Mollusks, levies, and earthquakes: A map of Mississippi’s aquifers answers unexpected questions

One of the largest groundwater surveys in the USA is also answering unexpected questions about infrastructure, habitats, and some yet to be asked.

Below the flowering crops in one of the most intensive agricultural regions in the United States is a valuable hidden resource: water. The future of food and economy here depends on managing it properly.

The United States Geological Survey (USGS) has undertaken an ambitious project — the largest airborne survey in the continental US for water resources. They want to see all the connections between water, from the rivers above to the aquifers below ground.

The team is mapping the Mississippi Alluvial Plain aquifer system, spanning Southern Missouri down to the coast in the Mississippi valley, using geophysical surveys to reveal the geology underground.

Specifically, they’re focused in on the shallow Alluvial Aquifer that feeds the lucrative farmland above.

“The groundwater in this region is supporting a $12 billion agricultural economy,” says James Rigby (JR), a research hydrologist at the USGS.

“So, this was a relatively minor investment in dollar amounts for a major resource that keeps an entire industry going.”

The massive subsurface map they’ve built has helped them understand the Alluvial Aquifer, but is having a far wider impact.

Their data is being used to answer questions they couldn’t have predicted and is revealing connections between seemingly unrelated fields of study.

Drinking water flows through the taps of local communities around Mississippi, so much was already known about the larger drinking water aquifers deep underground.

Yet, the shallower groundwater feeding the thriving agricultural industry had been largely overlooked.

“It’s been a pretty well-kept secret that the Mississippi Alluvial Plain is served by the Alluvial Aquifer and pumping about as much on water annually as the California Central Valley,” says JR.

“It does get a lot of rainfall and recharge but since it’s a thinner aquifer we have similar ground water decline issues.”

The Mississippi Alluvial Aquifer is very thin. On average, it’s only around 30 meters wide.

Understanding the human impact on this aquifer is key to supporting farming and food security. But the USGS didn’t stop there. They wanted to see how all the waters connected, from the surface to the deeper drinking water below.

“We put a lot of effort and energy into understanding the streams. We did a bunch of waterborne geophysics along several hundred kilometers of the streams before we even started the airborne,” explains Burke J. Minsley, research geophysicist at the USGS.

“We’ve collected a few thousand kilometers of data along some of the smaller streams and rivers in the network as well as the Mississippi and Arkansas rivers themselves for the sole purpose of helping to inform the connectivity between surface water and ground water.”

Where did all the waters meet? By using geophysics to see the geology between the underground water bodies, the team could make more accurate assumptions about where they were likely to connect.

“From the airborne, we couldn’t quantify the stream bed hydraulic connectivity. But what we could do is look at the material properties directly under the stream bed and see whether they looked like they would have higher connectivity or not,” explains JR.

A map of the most likely intersections arose, unveiling a network of links between aquifers and surface water.

“We were able to define several metrics that measure the connectivity between the shallow and deeper system, hoping to guide the groundwater models on understanding where those connections are happening,” says Burke.

Knowing what’s beneath our feet is not just useful for finding aquifers. The USGS team gets information requests for their subsurface data from some unexpected places.

“We get everything from communities that are looking to put in a landfill and they want to look at the airborne data and see what would be a good spot in their vicinity, to the Department of Environmental Quality or the Arkansas Geological Survey that they want to update their geologic framework of the region,” says JR.

The USGS has built a massive map of the subsurface in the area. It includes a variety of geophysics data – from EM, to radiometric, to magnetic– as well as historic geological and drilling data that others can use to make their own discoveries.

“We’ve mapped roughly 50,000 square miles using airborne EM [electromagnetics]. If you extend that out to the larger entire region and adjacent aquifer systems that we partner with – it’s over a hundred thousand square miles,” says Burke.

“The USGS has been doing airborne EM surveys for groundwater studies since the 80s, but this has more than doubled the amount of airborne EM data that we’ve collected in the previous 40 years just in the last four years.”

That huge amount of data keeps growing, and with it the number of new questions it can address.

“The Tempest system is flying again right now, extending coverage down to the Gulf coast where we’ll see more about salinity and expanding the footprint to the east and west in the recharge areas – so we continue to collect a lot more data,” says Burke.

“We also knew that there were going to be more questions that would come out of this data set in the future, but we didn’t know what those questions were at the time,” says Burke.

The Army Corps of Engineers even asked for their assistance to understand the levy system around the Mississippi River.

“I think what we’ve shown is that there are a lot of uses of airborne that we couldn’t have imagined to begin with until we had the whole survey just start working with,” says JR.

“For a habitat study, they noticed that there was a strong correlation between a high electrical resistivity portions of the riverbed and where they were seeing good habitat for mussels.”

Shellfish populations are even benefitting from the aquifer survey, but it doesn’t stop there.

“There’s also the hazards angle,” says Burke.

“We’re sitting inside the New Madrid seismic zone. There’s lots of interests in faults and historical seismicity in the region. And these data bring a lot of value to those studies as well.”

Besides flying the largest airborne study in the continental US for groundwater, the team sees this map and data being used beyond their application.

They want their project data to serve as a decision-making tool that others can use for their own studies.

“In addition to the airborne survey itself, we’re creating robust datasets that can address more problems than just one specific research question,” says JR.

“The more of the data that’s out there, I think the more applications people will figure out.”