UNIT 4 Water Issues
Aquifer overdraft
Tucson’s greatest water concern is the rapid depletion of groundwater from the Central Well Field. The wells in the center
of the city have provided Tucsonans with water since the 1930s. In fact, Tucson is one of the largest cities in the nation
to rely upon groundwater as its only water source. To keep pace with Tucson’s explosive population increase over recent
decades, Tucson Water has had to sink more and more wells. Tucson Water drills approximately five new wells each year
to keep up with demand. Presently new wells in the central well field are being drilled 800 – 1200 feet deep. It is impossible to calculate exactly how much water the aquifer
underlying the Tucson Valley holds, but it is well documented that twice as much water is being pumped as is
naturally being recharged. Correlated to this overdraft,
the Central Well Field water table has dropped almost
200 feet in the last fifty years, and continues to drop an
average of four feet a year.
The cost of working against gravity
Several problems are associated with groundwater
mining of this magnitude. Since the water table has
dropped so dramatically, wells must be drilled deeper,
Water table decline in Tucson’s Central wellfield at approximately
meaning that groundwater has to be lifted a greater
Grant Road
distance to the surface. Most of Tucson Water’s wells
operate using electric-powered pumps. More electricity is required to operate these pumps the deeper the water table
drops, resulting in increased expense for the water utility. If the water table continues to drop at current rates, at
some point it will become economically unfeasible to pump water. There is also a connection between the dropping
water table and water quality. The deeper sediments of the aquifer underlying Tucson contain more salts. There are
many different compounds generally referred to as “salts.” The table salt that we are most familiar with is sodium
chloride. Also, the more water that is removed from the aquifer, the greater the concentration of minerals in the
remaining groundwater.
Land subsidence
Soils are made up of different sized particles. The size of the soil particles relates to the size of the voids or pore spaces
that exist among them. Soils with large particle size have larger pore spaces. The pore spaces of dry, unsaturated ground
hold air. When the soil is saturated all the spaces are filled with water. What can happen when water is rapidly pumped
out of an aquifer is a collapse of the soil. This is called land subsidence. Because the water that once occupied the pore
spaces is gone, there is nothing to physically support the soil particles. Under this disturbed condition the soil can begin
to settle, filling in the previously open spaces.
It is important to understand that the degree of land subsidence that may occur depends both on the nature of the
strata making up the aquifer and the rate of groundwater pumping. In the cotton-farming region around Eloy, Arizona,
vast amounts of groundwater have been pumped to irrigate the cotton crops. As a result, the actual surface level of
the ground has dropped 20 feet over a 60-year period. This is an extreme example of subsidence. Because the region
where this occurred is largely rural, the consequences weren’t very grave. However, subsidence in urban areas can damage infrastructure. For example, underground water and gas pipes can break, foundations of buildings can settle and
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crack, and sewer lines and runoff drain lines engineered
to utilize gravity flow can be altered, causing water to
flow the wrong direction or back up.
The universal solvent
Most water users expect water to be colorless, odorless
and tasteless. However, most water contains substances
that may add some type of taste, odor and/or color. In
certain cases the substances found in water may have
resulted from human actions, but generally the substances are dissolved minerals such as salt. Water is
called the universal solvent because of its ability to dissolve many different solids. For example, water trapped
underground for thousands of years can cause minerals
to slowly dissolve from the surrounding rock. The presence of three minerals – calcium, magnesium and iron
– in water causes water hardness. Tucson’s groundwater
is referred to as being “hard” because it contains a great
deal of the minerals calcium and magnesium. Water is
“hard” in most parts of the southwestern United States.
Many factors, including high evaporation rates, low rainfall, and high mineral content in the ground, increase water hardness. When minerals are dissolved in water the water is called the solvent and the minerals are the solutes. The entire mixture is known as a solution. A solution consists of a solid, liquid or gas (solute) dissolved in a liquid (solvent).
Source: UofA Water Resources Research Center
Water contamination
When you consider that less than one percent of Earth’s water is fresh water, it’s difficult to believe that humans do not
take more care to maintain the quality of this precious resource. Ever since the Industrial Revolution, man has developed more types and greater quantities of potentially toxic, synthetic chemicals. Controlling and containing the release
of these substances into the environment poses a tremendous challenge. Because water is the universal solvent, it
is not surprising that water is particularly susceptible to
contamination.
In Tucson, one of the most serious threats to the quality
of the groundwater is contamination of the land under or
around the rivers in our valley. Sand and gravel pits have
always been located along the rivers because that’s where
the materials necessary for construction exist in quantity.
Unfortunately, in the past when sand and gravel pits were
retired, the vast holes left behind became garbage dumps.
Many old dumps are located along the Santa Cruz, Pantano Landfills and major identified hazardous plumes in groundwater.
and Rillito rivers. Water percolating down through these
landfills can dissolve all types of pollutants and carry them further down into the groundwater. Since access to groundwater is difficult and because it moves so slowly, pollutants are not easily removed and may stay in the groundwater
supply for many years, or even decades. Another challenging aspect of groundwater pollution is that as the water moves
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it carries the contaminants with it. This means that over time wells may be impacted that are far from the source of the
original contamination. On a positive note, we have learned from past errors; our present landfills aren’t located along
waterways and they are lined and capped with plastic or clay to avoid possible groundwater contamination.
Another source of aquifer contamination can occur from illegal dumping of toxins in and around recharge areas such as
washes, septic systems, sewers and basins. For example, many citizens remain unaware that motor oil or antifreeze that is
drained onto the ground or down storm drains, may penetrate deep into the ground, eventually entering the groundwater.
Even though significant penalties are imposed for those who are caught, illegal dumping continues to be a problem.
One test to detect if solids are dissolved in a solution of water is to allow a water sample to evaporate. If solid particles
remain after evaporation, this suggests that the sample was not pure. It is important to note that the evaporation method is only effective with solutes that are solid at room temperature. It will not work with liquid or gaseous solutes.
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Activity 4.1 What’s Left When the Water’s Gone?
*Activity adapted from "You Can Judge a Drop of Water By Its Spots," Water in Our Desert Community. 1994. Arizona
Municipal Water Users Association.
At a Glance
This activity engages the students in a simple water quality test using the scientific method. Students will place drops
of water samples on black construction paper squares. A lamp is used as a heat source to evaporate the water. After
the water has evaporated, the dissolved materials remain.
Arizona Department of Education Academic Standards
Please refer to the Arizona Department of Education Academic Standards section for the ADE standards addressed
by this lesson.
Learning Objectives
Students will be able to:
1) Compare water samples during a water quality test.
2) Infer that what appears to be pure water may include many different substances in solution.
Materials
data collection worksheet (Student Activity Book)
waxed paper
dark construction paper
permanent marking pen
ruler
five eye droppers
hand lens or magnifying glass
newspapers
samples of four premixed solutions, such as:
• salt water (as much as a 1/4 cup salt to 1 cup water)
• sugar water (as much as 1/4 cup sugar to 1 cup water)
• Kool-Aid, coffee, carbonated water, tap water, etc.
• distilled water
lamp (or allow samples to dry overnight)
Procedure
Introduce the activity by explaining that teams of students will take part in a very old and established problem solving
method: the scientific method. The scientific method includes the following steps:
1. identifying a problem or question
2. formulating a hypothesis
3. setting up an experiment to test the hypothesis
4. performing the experiment
5. making observations and collecting data
6. drawing a conclusion based on the results of the experiment
7. comparing the conclusion with the hypothesis
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The students are to detect the presence of contaminants in samples of water and determine which of the five water
samples is not contaminated with a solute. Since they do not know the nature of the solutes, it is not safe for students
to taste or even smell the solutions. Only by visual observation may they detect the presence of solutes. Based on their
visual inspection, students will first hypothesize which sample is “uncontaminated.” They will then detect solutes by
allowing the samples of water to evaporate.
Distribute the necessary materials to each team. Have students follow the steps in the Student Activity Book to carry
out the experiment.
When each team has completed the experiment, discuss conclusions. Which sample appears to have no solutes (contaminants)? Share the identity of each sample. Ask if students have ever seen residue like these in other samples of
water solutions.
Summarize by telling the students that water is known as the universal solvent because of its ability to dissolve a variety
of substances. You were presented with five liquid samples. By using evaporation, you successfully separated the dissolved solutes from the solvent. The water evaporated, leaving behind the residue of salt, sugar and other solutes. This
is only one method in which contaminants can be removed from water.
Extension
Use water test kits to sample for dissolved gases in water. Pool test strips easily identify the presence of chlorine,
bromine, as well as test for pH. Simple kits are also available to check for the quantity of dissolved oxygen and carbon
dioxide in water.
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Activity 4.2 The Unseen Unclean
* Activity adapted from "Hidden Dangers," Educating Tomorrow’s Hydrologists: Inquiry Based Curriculum on Hydrology.
1996. Arizona Hydrological Society.
At a Glance
Students look at the effects of an unseen pollutant and use problem solving methods to try to remove the contaminants.
Arizona Department of Education Academic Standards
Please refer to the Arizona Department of Education Academic Standards section for the ADE standards addressed
by this lesson.
Learning Objectives
Students will be able to:
1) Explain how groundwater can become contaminated.
2) Identify the locations and causes of groundwater contamination in Tucson.
3) Understand the difficulties in remedying groundwater pollution.
Materials
coarse sand
clear plastic liter soda bottles
cheese cloth or squares of nylon stocking
rubber bands
dark-colored Kool-Aid
historic “Tucson dumps” overhead transparency
Procedure
Begin with a teacher demonstration of simulated groundwater contamination.
Prepare a clear plastic liter soda bottle by cutting the bottom off and then attaching cheesecloth over the small opening at the neck with a rubber band. Invert the bottle and pour into it a layer of coarse sand several inches thick. Mix a
small amount of dark-colored, unsweetened Kool-Aid in with some sand and pour a four inch layer of that mixture on
top of the previous layer. This represents the landfill or hidden pollution. Pour several additional inches of “unpolluted”
sand on top of that. Place the bottle so that it will drain into a clear container (such as a plastic milk jug) in full view
of the class. Review the Activity 2.1 procedure for a system of setting up the bottle to drain.
Just before pouring water into the bottle, ask students to predict what will happen. Invite them to inspect the bottle
and report back as if they were going to drill a well. Students might assume that this is a review of the “How Much Can It
Hold?” activity and should tell you that the water will percolate at a fairly quick speed. They shouldn’t be able to detect
any possible sources of contamination.
Pour two cups of water through the system. As students become aware of the discoloration of the water, ask if they were
previously aware of anything unusual about the sand. Ask students, “How safe is groundwater from contamination?” Ask
them to identify possible sources of groundwater pollution (e.g., landfills, illegal dump sites, industrial waste disposal).
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Discuss the risk to humans of unknown pollution. Ask students to describe how their quality of life would be affected.
Brainstorm some ideas about how to clean up the polluted sand. Ask them how they think actual groundwater pollution
can be removed.
Break students into small groups. Hand out student lab sheets. Have students set up a “dump site” using any materials
you wish to have available. Possible types of easily visible “pollution” can include Kool-Aid, food coloring (which could
represent contamination from the surface), bits of colored paper, etc. Have students determine how many times water
must be flushed through in order to get clean water going into the aquifer.
Review how aquifers become contaminated. Discuss with students the difficulty in “cleaning” the soil and the aquifers.
Pose the question, “What can we do to protect our groundwater from contamination?”
Show students the overhead transparency of the historic Tucson dumps map. Ask students to explain the pattern of the
dumpsites. Ask students to explain how the location of dumpsites might affect the groundwater. Ask, “What can we do
to prevent landfills from polluting the groundwater?” (Don’t locate them near washes or rivers, use some sort of liner,
increased vigilance to intercept dumping of toxic materials.)
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