dowsing success is equal to or
less than pure chance in siting a well, although many people believe the
dowsers ability. Hydrogeological analysis looks for aquifers using a variety of
techniques, involving both surface and subsurface data, with success rates
superior to witching and chance.
15.3- ENVIRONMENTAL GEOLOGY – THE WALKERTON WATER TRAGEDY
In May, 2000, bacterial contamination of water supplies in the town of Walkerton,
Ontario caused 7 deaths and more than 2,300 cases of gastrointestinal illness.
Walkerton’s drinking water is supplied by municipal wells pumping groundwater and
the source of the bacterial contamination was determined to be from cattle manure
recently spread on a local farm. The bacteria had rapidly entered fractured limestones
underlying the area and was quickly transported in the shallow unconfined aquifer to
nearby wells. A number of lessons were learned from the Walkerton tragedy,
including the realization that clean drinking water cannot be taken for granted,
groundwater resources should be carefully managed and aquifer protection plans must
be based on thorough understanding of hydrogeological conditions.
15.4 – ENVIRONMENTAL GEOLOGY – THE LEGACY OF LANDFILLING IN URBAN AREAS
Many urban areas are affected by past land use practices that left behind
contaminated soils and groundwater. The Port Industrial District in Toronto is
underlain by severely contaminated landfill material and groundwaters migrating from
this area into Lake Ontario are affecting the quality of the lake waters. The material
used as landfill included garbage, street sweepings, scrap metal, pipes and fly ash.
Manufacturing and industrial activities added more contaminants to the area and these
chemicals are still present in soil and groundwater. A planned building development at
the site was halted in 1992 due to the prohibitive costs of decontamination. Parts of
New York City are built on similarly contaminated landfill materials.
Short Discussion/Essay
1. Draw a diagram to illustrate the type of geological setting in which a
perched water table may form.
2. How does the duration and periodicity of geyser eruptions reflect
permeability and porosity?
3. Explain how wells work in an unconfined aquifer using a straw in a cup filled
with ice and soft drink as an illustration.
4. Explain how a rock (e.g. pumice) could be porous and lack permeability, but
why the reverse is not possible.
5. Which groundwater contaminants come from natural sources?
Longer Discussion/Essay
1. Describe the steps in cave formation. Why are most caves initially filled with
water?
2. How does ground water get into a confined aquifer? What are the
environmental implications of this situation?
3. Explain how wells in unconfined and confined settings go dry. What is the
best means of preventing dry wells?
4. Explain why all areas with limestone bedrock do not exhibit karst
topography.
5. How can groundwater be protected from surface contaminants?
Selected Readings
There are many textbooks dealing with groundwater. Two of the more popular
ones are:
Fetter, C.W. 1988. Applied Hydrogeology. 2nd Edition. New York: Macmillian
Publishing Co.
Freeze, R.A. and Cherry, J.A. 1979. Groundwater, Englewood Cliffs, NJ:
Prentice-Hall
The Decade of North American Geology program of the Geological Society of
America produced an excellent volume on hydrogeology that treats
hydrogeologic regions, comparative hydrogeology, and groundwater
processes:
Back, W., Rosenshein, J.S. and Seaber, P.R. 1988. Hydrogeology. The Geology
of North America Vol. 0-2. Boulder, CO: Geological Society of America.
The Environmental Protection Agency has a two volume handbook that treats
groundwater, contamination, wells, and investigative techniques and
methodologies that is an excellent summary resource:
EPA Handbook. 1990. Volume I: Ground Water and Contamination.
EPA/635/6-90/-16a. Cincinnati, OH: Center for Environmental Research.
EPA Handbook. 1991. Volume II: Methodology. EPA/625/6-90/016b.
Cincinnati, OH: Center for Environmental Research.
Water chemistry is treated in:
Hem, J.D. 1989. "Study and Interpretation of the Chemical Characteristics of
Natural Water." U.S. Geological Survey, Water Supply Paper 2254 (Third
Edition).
Good general references on karst hydrology and geomorphology are:
Ford, D. and Williams, P. 1989. Karst Geomorphology and Hydrology. London:
Unwin Hyman.
White, W.B. 1988. Geomorphology and Hydrology of Karst Terrains. New York:
Oxford University Press.
For discussion of caves and geysers, it's probably best to go to descriptions of
the geology of Carlsbad, Mammoth Cave and Yellowstone National Parks.
Three good general references for national parks are:
Ellwood, B.B., 1996. Geology and America's National Parks. Upper Saddle
River, NJ:
Prentice-Hall, Inc.
Harris, A.G., Tuttle, E. and Tuttle, S.D. 1995. Geology of the National Parks.
5th edition. Dubuque, IA: Kendall Hunt Publishing Company.
Harris, D.V. and Kiver, E.P. 1985. The geologic story of the national parks and
monuments. 4th edition. New York: John Wiley and Sons.
The environmental effects of groundwater contamination in urban areas of Canada are
discussed in :
Eyles, N. (ed), 1997. Environmental geology of Urban Areas. Geological Association
of Canada, 590pp.