By Troy Gilmore, Groundwater Hydrologist
Traveling west toward Thedford, Nebraska, on Route 2, a visitor to the Sandhills may notice smooth-flowing water and patches of bare white sand where the Middle Loup River cuts into dunes to the south. Within eight miles of passing Thedford, the traveler crosses the river three times before the channel disappears across the dunes to the north and threads toward Seneca. If the traveler had the far-fetched idea (and, importantly, per-mission) to start walking upstream from Seneca, following every meander of the Middle Loup River, the South Branch of the Middle Loup River, and the South Fork of the South Branch of the Middle Loup River, they would arrive at the Gudmundsen Sandhills Laboratory (GSL) after a 60-mile journey. As the crow flies, that trip would be about 30 miles.
The traveler would see and hear many things along this walk, perhaps noticing wildlife, cattle, sandburs, the sound of wind, and the quietness of the grassy dunes. What they may not notice, however, is the lifeblood of this region—the mostly invisible groundwater that sustains life here. Well known by locals, this groundwater is the source of steady Sandhills streams and the cool water that flows from windmill to stock tank. In fact, by the time the traveler reached GSL, they would be standing over nearly 1,000 feet of saturated sand, silt, and gravel that make up the thickest part of the High Plains Aquifer. Stopping by the GSL dormitory for a drink, the traveler would hold up a glass full of groundwater pumped from about 600 feet below. They might be sur-prised to learn groundwater at that depth has been slowly creeping through the aquifer for thousands of years before being pulled through the well screen and jettisoned to daylight.
Sampling and measuring groundwater seepage into the South Branch of the Middle Loup River on Gudmundsen Sandhills Laboratory in May 2019. Image courtesy of Mikaela Cherry.
Sitting in the shade of the dormitory porch, the traveler might look across the valley and notice the rich green color of the groundwater-fed hay meadow contrasted with the greenish-brown dunes that rely more on the intermittent rain and snow for moisture. Mentally retracing their footsteps to Seneca, the traveler would first see the small channel of the South Branch of the Middle Loup River, fed by groundwater skimming closer to the top of the aquifer as it makes its way to the stream in just years or decades. Groundwater seeping into the stream originated as rain or snow that fell within the lifetime of local wildlife, cattle, and people. Further downstream, near North Hecla Road, the aquifer begins to thin slightly, and the stream channel is cut more deeply into the landscape. Groundwater, moving generally from west to east in this region, takes a much long-er and slower route to this part of the South Branch. Many human generations have passed while this ground-water crept through the aquifer for, on average, about 1,700 years before arriving and seeping slowly up through the streambed and into the sandy stream. A few seeps and springs along the channel might deliver “younger” groundwater to this section of stream. Finally, having traveled in their mind all the way back to Seneca, the traveler could envision the wide, flat water surface that roughly matches the invisible water table elevation beneath the nearby dunes. Here, even older groundwater is pouring through the coarse sand and gravel riverbed, feeding the Middle Loup River. This groundwater seeping into the river at Seneca approaches the “age” of groundwater the traveler holds in their glass at the ranch, with an average travel time through the aquifer of about 5,000 years.
Surprisingly, the traveler’s long walk from Seneca to GSL was a little like taking a groundwater tour. Starting at Seneca, where old groundwater emerges from the aquifer depths, to GSL, where younger groundwater feeds ditches and streams, the traveler covered not only 60 miles but also a great expanse of time. They also walked past, waded through, and drank groundwater that represents thousands of years of history. If you are interested in a more detailed aquifer tour, please visit our Research Story Map at https://go.unl.edu/gwss
Acknowledgments
Thank you to GSL Ranch personnel John Nollette, Jacki Musgrave, Andy Applegarth, Roger Carpenter, and Stacy Nollette for supporting our research. A special thank you to landowners who allowed us to access stream sites outside of GSL.
Special thanks to all who participated in field work and analysis of datasets, including collaborators at the University of Utah and North Carolina State University. Collaborators include Kip Solomon, Dave Genereux, Aaron Mittelset, Vitaly Zlotnik, Caner Zeyrek, Craig Jensen, Marcus MacNamara, Mikaela Cherry, Wil Mace, Galen Richards, Jens Ammon, and Mason Johnson. We appreciate USGS personnel who measured stream-flow during the project.
This material is based upon work supported by the National Science Foundation under Grant No. EAR-1744719 (UNL). Addiontal support was provided through a Daugherty Water for Food graduate student fellowhip, and U.S. Department of Agriculture - National Institute of Food and Agriculture NEB-21-177 (Hatch Project 1015698). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
The South Branch of the Middle Loup River looking west as it flows through GSL. Photo taken July 1, 2022.