In many years, heavy snowfall in the Midwest results in prolonged periods of flooding in the Lower Mississippi River Valley. But how much of that water, if any, flows into the aquifer underlying the Mississippi Alluvial Plain or MAP?

Scientists with the USDA-ARS Watershed Physical Processes Research Unit in Oxford, Miss., and USGS are studying recharge and other aspects of the physical properties of the aquifer that provides irrigation water for millions of acres of crops in the Mid-South.

Those scientists have used airborne electro-magnetic or AEM measuring to develop a new index that allows them to estimate “whether we think there is high conductivity or low conductivity between the rivers and the aquifer,” said Dr. J.R. Rigby, team lead and research hydrologist with the U.S. Geological Service’s Water Resources Mission Area and USDA-ARS.

Rigby, a speaker at the virtual Arkansas Soil and Water Education Conference last winter, discussed the Mississippi Alluvial Plain project, which includes flying 50,000 “line kilometers” across the Delta Region to gather more information about the MAP’s aquifer. The project began the flights in February of 2018.

Streambed study

“One thing we didn’t have in mind when we first flew the airborne electro-magnetic or AEM project, but has subsequently been a very interesting venture, was that we flew some of the rivers in the region to see what the beds looked like in terms of streambed material,” he said.

“On the left, you can see a map of all the rivers that were included in the original MERAS regional groundwater study,” he said, referring to a slide. “It’s about 43 records, and there was no information on the bed material from those rivers to help guide the calibration of the groundwater model to understand how much water was moving from the river into the aquifer.”

In the initial portion of the project, the scientists had the helicopter that tows the instrument package fly along some of the rivers just to get a picture of the bed material.

“What we realized is with 50,000 kilometers of east-west flight lines, we can just take those lines and select the points that occurred over the rivers and use that to map them,” he said. “So the figure on the right has taken the NHD plus flow lines that exist as a USGS database and intersected those with our flight lines.

“It gives us a much denser network of streams with information about the bed material. So everywhere you see a stream mapped in that figure on the right, we also have information from the airborne data. The blues and the yellows represent an index that we developed to estimate whether we think there is high conductivity or low conductivity between the river and the aquifer. This is already light years ahead of where we could have been with the previous model.”

Rigby said the scientists have been conscious of the return on investment. In contrast to previous studies that involved drilling about 10,000 boreholes in different states and for different purposes at an estimated cost of $200 million to $300 million, the current AEM effort has run about $1 million.