Scientists studying the groundwater reserves in the Mississippi River Valley alluvial aquifer have often wished they could peer into the ground and determine how much water was available for irrigation and other uses.
They still can’t do that, but they now have tools that are helping provide a better picture of the hydrogeologic framework under the Delta region and more clues about how much water is available.
“One of the frustrating things about working in groundwater, whether you’re doing research or trying to manage it, is that you can’t see it,” said James R, Rigby, a research hydrologist with the USDA Agricultural Research Service’s National Sedimentation Laboratory in Oxford, Miss.
“The extent of most of our knowledge about groundwater has been you put a straw in the ground and you pump and water comes out — or it doesn’t,” said Rigby, a speaker at the Arkansas Soil and Water Education Conference at Arkansas State University in Jonesboro.
In the last two decades, scientists have learned more about water availability in the region by gathering data from bore holes and using it to interpolate how much water might be available in the area surrounding the testing sites.
That information has been put into the U.S. Geological Survey’s Mississippi Alluvial Plain (MAP) Regional Water Availability Study, a joint project of the USGS and USDA Agricultural Research Service scientists.
“I’m with ARS,” said Rigby, “and we’ve been working closely with USGS on a couple of pilot projects, one in Mississippi, and we’re also looking at some inset modeling in Arkansas. But the regional study is this MAP study, and, specifically, we’re looking at an upgrade to the hydrogeologic framework.”
Rigby displayed a USGS map that he said “is one representation of the hydrogeologic framework from a 2009 study by Brian Clark. The alluvial aquifer is really this tan region up here at the surface, and that study wasn’t focused on the alluvial aquifer or the Sparta aquifer. It was trying to model all of the geological units through the Mississippi Embayment.”
Clark’s study was based on the information from a cross-section in Arkansas of a series of bore holes, some of them miles apart.
“Each of these orange and red columns is a bore hole that has been interpreted into sand and gravel fractions,” he said. “And this is about all you have to go on, and these come from lots of drillers with widely-differing recordkeeping techniques.”
Last year, researchers used a helicopter to tow a “torpedo-looking instrument” across the region. The instrument produced a much more detailed picture of the aquifer than before.
“This is a redistributing map for those of you who haven’t seen this before,” said Rigby. “Generally speaking, the reds and purples represent coarse materials like sand and gravel; the green and blues are finer material like silt and clay.”