Water Supply and Demand in Tucson

With a growing population and predictions of drought, will there be enough water in Tucson in the future? In this essay I review the supply and demand. The numbers are taken mainly from the Arizona Department of Water Resources (ADWR) website and from “Water in the Tucson Area: Seeking Sustainability” a 1999 report published by the Water Resources Research Center at the University of Arizona (WRRC). This review is confined to the Tucson Active Management Area (TAMA), an area of 3,866 square miles, which includes the Tucson Basin and the Avra/Altar Valleys – the areas from which we pump our water. CAP is the Central Arizona Project which imports water from the Colorado River. Most of the numbers refer to acre-feet (AF) of water. One acre-foot is 325,851 gallons.

The Demand

 In 1999, total usage in the Tucson Active Management Area was 323,000 AF according to WRRC. Municipal usage was 154,000 AF which included 17,000 AF used by golf courses (35% was effluent from the sewer plants), and 20,000 AF used by “turf” facilities such as parks, schools, cemeteries (33% was effluent). Agriculture used 132,000 AF (of which 20,000 AF came from imported CAP water). Mines used 39,207 AF, sand and gravel operations used 5,167 AF and “other” industrial use totaled 4,026 AF. Sewer treatment plants produced 70,000 AF per year and are projected to produce 115,000 AF by 2025. Currently 84% of effluent discharge is released into the Santa Cruz river channel where it infiltrates into a shallow aquifer. (Alert readers might notice that these official figures from 1999 add up to more than 323,000 AF, so some categories must have been counted twice.)

By 2003, total usage increased to about 350,525 AF. This is projected to rise to 396,000 AF by 2025 assuming increased municipal and industrial demand, and decreased agricultural use. Natural recharge provides only about 60,000 AF per year. In 2003, municipal usage totaled 185,199 AF. Municipal use includes all domestic and small business consumption. Industries used 47,430 AF; agriculture used 102,959 AF; Indians used 14,196; all others used 3,705 according to WRRC.

This total usage is about 169 gallons per day per capita, with residential use pegged at 110 gallons per day per capita, a figure that has remained constant for many years. In contrast, the Phoenix area uses 238 gallons per day per capita, but gets 73% of its water from “renewable” resources such as rivers, CAP, and effluent.

 The Supply

 In 2003, groundwater supplied 256,233 AF, CAP supplied 64,554 AF, use of effluent supplied 11,360 AF. The rest was due to incidental and natural recharge.

Tucson gets most of its water by mining groundwater stored in aquifers down to 1200 feet deep in the Tucson and Avra Valley basins. This is mainly fossil water deposited during the wet Pleistocene glacial periods. However, there is even more water in deeper aquifers, but as depth increases, water quality decreases, and water becomes briny with salts and toxic metals.

The center parts of Basin and Range valleys such as the Tucson Basin, are filled with porous sediments. The volume of these sediments is typically, 30 miles long, 5 miles wide, 1 mile thick — 150 cubic miles. The upper part of the aquifer consists of young water from the glacial epochs which is fresh, while deeper parts of the basin contain progressively older and saltier waters. The majority of water in the deeper parts of the Tucson basin contains high amounts of dissolved salt, gypsum, boron and lithium; difficult and expensive to treat into drinking water and expensive to pump.

 The 1999 WRRC report states that, “In 1940, when Tucson began to increase its groundwater pumping, these aquifers held approximately 70 million AF of groundwater at depths less than 1,200 feet below the surface.” This resource is equivalent to all the water in Lake Mead and Lake Powell combined. Since 1940, 10% of this groundwater has been withdrawn. Simple arithmetic implies at that rate, the remaining groundwater supply shallower than 1200 feet could last about 150- to 200 years. This time will be extended by increasing use of CAP water and effluent.

Pursuant to the 1980 Groundwater Management Code, Arizona was divided into five Active Management Areas to receive water from the Colorado River. The Tucson Active Management Area is allocated about 314,000 AF per year. Tucson area entities are to receive 215,000 AF of which the City of Tucson is allocated 138,920 AF per year. The statutory goal is to reduce dependence on groundwater and to make water usage “sustainable” by 2025. Tucson is currently using only a small portion of the allotment and political pressure is mounting to use it, or lose it. The recharge project in Avra Valley will eventually use all of our allotment. There are some problems associated with depending entirely on the anticipated CAP supply. The original allotments were meted out during years of abnormally high flow in the Colorado River so that these allotments amount to 150% to 200% of normal flow. There is also the danger that if the Biodiversity Treaty were ever ratified, then endangered species in the Colorado River delta and the Gulf of California would have precedence for much more water than they are currently receiving. The Treaty would supercede state laws.

The Avra Valley recharge project, called the Clearwater Renewable Resource Facility (CRRF), is a political compromise. It would be more efficient to use CAP water directly, but in 1992, when Tucson Water tried that, the improperly treated water gave many customers discolored, evil-smelling water as it dissolved the encrustation built up in the water delivery pipes and in home plumbing. Due to the damage, Tucson Water was sued and stopped delivery in 1994.

Conclusion

It seems that we have an adequate groundwater supply for about 200 years and our CAP allocation, if it materializes, will, alone, cover most of our projected needs. However, it isn’t quite that simple because having the water in the ground is quite different from getting enough of it into the distribution system at a reasonable cost and without causing dangerous subsidence. Current peak summer water demand in Tucson is greater than maximum well pumping capacity of 143 million gallons per day. The shortage is currently made up from small storage facilities filled by wells during non-peak periods and from summer monsoon rains. This seasonal shortfall is why Tucson Water promotes its “Beat the Peak” program. The question is: how shall we continue to meet this demand and at what price.

In more recent years Tucson Water seems to have gotten its act together. Their “Water Plan: 2000-2050http://www.tucsonaz.gov/water/docs/waterplan.pdf) seems to be on the right track to provide water for the long run. However, if we are to continue to “Beat the Peak” we must have more water available on a daily basis. Imagine what will happen to property values if the city cannot deliver sufficient water. Depending solely on recharge will not solve the delivery-rate problem. We need more wells (outside the Central Well Field) and perhaps some reservoirs or underground storage. And, there should be a plan to use properly treated CAP water directly in emergencies.

A recent WRRC report estimates that water resources in TAMA could support a population of 2.3 million people if proper conservation measures are used. However, those conservation measures must be sensible for there are already some unintended consequences. For instance, current city code requires new housing to have low-flow toilets. Harvesting gray water is also encouraged. But, according to the people at the sewer treatment plants, neighborhoods using these conservation methods do not send enough water through the system to properly flush the solids to the sewer plant.

And, we will eventually need to recycle the sewer effluent that currently goes down the Santa Cruz. Despite the “yuck factor” this is a growing resource and if treated properly can be merged with other sources of drinking water.

References:

Arizona Department of Water Resources: http://www.azwater.gov/azdwr/

Water Resources Research Center : http://ag.arizona.edu/azwater/

2 comments

Comments are closed.