U4735 - Lecture 8

U4735x Environmental Science for Decision Makers

Lecture 8: North American groundwater management issues.

Martin Stute

List of Projections:

1 - Total Water use in the United States in 1990
2 - Estimated Use of Water in the United States in 1990
3 - Arizona map/Tucson
4 - Water related newspaper articles, Tucson, AZ
5 - Geology of the Tucson Basin
6 - Tucson - climate
7 - Tucson - precipitation
8 - Tucson - hydraulic head distribution
9 - Tucson - map of wells
10 - Hydrograph of a well in Tucson
11 - Tucson - decline in water table
12 - Water supply and demand in the Tucson area
13 - Consequences of overdraft
14 - Subsidence in Tucson
15 - Subsidence fissures near Tucson
16 - Groundwater contamination in Tucson
17 - Growth of City of Tucson
18 - Population growth, Pima county
19 - Water options for Tucson
20 - The Central Arizona Project (CAP)
21 - Storage and recovery of CAP water, Avra Valley
22 - Providing a stable water supply for Tucson
23 - Population and water use in Arizona

Water use patterns in the United States

Most of the water in the United States is used for irrigation in agriculture, peaking in the western parts of the country (Projection #1). The source, use, and disposition of water are summarized in (Projection #2). Most of the water being used comes from surface water, the two major areas of use are irrigation and thermoelectric (cooling), and most of the water is not evaporated (consumptive use) but is returned to surface or groundwater bodies often at a lower level of quality, however.

Water use patterns in the Southwest - Tucson case study

The Tucson Basin is located in southern Arizona (Projection #3). Water issues in this area have been at the forefront for many years, as documented in a series of newspaper excerpts more than a century ago (Projection #4). The geological structure of the Tucson Basin is similar to a bathtub filled with sediments (gravel, sand, silt, clay) surrounded by several mountain ranges (Projection #5). Precipitation is about 12 inches per year in the city (versus 42 inches in NYC) and shows a large interannual and seasonal variability (Projection #6). 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 in the highest mountains (Projection #7).

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 (Projection #8). Most of the water currently being used in Tucson is pumped from the aquifers in several well fields (Projection #9) at rates considerably exceeding the natural recharge rate. 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 (Projection #10)and the entire basin (Projection #11). The balance between supply and demand can currently only be established by groundwater mining, i.e. pumping the aquifer at unsustainable rates (Projection #12). There are several major consequences of this overdraft (Projection #13) including subsidence (sinking) of the land surface (Projection #14) resulting in infrastructure damages (Projection #15).

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 (Projection #16). Countermeasures, such as pump-and-treat systems are currently in place.

The withdrawal rates will most likely increase as the city of Tucson grows (Projection #17, Projection #18). Several options to ease the problem have been evaluated (Projection #19). The most important 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, Projection #20). 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 (Projection #21).

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 (Projection #22). 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 (Projection #23).

REFERENCES

A very nice comprehensive overview of the Tucson Water issues can be found here:

Water in the Tucson Area: Seeking sustainability