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Phoenix Area Office

Central Arizona Salinity Study (CASS)



Local and imported sources are increasing the salinity of groundwater in localized areas and the salinity of reclaimed water in central Arizona. Since about 1985, the Phoenix metropolitan area has been accumulating salts at a rate of about 1.1 million tons annually. Currently, in the Tucson metropolitan area the salts are accumulating at about 100,000 tons annually but that will increase to a rate of about 200,000 tons annually when their CAP allocation becomes fully utilized.

Before the Salt River Project (SRP) system was constructed, the Salt and Verde Rivers transported salts through central Arizona to the Colorado River and then into the Sea of Cortez. Salts were naturally deposited along the way at low points in the river system. Upon construction of the SRP dams in the early 1900's and completion of an extensive canal system, most of the Salt and Verde River water was diverted for agriculture and domestic uses. With this water came an increase in the amount of salts accumulating in the metropolitan Phoenix area. The accumulation of salts in central Arizona was further increased in the mid 1980s when the Central Arizona Project (CAP) aqueduct system was completed, introducing Colorado River water and another source of imported salts to central Arizona.

Society's use of water adds additional salts into the water cycle through the use of fertilizers, water softeners, industrial water treatment, and many other activities. The sewer system carries salts from residential, commercial and industrial sources to the wastewater treatment and water reclamation plants. Typically, the total dissolved solids (TDS) concentration entering a wastewater treatment plant (WWTP) is between 300 to 500 milligrams per liter (mg/L) greater than the TDS concentration of the water supply entering the potable water system. These salts pass through the WWTP and remain in the effluent, which is discharged into a river or wash or is put to beneficial use, such as golf course irrigation, commercial agriculture irrigation, recharge, and power plant cooling water. Although the effluent is used in many different ways, ultimately a majority of the salts end up in the groundwater and soil.

The magnitude of the salinity issue is unclear and water providers in central Arizona decided to work together to assess the problem and, if necessary develop regional strategies for managing it.

To accomplish this, the Central Arizona Salinity Study (CASS) was initiated in 2001 to examine the problems created by the importation of salts into central Arizona. CASS began through a cooperative partnership between the Bureau of Reclamation and the Sub-Regional Operating Group (SROG), which is represented by the cities of Glendale, Mesa, Phoenix, Scottsdale, and Tempe, Arizona. After the first year of the study, the cities of Chandler, Goodyear, Peoria, Surprise, and Tucson; the towns of Buckeye, Gilbert and Oro Valley; the Arizona-American Water Company; the Arizona Water Company; and the Queen Creek Water Company joined CASS and financially contribute to the effort. The study consisted of two phases conducted over a four-year project term.

CASS Phase I

CASS Phase I identified that high TDS water has implications to all sectors of society - residential, commercial, industrial, and agricultural.

Residential Commercial (schools, hospitals, retail stores, etc.) Industrial Agricultural

Though the societal costs of high TDS water in central Arizona appears to be large, it has not yet substantively affected economic development of the region, as evidenced by continued growth and development in the region. The societal cost of salinity is very subtle and is dispersed among all sectors of society. CASS Phase I modeling equated an increase of 100 mg/L of TDS of the three primary surface water sources (the Salt, Verde, and Colorado Rivers) with approximately a $30 million annual increase in societal costs for the Phoenix Metropolitan area. However, given that the gross domestic product (GDP) for Maricopa County is about $93.6 billion, the relative economic impact at present appears to be minor. The societal costs are expected to increase in the future with the continued import and accumulation of salts and the additional salt loading as the population increases in central Arizona.

Phase I Final Report Documents


The focus of CASS Phase II was to evaluate a range of potential approaches to managing salinity in central Arizona. To accomplish this task, four subcommittees were formed and each subcommittee was tasked with examining a salinity issue in detail. Those subject areas were:

Phase II Final Report Documents

Last Reviewed:
March 28, 2014

Joseph J. Billerbeck - jbillerbeck@usbr.gov