Groundwater in many areas of the U.S. and across the globe is being depleted as pumping for drinking water and agriculture outpace nature's ability to refill aquifers. In the most severe cases this could lead to a point of no return, where an aquifer's physical structure collapses and it cannot be refilled.
But a new method of mapping aquifers could provide a better tool for managing them.
In a recent study published in Geophysical Research Letters , researchers at The University of Texas at Austin used remote sensing data and statistical methods to map a municipal aquifer in Nevada. Their goal is to create a lower-cost method to measure an aquifer's permeability, how easily water flows through rock, that could aid groundwater management.
"We think this is a really promising technique for aquifer characterization and management that could be tremendously useful and essentially free," said co-author Marc Hesse, a professor in the Department of Earth and Planetary Sciences at UT's Jackson School of Geosciences and affiliated faculty member at UT's Oden Institute for Computational Engineering and Sciences.
When water is pumped out of an aquifer it changes the pressure underground, which can cause the ground overlaying an aquifer to sink subtly, a few millimeters to a few centimeters over months. The study's lead author Amal Alghamdi, who conducted the research while earning a doctorate at the Oden Institute, said that this sinking contains information about the subsurface geology of the aquifer.
"The exciting question was how much we could work backward from that surface response to learn about what we cannot directly see underground," said Alghamdi, who is now an assistant professor at King Fahd University of Petroleum & Minerals.
Ground subsidence can be measured in different ways. In the study, the researchers used publicly available InSAR satellite data and a statistical method based on a Bayesian inference algorithm developed by co-author Professor Omar Ghattas' research group at the Oden Institute. They used the information to create a 3D model of how water is likely to flow and spread laterally in the aquifer. By using freely available satellite observations they mapped a range of possible permeability fields.
The Jackson School research builds on a study conducted by a different research group that calculated the overall hydraulic conductivity of the same Nevada aquifer using subsidence data from GPS monitors. While that research offered a single overall reading of the aquifer's hydraulic conductivity, a measure that's related to permeability, the spatial variations in the Jackson School's maps provide a more detailed view of how permeability changes in different places across the aquifer.
Understanding permeability gives water managers insight into how the water is flowing underground and can help them sustainably manage an aquifer.
These maps highlighted variations in permeability throughout the aquifer, including revealing a high permeability channel on the southern end of the aquifer and an area of low permeability that previous research has hypothesized as a geologic fault.
In addition, the research also compared permeability maps generated using GPS data from the earlier study, InSAR data, and a combination of the two. They found that high resolution data from a single InSAR map provided more detailed and accurate information than multiple GPS readings over time, at least at this location.
Last year, NASA launched the new NISAR satellite with the Indian Space Research Organization to monitor Earth's land and water processes, including vegetation change, sea level variation and groundwater movement. Co-author Jingyi "Ann" Chen, associate professor at UT's Department of Aerospace Engineering and Engineering Mechanics, said that this data can be used to track subsidence due to aquifer pumping and make inferences about permeability and potentially other characteristics of the ground below.
"It's exciting because that NISAR satellite operates at a short 12-day repeat cycle and longer wavelengths that can peer through plant cover," Chen said. "This allows us to retrieve deformation measurements with greatly improved quality over agricultural basins that rely heavily on groundwater pumping."
