Information for the Catalina Community

Impact of Arroyo Grande on Oro Valley/Catalina Water

The Arizona State Land Department (SLD) has held public meetings on Arroyo Grande, its conceptual plan to allow Oro Valley to annex 9,106 acres of currently undeveloped state trust land lying west of  Oracle Rd  to the Tortalita Mountains.  The annexation would extend from Rancho Vistoso northward to the Pinal County line – that is to about opposite the entrance to Eagle Crest.  The Plan’s target population is 38,313 people living in 15,964 dwelling units.  This compares to the following existing population estimates:  Oro Valley, 39,308 (2006); Catalina, 8,500 (mostly east of Oracle from Catalina State Park to Oracle Junction, including Eagle Crest); and SaddleBrooke, 6,840 (4,560 completed homes). 

Because Oro Valley would control actual development, and because the proposed land use categories include an allowable range of dwelling units per acre, it has been pointed out that up to 37,000 homes and 90,000 people could theoretically occupy Arroyo Grande.  However, experienced planners have also noted that this would be a higher population density than is normal for Oro Valley and might not be in the interest of developers  because high density would reduce the value per dwelling unit.

 The following table shows potential new water usage from this project: 

Groundwater (less any recycled effluent) is the only ready source of water for this project.  The increased pumping would be from the same aquifer on which surrounding communities now depend.  Oro Valley’s water table has been declining 5-7 feet per year and the community, along with Marana, is considering a $97 to 182 million joint project to treat and deliver Central Arizona Project (CAP) water to reduce reliance on groundwater.  Oro Valley’s CAP allocation is 10,305 acre feet (AF) per year.  Oro Valley’s current demand plus the added groundwater burden of Arroyo Grande would be larger than this CAP allocation. Thus, the ultimate outcome of adding Arroyo groundwater demand would be a continuing decline in water levels in Oro Valley area.  There is, therefore, speculation that the SLD might make available to Oro Valley a portion of its CAP allocation to satisfy some of Arroyo Grande demand.

If Arroyo will be dependent solely on groundwater, it could also lead to declining well water levels in the Catalina/SaddleBrooke/Oracle Junction area – an area that has thus far apparently been stable despite major increases in pumping.  Groundwater modeling by the Arizona Department of Water Resources (ADWR) indicates that, due largely to surplus direct mountain and local recharge, there has historically been a groundwater underflow passing southward under SaddleBrooke/Catalina and under the proposed Arroyo project area to the west.  In 1940 this underflow, at a location 1-2 miles south of the Pinal-Pima County line, was estimated to be 3,800 AF per year.   Lacking more precise hydrologic data, this underflow can roughly be viewed as the amount of water recharged  between the Tortalita and Catalina mountains in this “upper part” of the regional aquifer.  In the 1940s and 1950s, the pumpage in this upper Oro Valley area was so small that it probably had no overall impact on this underflow. 

However, this has changed.  In Townships 8 to 11 and Ranges 13 & 14, an area including Oro Valley north of Tangerine, plus Catalina and SaddleBrooke, groundwater pumping increased from 1,182 AF in 1985 (just prior to the start of  SaddleBrooke) to 8,640 AF in 2006.  This is a seven- fold increase and should have eliminated the estimated 1940 underflow, resulting in some decline in the water table in the Catalina/SaddleBrooke area.   That such a decline has not yet been detected could imply modeling error – i.e., there may be more recharge in this area than suggested by ADWR’s modeling.[2] An alternative explanation is that the “creeping edge” of the cone of depression[3] created by pumpage from Oro Valley[4] may not have yet progressed sufficiently northward to reach Catalina.  A recent map prepared by the ADWR shows 1995-2005 water level declines in the Oro Valley area of 50-70 feet in the central area (approximately the “Target Shopping Center”), 30-50 feet further north (Tangerine Rd./Rancho Vistoso), and 10-20 feet for two locations near Wilds Rd. (approximately 3 miles south of the Pinal County line). 

Whatever the explanation for the apparent stability in the Catalina/SaddleBrooke area, it appears certain that groundwater level declines will be experienced there as the ultimate outcome of a groundwater-dependent Arroyo Grande that increases the population of the area north of Rancho Vistoso by 200-300%.[5]  The amount and rate of decline could be predicted by professional hydrologists.  Such predictions, because of data limitations and varying assumptions, could vary markedly.  Ultimately, the ADWR would require the Town of Oro Valley to modify its Designation of Assured Water Supply to demonstrate that the town’s total groundwater demand, including Arroyo Grande, would not reduce groundwater levels to more than 1,000 feet below the surface in 100 years.   The hydrological analysis does not imply that water tables will behave uniformly over the 100 year projection.  Depending on the nature of the underlying aquifer, and future additional area population growth, water tables could drop more quickly early, or later, in the period.  If new demand for area groundwater is controlled to achieve a sustainable environment, water tables may never decline to the 1,000 foot limit.

What is certain is that once groundwater usage in an aquifer permanently exceeds annual recharge, water tables will decline until there is no more feasibly-recoverable water.  This can occur when depth to water and slow infiltration of water into wells makes pumping infeasible, and/or the costs of treating deep “old water” (with high dissolved solids and possible mineral-metal contamination) are excessive.  Even before this occurs, pumping may have to halt if land subsidence from declining water tables threatens roads, housing and other infrastructure.   Once this situation exists, the only remaining antidotes are reduced water consumption through conservation, and/or use of renewable surface water and effluent.  However, in the future, renewable surface water may already be allocated to other parties or otherwise unavailable – e.g., due to Colorado River water shortages.

Bob Simpson - April, 2008.  For more background on Arizona law and regulation of groundwater see “A SaddleBrooke Water Primer”, revised February, 2008, by the same author.

See also”A Layperson’s Guide to ArizonaWater”, http://ag.arizona.edu/AZWATER/publications.