Old Water and New Knowledge at Cienega Creek

By Dos Aguas

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, and sometimes granted. The discussion continues, although the academic issues are on a slower timeline than the political.

The Cienega Basin

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

The Cienega Basin runs northward between several sky island mountains. 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, the Chiricahua Leopard Frog, and the Yellow-billed Cuckoo, among them). But it’s not pristine wilderness. Barbed-wire fences divide the grass into manageable pastures, rough ranch trails wind across the foothills, stock tanks are scattered over the 45,000 acres, now owned and administered by the Bureau of Land Management (BLM).

USGS Streamgage on Upper Cienega Creek (Rick Bowman)

Everything in Cienega Basin depends on the water – the insects, the fish and frogs, trees and shrubs and grass, the cuckoo, 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. The daily flow at this spot, draining about three hundred square miles, averages about 0.9 cubic feet per second (cfs), but there is much variation. The 2016 low was .09 cfs (in July, a hot summer day before the large monsoon rains), and the high was 194 cfs (in September, immediately following a monsoon downpour).

The Mine

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.

New Data

The groundwater models created for the Rosemont Project predict that even though groundwater levels will be lowered significantly, the environment at Cienega Creek will survive. Not everyone agrees, but until now no one has had solid evidence to make the assumptions of the models more accurate.

In her bright and sunny office Dr. Jennifer McIntosh greeted Dos Aguas with a warm smile and a strong handshake. 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. Their method of study uses isotopes of several elements (Oxygen, Hydrogen, Carbon, and Tritium among others), along with chemical tracers, to extract a “signature” from a water source, and then to track its movement through an aquifer. 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.

Dos Aguas hosts an online journal of water stories. For more stories, click here.

Changing Planet

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