A recent analysis by The New York Times of 84,544 monitoring wells across the nation revealed a developing crisis of over-pumping of groundwater resources. Left unchecked, this drawdown will eventually threaten our standing as a world food superpower.
To better understand what is going on with groundwater in southeastern Arizona, Dr. Jennifer C. McIntosh, a Distinguished Scholar and Professor of Hydrology and Atmospheric Sciences and a Joint Professor of Geosciences at the University of Arizona, is currently conducting a survey of private wells in the Sonoita-Elgin area. The study, funded by a grant through the Desert Botanical Gardens, is led by Dr. Andrew Salywon, a plant biologist.
“Groundwater is essential not only for human life, but also for unique habitats, like streams and wetlands, that support a great diversity of both plants and animals,” Dr. Salywon said. “We are interested in establishing a baseline for the Sonoita-Elgin area so that we can infer/predict how groundwater resources might change with increasing groundwater pumping, drought and or heat extremes. This insight will help guide the best ways to use the groundwater resources in a sustainable manner.”
Dr. McIntosh answered further questions about the study and general water issues in the region. The following has been edited for brevity and clarity.
One of the things that you are doing in your survey is testing for the age of the water. How do you determine the age of water?
It is done with radioisotopes. We use two tracers. One dates what we call ‘modern water,’ the water that was recharged since the 1950s when tritium was released into the atmosphere during nuclear weapons testing. Tritium is not harmful in groundwater but we can use it as a tracer of modern water and that’s important because it tells us that water is being actively recharged, at least within the last 60 to 80 years. It also tells us which groundwater is more vulnerable to contamination from the surface because there is more of a connection. We use radiocarbon to date much older groundwater, which is very common in southern Arizona. Most groundwater is beyond the limits of tritium dating. We turn to Carbon-14 which dates groundwater between 500 years and 40,000 years old.
If water is determined to be 500 years old or a thousand years old, is there an implied recharge rate?
Yes, if the water is young that means it was recharged more recently and we can determine the recharge rate. The problem is that we, in many locations, are pumping the groundwater out faster than it would be naturally replenished. Knowing the age of the water doesn’t help so much in terms of determining how vulnerable your well would be to draw down. We use groundwater age to better understand the system—for example, if we find that most of the groundwater is fossil, and it is thousands of years old, then that tells us something about how groundwater is moving underground. Then if we see that old groundwater emerging into a wetland, it tells us how far that water has traveled, and what might be sustaining this wetland. Is it a local recharge? Is it something that comes from much further away?
Will two wells, in the same neighborhood, drilled to different depths, produce water of different ages due to being drawn from different levels of the aquifer?
Possibly. In general, groundwater age increases with depth, but what we find throughout Southern Arizona is that most of our recharge happens in the mountains. So, for example, groundwater downgradient from the Santa Rita Mountains is recharged from the mountains. If your well is shallow and your neighbor’s is deeper, both of these groundwaters are probably old and probably came from the mountains. There is not a lot of local recharge in the center of valleys in general. This is our first study in the Sonoita/Elgin area, so this is one of our first questions: Where is recharge occurring? Where is groundwater flowing and, ultimately, where is it discharging, and is it supporting these wetlands?
Water mixes. How can you differentiate between a source of water that is one age and newer water that is penetrating the surface and blending with the older water?
That is something we purposely look for, because we know that groundwater mixes in the subsurface and that, also, wells allow for mixing. Your screen on your well might be ten or a hundred feet deep, and so you may have groundwater of a younger age and older groundwater of an older age and that is what we would expect to find. That is why we do at least two different age tracers.
South32 wants to pump up to 4,500 gallons of water per minute for the life of its Hermosa mine, estimated at 60 years. Is there even that much in the aquifer?
That’s beyond my area of expertise. I am on the water chemistry, and water quality, side. I did have a recent Master’s student, Sean Schrag-Toso, who published his master’s thesis on the hydrology of the Patagonia Mountains. Using isotopes and chemistry we were able to say some things about the hydrologic connection between the mountains, where they are going to be mining, and the Sonoita Creek aquifer system.
There is a big fault right in front of the Patagonia Mountains that separates the mountains from the creek and we found that the groundwater in the basin was much older than the groundwater in the mountains. That tells us that the mountains are not feeding the aquifer directly. Rather, we found the younger, more recent recharge waters traveling down the washes to meet Sonoita Creek.
Does groundwater pumping in Sulphur Springs Valley deplete water from the Cienega Creek Valley?
No. They are completely separate groundwater basins. They are separated by two mountain ranges and the San Pedro basin.
To learn more about your own well’s history, consult the Wells 55 website at azwatermaps.azwater.gov/wellreg