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The Frugal Geophysicist: Airborne Geophysical Strategy Helps Solve Mystery


by Marc Airhart
June, 2008

Water in West Texas is, as the old saying goes, scarce as grass around a hog trough.

With speculators planning to sell rural West Texas water to thirsty cities, State water regulations that have gone largely unchanged for over a century, as well as the specter of more extreme weather courtesy of climate change, prospects don’t look much better for the future. And you can add one more threat to the list—salinity.

In 2000 the State of Texas released a report describing water quality along a stretch of the Colorado River northeast of San Angelo as “impaired” for general uses, including contact recreation, public water supply, fish consumption, and aquatic life. Specifically, this segment of the Colorado River fails to meet Texas Surface Water Quality Standards for levels of salt (chloride) and total dissolved solids (TDS).

Farmers and water suppliers in the region began to blame oil producers, oil producers blamed natural geological processes, and others blamed an invasive plant.

“Saline water doesn’t taste very good, and it isn’t very good for farming or some desirable aquatic species either,” says Dr. Jeffrey G. Paine, a research scientist at The University of Texas at Austin’s Bureau of Economic Geology. “The Colorado River provides drinking water for hundreds of thousands of people. So there’s a compelling reason to do what you can to protect that resource.”

Jeff Paine
Bureau scientists developed a faster, cheaper way to map saltwater pollution hot spots on the Colorado River in West Texas. Image: Jeff Paine (right) and Geophex geophysicist Dak Darbha with electromagnetic induction instrument.

With funding from the Environmental Protection Agency (EPA), the Texas Commission on Environmental Quality (TCEQ)—a public agency charged with protecting the state’s air and water—began an investigation into sources of the salinity and TDS.

Paine, who uses geophysical techniques to study environmental contamination, had an idea for a new strategy to measure water chemistry from the air quickly and economically. The mystery of the Colorado River’s elevated salinity presented the perfect opportunity to try it out.

Up, Up, and Away

Zeroing in on culprits in reduced water quality can be a difficult task. For one thing, water is slippery. It smears out across the landscape, making it hard to pin down its point of origin. For another, there are many natural and human sources of salts in the environment.
Airborne Survey  

One possible source is abandoned oil and gas wells. During drilling, salty water at depth is brought up to the surface. Before the practice was banned in Texas in the late 1960’s, brine was often dumped into shallow, unlined pits. Today the brine is typically reinjected deep underground or is stored in special tanks.

Natural geological processes might also be involved. The watershed has significant deposits of gypsum—a salt—at the surface. Runoff at the surface can carry salts into streams and rivers. Underground mineral deposits can also dissolve into flowing groundwater that eventually discharges to the surface.

Yet another possible source is salt cedar, an invasive plant that is spreading quickly across West Texas. The plant uses groundwater much more rapidly than native plants. As it transports slightly salty groundwater to its leaves, salts accumulate. When the leaves drop in autumn, they contribute to saltier surface water through runoff.

To save time on the ground, scientists often start with airborne surveys, using geophysical instruments that can measure thermal, electrical, or seismic properties of the soil and water to help narrow the search to a manageable number of smaller hot spots. This survey is typically done from a helicopter or fixed-wing aircraft flying back and forth in a large grid.

“Most airborne surveys are like mowing a yard,” says Paine. “You have parallel flight lines in a grid. In salinity studies, 95 percent of the area isn’t really germane to the groundwater or surface-water problem. You’re acquiring a lot of data you don’t need.”

That’s because the areas of interest—rivers and streams—are long and sinuous. Fixed-wing aircraft have a difficult time flying twisty, curvy paths. Paine hit on the idea of acquiring a geophysical log of the riverbed using helicopters, which can follow rivers and streams more easily than other aircraft, to collect only data of interest. He applied for and received a grant from TCEQ to use the new technique to locate sources of increased salinity on the Colorado, as well as a second waterway in South Texas.

Paine and his colleagues teamed up with an independent helicopter crew from Oregon to survey the Colorado using an instrument that measures electrical conductivity of soil to depths as great as 50 meters.

“So the helicopter flies along at low altitude over the river, towing a big white tube with fins,” says Paine. “It looks like a cruise missile.”

The 5-meter-long tube is actually an electromagnetic induction (EM) instrument. It creates a changing magnetic field, which induces electrical currents to flow in the soil. These, in turn, create a secondary magnetic field. A receiver in the instrument measures the induced field, the strength of which is proportional to the conductivity of the soil. And because conductivity rises with increased salinity of water in the soil, the magnetic field also provides a measure of salinity.

Over 2 days in February 2005 the crew surveyed 400 kilometers (250 miles) of the Colorado River and its major tributaries.

“Most airborne surveys are like mowing a yard,” said Paine, describing the standard grid approach. “You’re acquiring a lot of data you don’t need.” His innovation was to use a helicopter to follow the twists and turns of the river.  

Grounded

The airborne geophysical data showed that there were four regions of elevated conductivity along a 66-mile stretch of the Colorado River below E.V. Spence Reservoir. With the areas of interest whittled down to a manageable size, Paine and his colleagues at the Bureau, Seay Nance and Eddie Collins, took to the ground to search for specific sources of salinity.

Nance, who has worked as a geologist at the Bureau for over 25 years and is now finishing a doctorate in hydrogeology, took 2-liter samples of water at 18 spots along the impaired river.

“It was up to me to show chemically how the surface water was evolving along its flow and why it was evolving the way it was,” says Nance.

He adds that without the airborne geophysical work, the task would have been much more time consuming. And with a cost of about $200 to analyze each sample at an independent lab, the team had to be judicious.

  Study Area
  In 2000, the state of Texas declared a stretch of the Colorado River northeast of San Angelo as “impaired," meaning salts and total dissolved solids exceeded levels safe for swimming, fishing, drinking, and farming. (click image to enlarge)

“If we didn’t know anything at all, I’d have to go up and down the river who knows how far,” he explains. “It definitely focused our sampling.”

Chemical analysis showed that signatures of each “hotspot” differed. One area near an abandoned oil field had elevated levels of sodium and chloride. The team concluded that deep groundwater brought to the surface by oil and gas production was the most likely source. Ground and borehole EM induction measurements helped pinpoint specific point sources and groundwater pathways within this hotspot.

Not all increased conductivity along the evaluated part of the Colorado was due to byproducts of oil and gasproduction, however. Some came from elevated sulfate levels, which the team concluded came from natural dissolution of minerals such as gypsum. Some local, shallow accumulations can be attributed to salt cedar, but salt cedar serves only to concentrate salinity already in the system.

Survey results  

One of the significant threats to water quality below Spence Reservoir appeared to come from the Wendkirk oil field, which had been in operation for many years under numerous operators.

“Suffice it to say, it is relatively typical of many West Texas oil fields,” says Paine. That is, produced water was simply dumped at the surface in unlined pits. The practice was common and allowed by regulators until the late 1960’s. He adds, “With half of Texas oil and gas production occurring before then, there’s a lot of salty water out there from oil and gas production.

”Today, petroleum companies pay into funds to help clean up sites that have no responsible parties, such as Wendkirk. Without intervention, Nance estimates that it might take hundreds or even thousands of years for excess salts from oil and gas production in West Texas to be flushed naturally out of the system.“It’s a great cautionary tale that people need to be more careful developing oil and gas resources near water,” says Nance.

One of the significant threats to water quality on the Colorado came from the Wendkirk oil field. Electrical conductance measurements (in millisiemens per meter) indicated the highest levels of salinity in red. (click image to enlarge)

 

Future Work

The Railroad Commission of Texas is now looking at ways to remediate groundwater contamination at the Wendkirk site. One option might be to dig interceptor trenches and pump out saltwater to halt its migration to the river.

Paine and his colleagues applied the same airborne geophysical techniques to studying elevated salinity in Petronila Creek, a 44-mile-long freshwater stream southwest of Corpus Christi, Texas. As with the upper Colorado River study, oil and gas production was identified as the major source of salinity at Petronila Creek.

Paine says that his strategy for quicker, cheaper airborne geophysics could be effective in assessing and mitigating water contamination in other parts of Texas and around the world.

Jeff paine
After the airborne geophysical data indicated hot spots, Jeff Paine used a ground conductivity meter to study specific areas up close.

Several other major river systems in Texas are affected by natural and oil-field salinity, including the Canadian River in the Panhandle, the Red River in the north, the Pecos in the west, and the Brazos flowing from the north to the southeast. Paine says that he would like to survey them from the air but does not currently have the necessary funds.

In Australia, irrigation practices and climate change are increasing salinity of shallow soil and water, threatening food crops. Paine says that researchers there are testing his technique as a more focused, less-expensive way to monitor saline-water inflows into rivers.

With the kind of information that Paine and his colleagues gather, policy makers and stake holders can move beyond the blame game and start the long, difficult task of cleaning up impaired waterways.

   
 
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©2008 Bureau of Economic Geology, The University of Texas at Austin