In 1931, the Texas historian Walter Prescott Webb called windmills “the life-savers of the Plains,” one of three technologies – along with the six-shooter and barbed wire – that allowed white settlement of the arid West. Today, access to underground water is as vital here as it was for the first ranchers. From Amarillo to Alpine, aquifers are a prime source for drinking water, agriculture and industry.
Yet groundwater is, by its very nature, mysterious. How do scientists understand this hidden but essential resource? A new study in our region sheds light on the subject.
Bounded by the Guadalupe Mountains on the east, the Salt Basin stretches from New Mexico's Sacramento Mountains south nearly to Van Horn – encompassing gypsum dunes, volcanic hills and desert grasslands. The basin also contains a prolific aquifer. Since the 1950s, farmers in Dell City, Texas have irrigated from that aquifer – pumping more than 60,000 acre-feet annually. By comparison, that's more than 10 times the water used in Brewster County – for all purposes – each year. It exceeds the annual water use in Midland County.
Water is always political here. In recent years, the City of El Paso has acquired extensive holdings in Dell City, with plans to import Salt Basin groundwater for municipal use. New Mexico has a stake in the resource, and there are questions about how new petroleum development could impact water quality.
Amidst that charged context, a group of researchers is doing the hard science. The New Mexico Bureau of Geology, the U.S. Geological Survey and other agencies are seeking insight into basic questions – including the source, or origin, of the desert basin's abundant groundwater.
Dr. Talon Newton is with the New Mexico Bureau of Geology.
“What we think is that it is fairly discreet,” Newton said, “and it could be defined by elevation, basically. You could identify an area in the Sacramento Mountains where you see a lot of perched aquifers, that are very connected to the mountain streams there. And that really seems to be the recharge zone.”
With students and colleagues, Newton's analyzing water from Salt Basin wells – looking at trace metals and isotopes, even at the hydrogen and oxygen that comprise the water molecules. Each water sample, Newton said, has a unique chemical signature or “fingerprint” – revealing its age, and its source.
Listeners familiar with the Texas Hill Country – “the Land of 10,000 Springs” – may have intuited a hydrogeological fact. That region is one of spring-fed pools and rivers – and of limestone landscapes. Limestone erodes into fissures and caves, providing routes through which underground water can flow. This same limestone, or “karstic,” phenomenon is at work in the Salt Basin.
Some 60 miles north of Dell City, the Sacramento Mountains rise to nearly 10,000 feet. And the range is comprised almost entirely of limestone.
“The Sacramento Mountains – they stand out in so many ways,” Newton said. “There are springs everywhere. There's lots of wildlife. Even if you look on Google Earth, they stand out as this island of green in the middle of the desert, and a lot of that has to do with the geology, with the karstic aquifer system that exists up there.”
In the heights of the Sacramentos, amidst pines, firs and aspens, a geologic strata called the Yeso Formation is exposed. This formation, scientists say, is the site of recharge for Salt Basin groundwater. Here, snowmelt in mountain streams enters the subsurface. Then it flows through buried limestone strata – with names like the Victorio Peak and Bone Springs formations – on a long journey beneath mountain, mesa and desert.
The water Dell City farmers use today to grow alfalfa and chiles began as Sacramento snowmelt more than 10,000 years ago.
A famous 1904 Texas Supreme Court decision concluded that the “movement and course” of underground water are “so secret, occult and concealed” as to defy comprehension. But, today, scientists have techniques for mapping those hidden movements.
Dr. Shari Kelley is leading the “geophysical” aspect of the Salt Basin project.
“This is actually the first publicly funded geophysical survey of this area,” Kelley said. “Because it's not been done before, we're not exactly sure what we're going to find.”
Kelley and her team are using a technique called audio-magnetotellurics to “sound” the Earth's depths.
The technique relies on what's called the solar wind. Our planet is constantly being bombarded by a stream of charged particles from the Sun. These electromagnetic waves interact with the Earth, generating magnetic fields and electrical currents in rock strata. Some strata conduct electricity well – while other rock types are more resistant. These variations are the key to buried secrets.
“So if you've got limestone full of freshwater, it's going to be very resistant,” Kelley said. “If you've got clays that are full of salty water, they're going to be very conductive. And if you have limestone that has some salty water in it, it will come out in the middle. We're going to try to use these signals to map out the geology in the subsurface.”
At locations across the Salt Basin, Kelley's team will spread copper wires and antennae across the ground – to capture electromagnetic activity below the surface. With that data, and high-powered computing, they can generate three-dimensional models of the subsurface – and identify the “flow path” of groundwater through limestone.
Water-bearing limestone in the Salt Basin was created some 270 million years ago, in the same Permian sea that produced our region's petroleum resources. But the Salt Basin has been through a lot, geologically speaking. Ruddy hills and outcrops testify to volcanic activity. Rock strata were bent and broken by the tectonic forces that produced the Rocky Mountains, and the Rio Grande Rift. More than 300 million years ago, the fusing of the planet's land masses to form the Pangaea supercontinent raised high mountains here, which geologists call the “Ancestral Rockies.”
The faults and fractures from all these events dictate the flow of groundwater.
Underground water has always captivated humankind. Springs and seeps are the sites of ancient shrines. The practice of dowsing, or “water-witching,” is a venerable Western tradition. Even for scientists who dismiss such divination, groundwater remains the least-understood aspect of the water cycle.
Yet as the Salt Basin research shows, the study of this resource is advancing. In an arid region like ours, it's a matter of survival.
