Hydrology EESC BC 3025
Water for Tucson, AZ
(Fig)(Fig)(Fig)(Fig)(Fig)(Fig)(Fig)(Fig)(Fig)(Fig)(Fig)(Fig)(Fig)(Fig)(Fig)(Fig)(Fig, Fig)(Fig)(Fig) (Fig)(Fig)(Fig)
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The Tucson Basin is located in southern Arizona (Fig).
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Water issues have been at the forefront fo many years, as documented in
a series of newspaper excerpts (Fig).
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15 min documentary
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The geological structure of the Tucson Basin is similar to a bathtub filled
with sediments (gravel, sand, silt, clay) surrounded by several mountain
ranges (Fig).
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Precipitation is about 12 inches per year in the city (versus 42 inches
in NYC) and shows a large interannual and seasonal variability (Fig).
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Most of the precipitation falls in the summer monsoon season during major
storms and during the winter low intensity rainy season. Precipitation
shows a steep increase with elevation reaching levels similar to NYC inn
the highest mountains (Fig).
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The sedimentary aquifers in the basin are being recharged through fractures
in the mountains that are connected to the aquifer and through recharge
underneath the washes, carrying mountain precipitation to the valley. Some
recharge occurs directly in the foothills. The main groundwater flow direction
as indicated by the hydraulic head distribution is from the SE to the NW
(Fig).
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Most of the water currently being used in Tucson is pumped from the aquifers
in several wellfields (Fig)
at rates considerably exceeding the natural recharge rate.
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As a result of this overdraft, the water level of the aquifers are dropping
rapidly, here illustrated for one well in the center of Tucson (Fig)and
the entire basin (Fig).
The balance between supply and demand can currently only be established
by groundwater mining, i.e. pumping the aquifer at unsustainable rates
(Fig).
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There are several major consequences of this overdraft (Fig)
including subsidence (sinking) of the land surface (Fig)
resulting in infrastructure damages (Fig).
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Some parts of the aquifers in the Tucson Basin are contaminated by landfill
leachates and percolation of organic contaminants (mostly TCE, Trichlorethene,
a cleaning agent) originating at the airport (Fig).
Countermeasures, such as pump-and-treat systems are currently in place.
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The withdrawal rates will most likely increase as the city of Tucson grows
(Fig, Fig).
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Several options to ease the problem have been evaluated (Fig).
The most importnat aspect of any solutions are reduction of groundwater
pumping rates in the city of Tucson and increased reliance on Colorado
River water transported to the Tucson area by the recently completed Central
Arizona Project (CAP, Fig). CAP
water had been used before in Tucson, but was rejected by the population
because of its taste (high salinity) and color (mobilization of deposits
in the pipes caused by changed chemistry). The current plan being
implemented right now is to recharge CAP water into the aquifer, let it
mix with groundwater and then pump the blend from the aquifer for
delivery to the population (Fig).
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For the moment and some years to come the use of CAP water can perhaps
eliminate the overdraft and balance supply and demand. However Tucson is
one of the fastest growing areas in the country and further savings need
to be implemented, and it is not clear how the long-term demand will be
met. All resources, groundwater, CAP water and reclaimed waste water will
be needed (Fig). Population
and water demand data for the state of Arizona seem to indicate that a
decoupling of population growth and water demand might be possible to achieve
(Fig).
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student excercise: water demand and supply projections
for the future
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download the TAMA (Tucson Active Management Area)
water use data (TAMA_projections.xls)
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make (linear?) projections for population and related
water demand for the next 50 years
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play with different assuming you (a) do and (b) do
not have access to CAP water.
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where do you need answers?
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what policy recommendations would you make?
REFERENCES
A very nice comprensive overview of the Tucson Water issues can be found
here:
