Hydrology EESC BC 3025
Case studies
Rockland
County, NY
- north west of New York City (Fig)
- water supply sources (Fig)
(Fig)
- groundwater resources (Fig)
- age of groundwater (Fig)
- droughts in New York Metropolitan area (Fig)
- slides
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).
- 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).
-
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).
-
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).
-
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 (C2HCl3),
Trichlorethene,
a
cleaning agent) originating at the airport (Fig).
Countermeasures, such as pump-and-treat systems are currently in
place.
-
The withdrawal rates will most likely increase as the city of
Tucson
grows
(Fig, Fig).
-
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).
-
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).
- water levels in Tucson are recovering (Fig)
- student excercise: water demand and
supply
projections
for the future
- download the TAMA (Tucson Active
Management
Area)
water use data (TAMA_projections.xls)
- make (linear?) projections for
population
and related
water demand for the next 50 years
- play with different assuming you (a)
do and
(b) do
not have access to CAP water.
- where do you need answers?
- what policy recommendations would
you make?
REFERENCES
