The Mississippi Alluvial Plain runs from southeast Missouri through Arkansas, west Tennessee, Mississippi and Louisiana. For decades the shallow aquifer beneath the MAP has provided irrigation water for millions of acres of corn, cotton, rice and soybeans.

But declining water levels and increasing energy costs are leading scientists, such as Dr. J.R. Rigby, a team lead and research hydrologist with the U.S. Geological Survey, to take a closer look at just how plentiful and cheap water from the aquifer may be in the future.

Rigby, who spoke at this year’s virtual Arkansas Soil and Water Education Conference, discussed how scientists with the USGS and USDA’s Agricultural Research Service are using new digital tools to explore the Mississippi Alluvial Plain and its aquifer in a much more comprehensive and much less expensive manner than in the past.

“One data set that we think is a long-term investment for the region is airborne electromagnetic surveys to map the entire aquifer system from Missouri to Louisiana,” he said. “This is the largest airborne electromagnetic effort for water resources mapping in the continental United States to date with a total planned flight line distance of 50,000-line kilometers.”

Three phases

Rigby said the AEM efforts are being flown in three phases that began in February of 2018 and will culminate this spring when the USGS will fly another 5,000 line kilometers of resolve surveys. They will follow that up with the final component of the survey.

“I think we're going to see that this data set is a game changer in terms of the robustness for applications in all kinds of areas.,” he noted. “It will have applications as broad as earthquake histories to the early geology of the region to its intended purpose – better mapping of water resources and flow pathways.”

AEM provides three data sets in one from the three instruments on board the aircraft. “The first gives you the relative concentrations of uranium, thorium and potassium in the upper 30 centimeters or the upper one foot of the soil,” he said, referring to a slide comparing the data sets.

“This is just giving us the mineralogy of the soils at the surface, and it corresponds very closely to existing soil surveys from USDA’s Natural Resources Conservation Service. In the middle graphic, you see the electromagnetic resistivity. It gives us a three-dimensional picture of the aquifer down to about 200 meters below the surface.”

The third graphic on the far right of the slide provides the magnetic data as measured by a magnetometer on board the aircraft. “It is responding to changes in the rock density very deep below the surface,” he noted. “This is on the order of kilometers below the surface and shows some of the geometry of the basement rock.

Applications

“As I mentioned, there are a wide variety of applications. One of them is mapping the base of the aquifer, and we've done some preliminary comparisons comparing the airborne data with existing information on the aquifer derived from boreholes in the left figure.” (The boreholes were drilled over decades at a cost of between $200 million and $300 million.)

The two data sets don't look very different at first glance, he said. “But if you take a difference map, just subtracting the elevation of the base between those two surfaces, you get the figure on the right where the dark reds represent an elevation difference of 20 meters or more, and the dark blues represent a positive difference of 20 meters or more.

“Throughout the aquifer the average thickness is about 35 meters. Anywhere you see differences on the order of 20 meters, we’re over 50 percent different in the total potential saturated thickness of the aquifer between those two estimates. So the airborne has the potential to revise our understanding of the geometry of the aquifer substantially.”:

To watch the entire presentation, visit www.arkswec.com and go to the 2021 Conference heading.