Under a state groundwater law that may be the toughest in the nation, fast-growing urban areas like Tucson have to make sure that by 2025 they're not pumping out more groundwater than is being recharged.
Thirty years after that law took effect, metro Tucson is succeeding - and failing - at meeting the Groundwater Management Act's goals.
A map released as part of a federal study of Arizona's aquifers shows large patches of blue that indicate the groundwater table is rising in Tucson's urban core.
Tucson Water has significantly reduced groundwater pumping there since it started using renewable Central Arizona Project water from the Colorado River.
At Speedway and Swan Road, for instance, the water table has risen nearly 50 feet since 2000, Tucson Water says.
Outside the city, the blue areas on the U.S. Geological Survey map include several large, artificial groundwater-recharge basins dug south of Tucson and in the Avra Valley and Marana areas.
That's where CAP water from the Colorado River is being placed on the ground and allowed to sink into the aquifer.
As a result, after decades of running up big deficits, in several recent years the Tucson area had a groundwater surplus- although not enough to meet requirements known as "safe yield" under the state groundwater law.
Falling in some areas
While some areas of the map are blue, others are covered with red blotches showing where the water table is still falling.
These include areas occupied by some copper mines and farms that pump groundwater, along with suburban areas where municipal and private water utilities aren't yet delivering CAP to local homes and businesses.
The water table is falling by as much as 9 feet a year in a section of Oro Valley, and from 1 to 8 feet a year in many other suburban areas, including the Catalina Foothills, Casas Adobes, Catalina, Green Valley, Vail and other southeastern suburbs.
On average, water levels fall at about 3 feet a year in Oro Valley wells and about 2 1/2 feet a year in the neighboring Metro Water utility's wells on the northwest side, their officials say.
These and other entities, like Tucson, recharge CAP water into the ground. But unlike Tucson, they leave most of it there. That's permitted under state law, but it's at the heart of why the water table is falling in some places.
The problem is that nearly two decades after the CAP went on line here and more than four decades after Congress authorized the water project, the region's cities, farms and other entities still aren't directly using about one-third of the Tucson area's entire CAP allocation of 196,000 acre-feet. That's because most water users have no simple, inexpensive way to build the pipes and pumps needed to deliver CAP water 10, 15 or more miles from the main canal to their systems - and ultimately to their customers.
"It's a challenge," CAP spokesman Mitch Basefsky said. "We don't know how it will be solved. Infrastructure is very expensive."
Pumping costs rise
Typically, scientists who study water are concerned that if an aquifer gets too low, the cost of pumping can become prohibitive, or the water quality diminishes. Eventually, overpumping can lead to subsidence, where the ground collapses and can form fissures.
Because of extensive CAP recharge in this area, the rate of subsidence has slowed greatly. It's still going on in parts of the Sahuarita-Green Valley area, primarily in the pecan groves north of Green Valley, said Rob Carruth, a Geological Survey hydrologist.
In some places near the region's major recharge areas, USGS officials are detecting slight rises in ground surface levels that used to be sinking due to pumping.
In other areas with falling water tables, such as Oro Valley and Vail, aquifers aren't as susceptible to subsidence because they're not as large and thick and don't have fine-grained, saturated clays and silts that are prone to subsidence at the ground surface, Carruth said.
As water tables rise in areas such as the urban core, where pumping has stopped, it's not unthinkable that someday the riverfront riparian vegetation that died off decades ago due to overpumping could someday be restored, said Wally Wilson, Tucson Water's chief hydrologist.
But "we're talking 100 years" before the recovery of groves of cottonwood, willow and mesquite trees could happen, Wilson said. A large "hole" of unsaturated soils and sands in the still-depleted aquifer must be filled first, he said.
Successes and problems
Six big, oblong-shaped recharge basins in Marana are symbols of the successes and problems with recharge.
The basins span more than 40 acres in Marana, about five miles west of Interstate 10. They take in CAP water at about a half-foot daily. Because the water sinks in so fast, the ponds are never more than 3 feet deep.
Lying a half-mile from the CAP's concrete canal, these basins were built here, at a total cost of $8 million, because the underlying soils are porous sands and gravels through which water can easily seep into the aquifer.
This practice is occurring not just in the Tucson area but all over the state. Statewide, about 7 million acre feet of CAP water is being recharged or stored by farms. That's more than four years of the entire state's annual CAP supply.
Locally, the CAP recharge is bringing the water table up by 6 to 7 feet a year in the Lower Santa Cruz and Avra Valley project areas. Farther south in the Avra Valley, the water table is rising even faster at two city of Tucson recharge projects.
Between the two, the city this year will put into the ground 140,000 acre feet of CAP water and take out 80,000. This has raised the water table 9 feet a year at the city facility in the central Avra Valley facility for the past decade and 140 feet in the three years that a second city recharge facility and well field has existed in the southern Avra valley.
It's been 11 years since Tucson started buying, recharging and using CAP. But this is the first year the city has bought its entire CAP allocation of 140,000 acre-feet. That's partly because of the Colorado River water's high cost of about $137 an acre-foot to regular customers, and partly because of the time it took to build enough recharge basins to store that much water, said Wally Wilson, Tucson Water's chief hydrologist.
Because of this and the other recharge occurring in the Tucson area, it is relatively close to reaching the groundwater law's goal of balancing pumping and recharge.
For four of the five years from 2005 through 2009, the State Department of Water Resources classified Tucson as being in a state of groundwater surplus, as more and more CAP water was recharged and farms slowly cut back their pumping. By contrast, as recently as the late 1990s, the region's groundwater deficit was as large as 180,000 to 190,000 acre-feet - meaning enough groundwater had to be pumped to serve more than a half-million homes.
The only other surpluses since 1941 - when World War II and then the postwar boom ushered in more than a half-century of groundwater overuse - came in the record-breaking flood years of 1983 and 1993.
The areas where the water table is falling are being hurt by what CAP spokesman Basefsky calls a regulatory flaw in state law.
That law, passed in the mid-1990s, says water companies, cities, developers and other entities building or serving new subdivisions can develop and pump their aquifer down if they show the aquifer doesn't drop more than 1,000 feet over 100 years.
In return, they must replenish the aquifer or pay someone else to replenish it from sources such as the CAP. They only have to recharge somewhere within the 3,866-square-mile Tucson Active Management Area set by the state water department. Or they can use CAP water rather than groundwater on crops.
Some water experts, such as Grady Gammage Jr., a Phoenix attorney and former CAP board member who has written a study on Arizona's water supplies, say it may be time to revisit the law.
But Oro Valley water director Philip Saletta said a change in the law could be a controversial if it would require pumping and recharge in the same place, even by utilities whose water table isn't falling much.
Overall, this is one of the area's biggest water management challenges, said Sharon Megdal, director of the University of Arizona's Water Resources Research Center and a current CAP Governing Board member.
"The fact that the law allows us to pump to 1,000 feet below, that concerns me, because we know that we are getting into fossilized groundwater, using it up, and it's not going to find its way back down there again," Megdal said. "On the other hand, I wouldn't argue we should have no declines. We do have a vast store of water.
"I wouldn't say it can't be used to meet some human needs. What's the balance?"
On StarNet: Find stories on nature, wildlife and the environment in Southern Arizona at azstarnet.com/environment
HOW MUCH WATER IS UNDER YOUR NEIGHBORHOOD?
Find water table trends on a map prepared by the United States Geological Survey. Go to az.water.usgs.gov/projects/amatrends. Turn off your computer's pop-up blocker and click on the site's large green button that says, "Go to trends in groundwater levels in Arizona AMAs."
It will take you to another USGS site where the map is. Then, of five active management areas, choose the Tucson AMA.
Check the box at the bottom of the site that says 2000-08. Zoom into the area you're interested in, and then click a hyperlink tool, shaped like a lightning bolt, on the right column. Then click on any red or blue area and you'll see a graphic that shows how fast the water table there rises or falls per year.
WATER TABLE HOT SPOTS, 2000-08
• Fifth Street and Swan Road: Rising 4.1 feet a year, now 325 feet deep.
• Camino de Oeste and Gates Pass Road: Dropping 4.5 feet a year, now 250 feet deep.
• Cortaro Farms and Thornydale roads: Dropping 7 feet a year, now almost 250 feet deep.
• Oracle Road and First Avenue: Dropping 9 feet a year, now 225 feet deep.
• Avra Valley and Sandario roads: Rising 6.1 feet a year, now 210 feet deep.
• Canoa Ranch area south of Green Valley: Dropping 5.1 feet a year, now 250 feet deep.
• Interstate 19 and Pima Mine Road: Rising 8.3 feet a year, now 350 feet deep.
Source: United States Geological Survey data.
Water utilities and other users explain why they're taking so long to use Central Arizona Project water, and how they're going to put it to use.
Contact reporter Tony Davis at firstname.lastname@example.org or 806-7746.