Tucson’s Groundwater Management Plan and How Outlying Developments Can Fix its Holes

Tucson from Mt Lemmon

Since the passage of the 1980 Arizona Groundwater Management Act (see current water code here), the City of Tucson has steadily worked to combat overdraft.  Is this enough to sustain the greater Tucson area for the foreseeable future and beyond?  Or is Tucson’s groundwater management plan only another band-aide on a growing water crisis in the west?

For decades preceding the Groundwater Management Act, Arizona has struggled with groundwater overdraft.  Overdraft occurs when groundwater pumping outpaces recharge.  Groundwater is held in saturated zones in the soil called aquifers, the top of which is referred to as the water table.  Aquifers are recharged by surface runoff from annual rain and snow.  Groundwater in the Tucson area is tens of thousands of years old due to the slow recharge rate in the arid southwest.  Average annual precipitation in Tucson is 12 inches increasing to 29 inches on the highest peak above the city, Mount Lemmon.  Such low precipitation amounts also mean low annual aquifer recharge.  With modern well technology, groundwater can be pumped at high flow rates that can quickly lower the water table.  A declining water table has been a problem for Tucson, which has historically relied exclusively on groundwater to supply its citizens.  Tucson’s central well field site has seen a 200 ft. reduction in the water table from 1983-2013.

The Groundwater Management Act led to a variety of regulations for groundwater pumping including mandatory conservation and recharge programs.  Tucson and other populated areas in the state were designated “Active Management Areas” (AMAs) under the Act.  Tucson’s AMA goal is safe yield by the year 2025, which occurs when annual replenishment equals annual withdrawal of water from the aquifer.  An analogy would be a bank account where withdrawals are equal to deposits.  According to the 4^th Management Plan for the Tucson AMA published in 2016, Tucson has been able to achieve safe yield in 2011, 2012, and 2013 due to higher incidental recharge along with reductions in withdrawals from the aquifer that supplemented natural recharge.  Incidental recharge is water that finds its way back to the aquifer after agriculture, municipal, or industrial uses compared to natural recharge from precipitation. 

In addition to incidental recharge and reduction in groundwater withdrawal, much of the success of the Tucson’s AMA is attributed to recharge using Central Arizona Project (CAP) water in the Avra Valley.  The CAP canal reached Tucson in the early 1990s, but the integration of CAP water into the city’s distribution system wasn’t smooth.  CAP water is water from the Colorado River and has a different composition than groundwater.  The Colorado River has a high concentration of dissolved solids known as “salts” acquired after flowing through the erosion prone soils of Utah and the Grand Canyon.  When Tucson tried to distribute raw CAP water, many Tucsonans experienced brown water coming out of their tap as the water reacted with and loosened corrosion buildup in galvanized steel distribution pipes.  To solve that quality problem, Tucson began injecting CAP water into the aquifer at the Central Avra Valley Storage and Recovery Project (CAVSARP) and later the Southern Avra Valley Storage and Recovery Project (SAVSARP) to dilute the dissolved solid rich CAP water with purer groundwater.  Since then, Tucson has been relying primarily on the CAP, considered a renewable supply because it is surface water recharged by precipitation runoff, to achieve safe yield by 2025 consistent with the AMA’s goal.

Is this practice sustainable?  Tucson is a municipal customer on the CAP meaning that it has some of the highest priority water rights in the hierarchy of rights among CAP customers who include tribes, irrigation users, and other municipalities.  Even in times of shortage on the Colorado River, municipalities are entitled to full water deliveries while junior right holders will be forced to take cuts.  Tucson’s supply as a municipality appears to be safe for the foreseeable future.

However, the effect of CAP recharge on all areas of the Tucson AMA has been mixed.  Groundwater levels near SAVSARP and CAVSARP have risen considerably in recent years.  Groundwater has risen in central Tucson and south along I-19 in the last 18 years according to a 2019 groundwater study conducted by the City.  However, other parts near the border of the Tucson aquifer haven’t been so fortunate.  Saddlebrooke Ranch, a community of 5,400 homes, does not benefit from recharge and reduced groundwater pumping because it is situated uphill from most of the aquifer and is not connected to Tucson municipal water.  Groundwater still flows with the terrain and higher elevation groundwater will slowly flow underground to the lowest point in the valley. 

Consequently, as more homes and golf courses are built and supplied by finite groundwater, high rates of withdrawal lead to a falling water table and subsequent subsidence concerns.  CAP recharge through SAVSARP and CAVSARP will not be able to help communities like Saddlebrooke that are not connected to City distribution systems.

What can communities like Saddlebrooke do to ensure water for residents?  Under the Assured Water Supply (AWS) Rules, passed in 1993, developers must demonstrate an assured water supply for their community for 100 years.  Another requirement of AWS is that a community’s water supply must not be based solely on groundwater or if groundwater is the sole source, the community must be a member of CAGRD.  CAGRD stands for Central Arizona Groundwater Replenishment District and is the managing authority for AMAs.  A community that is a member of CAGRD must not pump groundwater above a limit that jeopardizes the AMA goal and if this occurs, the community must pay CAGRD to replenish the amount above the limit that was withdrawn from the AMA.  But replenishment doesn’t have to occur in the immediate area of pumping, which is why Saddlebrooke’s water table is dropping 2-7 feet per year in area wells.  Such a drop, if left unchecked will require drilling deeper wells to mine groundwater which poses significant expense.  Further, a declining water table leads to subsidence which can destabilize building foundations as the soil particles compact to fill the void left by extracted groundwater. 

The ability to extract water in one area and recharge it in another area of the aquifer is a hole in the 1980 Groundwater Management Act that is still present today.  The hole is created by the language that safe yield can occur when the entire aquifer is net zero based on recharge minus withdrawals.  Thus, to solve groundwater overdraft for all communities in southern Arizona, including those unincorporated by a municipality, replenishment will have to occur in the localized area where withdrawals are high.  Under the AWS Rules, replenishment can come from a source of renewable water (surface water) or reclaimed water, but not groundwater elsewhere in the AMA.  Aside from using less water, the latter appears to be the best option for communities like Saddlebrooke.  The best management tool Saddlebrooke can use is to treat wastewater from the community and capture runoff from golf course irrigation and inject that water back into the aquifer through dry wells.  Certainly not all water used will be returned to the aquifer due to evaporation losses, but this will slow the rate of decline to a rate that is more geologically and financially stable. 

Using CAP water exclusive of other management tools to recharge the aquifer and attain safe yield won’t help all of Tucson and surrounding areas because safe yield views the aquifer as one whole “account” despite its diverse portfolio of communities, geology, and well fields.  Ultimately, new real estate development will have to either invest in City services or get creative with recharging their local aquifer with reclaimed water.  Together with recharge and reduced use, sustainable groundwater pumping can occur throughout all areas of the Tucson AMA, but the time to implement these tactics is now, not when it becomes cost prohibitive to drill deeper wells and mine deeper water.