Old Water and New Knowledge at Cienega Creek

The Questions

“How old is your water?” That’s not a common question among water users, or even in water education, yet it’s high on the list for Dr. Jennifer McIntosh. She’s an Associate Professor in Hydrology & Atmospheric Sciences at the University of Arizona whose focus is the elemental and isotopic chemistry of water. For her, estimating the age of water can be a key tool in understanding the structure and functioning of aquifers. In the Cienega Basin in Southern Arizona, her hydrology group is using spectrographic and chemical analyses to research the “signature” of water flows, including the age of the water, and ultimately to expand knowledge of how the hydrology of this basin functions. Cienega Creek is a special and noteworthy place, one of the few remaining perennial streams in Southern Arizona which has been the subject of heated debate and detailed study for nearly two decades.

Could a proposed new copper pit mine in Southern Arizona damage water resources in the Cienega Basin, either above or below ground? For years data have been gathered, models have been created, permits have been requested and reviewed, some have been granted. The discussion continues, although the academic issues are on a slower timeline than the political. In addition to the new hydrological studies underway, the Army Corps of Engineers is preparing to issue a ruling on the last major Rosemont Project permit under Section 404 of the Clean Water Act (regulating discharge of fill material into adjacent canyons).

The Place

From the west the route to the perennial stretches of Upper Cienega Creek begins at Arizona Highway 83. The blacktop angles east and north across the open grassland to the Empire Ranch headquarters. The land was quiet on the bright January morning Dos Aguas drove in, a red-tailed hawk staring out from a the top of a mesquite, a small group of pronghorn grazing across the wash. A few of the pronghorn turned to watch the truck pass, but no animal moved. The dry and brittle grass waved in the breeze and was dotted with numerous dark clumps of invading mesquite growth. At the top of the first rise the Whetstone Mountains dominated the horizon to the east, and the ribbon of water-fed cottonwoods and willows was visible winding through the lowlands.

Winter Flow on Upper Cienega Creek (Rick Bowman)

Soon the blacktop yielded to gravel, and continued past the ranch buildings toward the first slopes of the Empire Mountains. Then the road curved across a dry section of the creek, and turned north along the east bank. Gravel gave way to two tracks across the desert, rough hardpacked ground with occasional muddy puddles from the winter rains. To the left the creek bed was marked by a zigzagging file of cottonwoods towering over the desert scrub. The cottonwood canopy looked hazy in the morning light, as if covered in frost. Through the binoculars the haze proved to be the yellowish green of new buds on the top branches.

The road bent away from the creek, climbing the slope of the Whetstone Mountains to the east. On the ridge beyond Mattie Canyon the tracks swung down again toward the creek. The truck was forced to a crawl across the rocky washboard slope descending to the bottom land, passing through multiple wire gates marking off the pastures allocated to cattle, scrapping past brush grown into the trail, skirting the mud holes (or speeding hopefully through the center). Other than the fuzzy, greenish cottonwood tops, the landscape was dominated by winter browns and greys.

As the trail neared the cottonwoods, Dos Aguas abandoned the truck. Sink holes, from a receding water table, have claimed much of the roadbed, some of them large enough to swallow a truck. Walking down the last slope to the bottom gurgling water could be heard, being pulled by gravity along the channel, over rocks, around fallen trees, past the dead-looking brush and tufts of grass with a few green shoots standing tall. Cienega is a classic creek, not so big, water clear and cold, a steady current bubbling in the shallows and waving the strands of moss at the bottom of the pools.

The old road up to the Total Wreck Mine crossed the creek just below these pools. On that January day water flowed two feet deep over the rocky base of the crossing. The place was deserted. No vehicles, no hunters or hikers, no cattle or cowboys, no land managers or Indians. Not even any fish or frogs were visible, all sequestered away on a cold morning. But the water flowed, morning or night, warm or cold, rain or shine.

Scale Model of Rosemont Mine, viewed from the South. The pit is colored white; the waste rock and tailings are pink. (Courtesy Pima County Administrator’s Office)

The Mine

[+]The proposed mine pit…

The Rosemont Copper Project, now owned by the Canadian firm Hudbay, Inc., has proposed a new open pit mine into a major porphyry copper ore body in the Santa Rita mountains about 30 miles southeast of Tucson, AZ. If approved as designed, the Rosemont operation would be the third largest copper mine in the US, producing an estimated 230 million pounds of copper annually. The pit would sit high on the east side of the Santa Rita ridge. During the twenty plus years of active mining any water draining into the pit would be pumped out to facilitate operations. After mining ceases, the pit would gradually fill with water, fed both by precipitation and by groundwater infiltration.

Hydrologists who have focused on this project agree that the pit will essentially suck water from the surrounding aquifers, creating a “cone of depression” in the groundwater levels. This cone will grow for several hundred years until the flows in and out of the pit stabilize. Key environmental questions are how much lower will groundwater levels be by the time a new equilibrium is reached, and what impact will the lowered groundwater levels have on surface waters.

The Santa Rita Mountains viewed from Las Cienegas grasslands (Rick Bowman)

The Cienega Basin runs northward between several sky island mountains, the Santa Ritas and Empires on the west and the Whetstones and the Mustangs on the east. The valley floor, just above four thousand feet, hosts its two central and unusual features: Cienega Creek, about thirty miles of narrow riparian corridor, of which about thirteen are perennial; and rolling sacaton grassland, the surviving several thousand acres spreading out from the creek toward the foothills.

This valley is the heart of Las Cienegas National Conservation Area, home to nearly three hundred native species of mammals, birds, reptiles, amphibians, fish, and insects, including several threatened or endangered (the Gila Chub and the Gila Topminnow; the Chiricahua Leopard Frog; the Southwestern Willow Flycatcher; the Yellow-billed Cuckoo). Portions of the creek have been designated as Outstanding Arizona Water (Tier 3) by the Arizona Department of Environmental Quality, granting this resource the highest level of protection against degradation. But it’s not pristine wilderness. Barbed-wire fences divide the grass into manageable pastures, rough ranch trails wind across the foothills and the washes between them, stock tanks are scattered over the 45,000 acres, now owned and administered by the Bureau of Land Management (BLM). Before the arrival of settlers, miners and ranchers from Mexico and the US, there were thousands more acres of grassland in the valley. Subsequently, in the early days of Empire Ranch beginning about 1880, the valley was often filled with cattle, sometimes up to 40,000 head at once. Both grass and water resources suffered. Nowadays a few thousand cattle are permitted, whose grazing is timed and rotated to preserve what remains. (A close look at the full-screen version of the accompanying grassland photo – accessed by clicking on the image – will reveal one white face visible in the center. It is a Hereford cow staring above the grass at the photographer.)

USGS Streamgage on Upper Cienega Creek (Rick Bowman)

[+]USGS Gage 09484550…

Everything in Cienega Basin depends on the water – the insects, the fish and frogs, trees and shrubs and grass, the hawk and the cuckoo and the pronghorn…as well as the cattle, and the humans. How much water is there? One measure is the streamgage of the US Geological Survey which sits on a perennial section of the creek just north of Sanford Canyon, and reports its quarter-hour measurements via satellite.

Dos Aguas hiked north along the bank to the site of the gage, a metal weir wedged into a rock passage and reinforced with cement. The water splashed steadily over the weir, throwing occasional droplets onto the nearby ledges and slowly melting the patches of ice formed overnight. An insulated wire snaked up the hill from the channel to a metal box and satellite dish in the trees at the top of the rise:
9am – 1.1 cubic feet/second (cfs). 10am – 1.1 cfs. 11am – 1.2 cfs. 12pm – 1.1 cfs.

Models and Opinions

[+]Formal opposition…

What influence would the proposed mine have on the flow in Cienega Creek? To predict the future, three separate groundwater models of the Rosemont area have been created, and subjected to peer review, by hydrologists contracted by various stakeholders. Each model projects the impacts of the mine on area waters over approximately 150,000 acres and up to 1,000 years into the future. The models vary in their specific predictions, however a few patterns are common to all three:
*Groundwater will be lowered significantly. All of the models predict a major decrease in groundwater level. The question is over how large an area. The 150,000 acre analysis area is defined approximately by the extent of a drawdown of at least five feet.
*Groundwater knowledge is too sketchy for accurate predictions. All stakeholders and participants agree that no one has enough information about the hydrogeology of the region to make accurate estimates of groundwater drawdown.
*The closer to the mine, the higher the impact. Empire Gulch, for example, just below the Empire Ranch headquarters, will experience greater impacts from groundwater drawdown, and earlier in time, than the slightly more distant Cienega Creek.
*The probabilities of days of low streamflow and days with no water are significantly increased. At Empire Gulch, one model predicts at 150 years in the future the count of such days per year to go from the current seven to 315. At 1000 years, all three models predict Empire Gulch will be “ephemeral” (i.e., flow only in response to storms). Under those conditions the pools, the fish, and the amphibians will be gone, the cottonwoods and willows will disappear, the wildlife dependent on riparian habitat will move elsewhere. However, even with a lower flow, the models predict the environment at Cienega Creek will survive. Not everyone else agrees, but until now no one has had solid evidence to make the assumptions of the models more accurate.

Of course, any negative effect of this mining project on water resources is added to the pressure already exerted by drought (significant in recent decades) and by increased use of basin water for other purposes (up to now, slight to moderate).

Just steps from the noisy rush of University of Arizona students on Tucson’s Second Avenue, the inside of the JW Harshbarger building is an oasis of quiet. Dos Aguas followed narrow hallways with worn stone floors up to Room 318 and its vestibule, an airy, empty space. From an open door in the corner of the vestibule, 318B, came a warm light. Inside Dr. Larry Winter, Professor in Hydrology & Atmospheric Sciences, was seated at a wooden desk under an incandescent lamp, an Apple laptop in front and wooden bookshelves in back. He has a ruddy face, and a strong handshake.

“I’m not looking for a fight,” he said when asked about the groundwater models done for the Rosemont Project. “They are good hydrologists. Some of them were trained in this department, and we’re proud of them… They’re correct to claim their models are standard hydrological practice…but they [the models] are wrong. I use this example in my groundwater class to show how not to do things.

“They’re wrong because they oversimplify,” he continued. “They all assume uniformity of conductivity in the aquifer, or uniformity of transmissivity, which is almost the same thing.” He looked at me expectantly, as if waiting for a question, or a contradiction. “Their own data show that conductivity is not uniform.” This lack of uniformity means, he explained, that the water is traveling through fractures in the material rather than through pores, it means there is uncertainty in the predictions, which everyone has acknowledged but no one has estimated. “We know how to do that,” he added. “To estimate the extent of the uncertainty, and its probability, would have added some cost, but probably not even up to double what they spent.”

New Data

[+]Tracing of water sources…

Room 322B is just down the hall from Larry Winter’s office. Dr. Jennifer McIntosh greeted Dos Aguas there with a warm smile. She’s familiar with the groundwater basins of the desert southwest, and is among the authors of a recent major study of the San Pedro Basin, one mountain range east of Cienega Creek. Jennifer now has a graduate student, Rachel Tucci, working on water isotope studies in the Cienega Basin, and the initial results are unexpected: unlike the San Pedro Basin (to the east) and the Tucson Basin (to the west), the springs, alluvial aquifers, and small but steady perennial flows in the Cienega Basin are consistently dominated by old water. How old? It’s early in the study, the data are not all analyzed yet, so Jennifer hedges any conclusions. It’s “groundwater…recharged prior to the 1950s,” she has written (since most all samples have no detectable levels of tritium). But in person she emphasized that the water is really old. “…it’s on the order of tens to hundreds of years old, or hundreds to thousands,” she told Dos Aguas, “with almost no input from local precipitation.”

There are many questions yet to answer. How high in the mountains was this old water originally recharged? How long is the transit time from recharge to discharge? And where is the water from local precipitation ending up? Only then do the next set of challenges begin: How does this new knowledge affect the predictions of the models? What policy decisions make sense in light of any revised predictions? The investigation of Cienega Creek’s old water is just beginning, but the sleuths are persistent.


For an abbreviated version of this story, see National Geographic Water Currents.

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