Groundwater Research at GSL
by Troy Gilmore, Groundwater Hydrologist
The beauty of the Sandhills includes meandering streams, wet meadows, and flowing springs, which all have a connection with groundwater. Groundwater is so close to the land surface in much of the Sandhills that it easily seeps out where we can see it. Groundwater in wet meadows, wetlands, lakes, and streams affect everyday life and alter plans for ranch operations. This leads to important questions about how water moves through the Sandhills, both on the land surface and in the deep aquifer below.
GSL staff has measured groundwater levels since the late 1990s, giving us an understanding of how groundwater levels change seasonally under dunes and valleys. Figure 1 shows these variations based on sensor measurements (courtesy of Dr. Dave Gosselin) complementary to the many manual measurements made by ranch staff. We can see a quick and strong response to precipitation in the shallow groundwater just below the wet meadow, while the groundwater variation below the dunes is much more subdued. There is also a rise in groundwater levels in the valley during winter months as deep-rooted plants in the wet meadow use less groundwater. A slight rise under the dunes is seen during winter, but it is subtle and the peak groundwater level seems to occur a couple months after the valleys.
Groundwater levels help us visualize how groundwater gradually flows beneath the hills and wet meadows before seeping out into streams. Over the last few years, we have been working on projects to help us understand where and how much groundwater seeps into streams and how long it takes to get there.
We’ve probably all stepped into a creek in the summer and felt how cool the streambed is compared to the stream water. This temperature can vary within the streambed. Cooler streambed zones are likely to have more groundwater seeping into the stream over time. In June 2017 we measured streambed temperature over several days (Figure 2). To measure temperature, we used a special fiber-optic cable placed on the streambed. We also made physical measurements and confirmed that more groundwater did tend to enter the stream in cooler areas. Results of this study are freely available at https://www.mdpi.com/2073-4441/11/11/2312.
Building on the past decades of GSL water research, we were awarded a National Science Foundation (NSF) grant in 2018. We collected groundwater samples from over 100 locations in the Middle Loup streambed between GSL and Seneca, NE in 2019. These samples are being analyzed to determine the “age” of groundwater that seeps into Sandhills streams. Results of this study will help us understand how groundwater in the Sandhills is replenished, and how long it moves in the aquifer before it seeps out into the streams. This work will benefit Sandhills ranchers through improved understanding of flooding events and of water availability during dry periods. We are grateful to the GSL staff, neighbors, and other UNL scientists for accommodating our work. Thus far, seven graduate students and two undergraduates have had the opportunity to work in the Sandhills. Four graduate students are working full time on the NSF project. We hope to continue sharing results as they develop.
Figure 1. Groundwater levels in the wet meadow and under a dune in the eastern part of GSL. Precipitation is also shown. The largest precipitation event of about 90 mm is equivalent to 3.5 inches of rain. Most events are less than one inch (25 mm) of rain. Note the quick response to precipitation in the wet valley (wet meadow).
Figure 2. Streambed temperature showing cool zones on the streambed. The inset photo shows the fiber-optic cable on the streambed. Trucks in the image provide a sense of scale. Drone imagery courtesy of Dr. Jesse Korus (Conservation and Survey Division).