Groundwater shapes the Sandhills in many ways, both visible and invisible. Not far below the surface—and at the surface in lakes and wet meadows between the dunes—the spaces between sand grains are filled with water instead of air. If you dig in these places, the hole will fill with water. This is called an aquifer: an underground unit of rock, gravel, or sand that stores water and allows it to flow beneath the land surface.
The Sandhills are part of the High Plains Aquifer, a large system of several interconnected layers of sediment that store and transmit water and that stretches from southern North Dakota to south of the Texas Panhandle. About two-thirds of the water in the High Plains Aquifer is in the Sandhills. The largest geologic unit of the High Plains Aquifer is the Ogallala Group, and the aquifer is often referred to as the Ogallala Aquifer. The Ogallala Group is made up of sediments that eroded from the Rocky Mountains and were carried by streams from present-day Colorado and Wyoming. As the mountains rose, gravel and sand were washed down the slopes to create an extensive blanket of sediment.
In most places, a large proportion of water from rain and snowmelt “runs off” (flows across the land surface to a stream channel) or evaporates from puddles. The rest of the water soaks into the ground and, as it trickles through the soil, is taken up by plant roots and transpired through leaves. Usually, only a tiny percentage of the water in soils makes it past the root zone and into the deeper unsaturated zone, where it eventually reaches the water table and becomes what is known as groundwater recharge. In the Sandhills, this precipitation partitioning is very different than most regions. The Sandhills is like a giant sponge. When rain falls, almost all the water quickly sinks into the sandy soil rather than running off into surface depressions or streams. Although other parts of the High Plains commonly have more rain and snow than the Sandhills, the Sandhills receive by far the greatest proportion of precipitation flowing to the aquifer, because water moves through the coarse sandy sediments so quickly. This abundant infiltration keeps the water table shallow, close to the surface of the landscape.
Groundwater flows down a slope, just as surface water flows downhill. When the water table intersects the land surface, a natural spring forms. In the Sandhills, this often takes the form of wet meadows in low spots between dunes. Wet meadows are a common, visible, and ecologically sensitive consequence of the water table’s proximity to the land surface. Like wet meadows, many regional streams and rivers of central Nebraska also are fed by cool groundwater from the Sandhills. These streams act like giant drains, preventing groundwater from accumulating and turning the region into a single vast wetland. Occasionally the sandy soils cannot absorb all the water from a rainstorm or blizzard, and large floods occur. When this happens, the Sandhills can stay flooded for months at a time as the water slowly evaporates or drains to streams.
In addition to having a high water table, the Sandhills sediment extends deep below the surface. Most aquifers have a base of less-permeable rock that serves as a barrier to deeper water infiltration, and the Sandhills is no exception. But while other areas of the High Plains Aquifer have an impermeable layer up to a few hundred feet below the land surface, the base of the Sandhills aquifer is generally more than 500 feet below the surface. In some places, the water-bearing sediments are more than a thousand feet thick. This is another reason that the Sandhills store so much water. Much of this water is likely to be thousands of years old, but its age is hard to determine and varies with depth.
Sandhills groundwater flows in the same general direction as the rivers, from west to east, but much more slowly. According to measurements taken by the U.S. Geological Survey, the groundwater in the Sandhills may travel about 30 feet per day, much faster than groundwater in most places. This west to east flow is why declining groundwater levels to the south in Kansas and Texas have no effect on Sandhills water levels.
Groundwater withdrawals from the High Plains Aquifer are greater than groundwater withdrawals from any other aquifer in the United States. About 97 percent of the water in the High Plains Aquifer is used for agriculture, and the aquifer is known for its declining water levels due to irrigation of corn, soybeans, and cotton. Yet in contrast to other parts of the aquifer, the water level in the Sandhills has remained fairly stable since recordkeeping began. One likely reason is that the Sandhills is not a profitable place to grow crops because the sandy soil does not retain sufficient nutrients to support intensive agriculture.
The large quantity of water stored in the Sandhills has led to suggestions to pipe the water to other parts of the aquifer where water availability is declining because of excess withdrawals. Despite its huge water storage, the Sandhills is a fragile, groundwater-dependent ecosystem that would be damaged irreparably by significant withdrawals. For example, along the cliffs of the Niobrara River, there are many springs where groundwater meets the surface and flows into the stream channel. Other areas of the High Plains once had many of these springs that provided water for plants and animals, but most stopped flowing when the water table declined. If this happened in the Sandhills, rivers like the Loup, Dismal, Elkhorn, and even reaches of the Niobrara could stop flowing and become dry, and the wet meadows would disappear.
Maintaining Sandhills groundwater quality is a major environmental challenge. The close relationship between groundwater and surface water makes this area vulnerable to contamination. Fertilizers, such as nitrate, and pesticides, such as atrazine, move readily with water through the unsaturated zone. In other regions, it is estimated to take decades for a drop of water to travel from the land surface to the underlying aquifer. In the Sandhills, the surface water—and anything in the water—goes straight into the water table. When it reaches the aquifer, it may travel to a nearby stream in a matter of a few years. Once contaminated, groundwater is difficult to clean up. Despite this vulnerability, groundwater in the Sandhills remains cleaner than in other areas of the state with more intensive agricultural and urban development.
Next time you pass through the Sandhills, notice the wet meadows, the irrigation pivots, and the windmills, many of which have a small stock tank at their base. The windmills are used to transport water from the aquifer to the surface. This was the first technology used to pump water from the High Plains Aquifer, and these small wind-powered wells are still a common sight across the region. The windmills, pivots, and wet meadows are reminders of the ways in which groundwater has shaped this unique landscape over thousands of years and continues to do so today.