The High Plains aquifer, part one

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By Jason Gurdak, Ph.D.

In the book “Ogallala: Water for a Dry Land,” John Opie writes that “The enormous Ogallala aquifer is groundwater trapped below 174,000 square miles of fertile but otherwise dry plains farmland. Unlike most of the world’s water supplies, Ogallala groundwater is largely nonrenewable because its sources were cut off thousands of years ago. It is essentially ‘fossil water’ that was generated ten thousand to twenty-five thousand years ago from the glacier-laden Rockies....However, more than three billion acre-feet (an acre-foot is a foot of water across one square acre, or 325,851 gallons) had been deposited under the plains.”

Opie was right.

The High Plains aquifer (known locally as the Ogallala aquifer) contains an enormous volume of fresh groundwater, making it one of the largest aquifers in the world. Use of that water for irrigation has transformed the High Plains into an important agricultural region that sustains more than one-fourth of the nation’s agricultural production, earning it the nickname “breadbasket of the world.”

Unfortunately, there has been a cost for irrigating the breadbasket.

Water levels in the aquifer have declined substantially since the mid-1950s because groundwater withdrawals have greatly exceeded recharge (groundwater replenishment) across much of the aquifer. The largest water-level declines range from 50 to more than 150 feet, primarily across parts of Kansas, Oklahoma, New Mexico and Texas. The saturated thickness of the aquifer has decreased by more than 50 percent in some parts of Kansas and Texas. This groundwater depletion has led to increased pumping costs and a reduction of water discharging to streams, among other things. Ecosystems along river corridors that rely on groundwater discharge are adversely affected by even small volume changes in the groundwater system.

The concern is that groundwater from the High Plains aquifer is largely a nonrenewable and limited resource. Declining water levels mean that there is less groundwater supply in the aquifer now than there was prior to the onset of agricultural activities. The current and future supply of groundwater is central to the overall health of the High Plains agricultural economy, viability of its cities and rural communities and environmental well-being of the landscape.

What Opie doesn’t mention is that groundwater quality (chemistry) can largely determine how groundwater can be used for irrigation, drinking and livestock. So groundwater quality (chemistry) is an equally important consideration as groundwater quantity (supply) for the High Plains.

The good news is that according to a recent U.S. Geological Survey (USGS) report, water produced by the High Plains aquifer is generally acceptable for human consumption, irrigation and livestock watering. However, some USGS findings indicate that the groundwater quality may not be suitable for those intended uses indefinitely.

The USGS study assessed water-quality conditions of the High Plains aquifer for the period 1999–2004 and provides the first comprehensive regional assessment of water quality in this nationally important aquifer. Results of this study show where, when, why and how specific water-quality conditions occur in groundwater across the High Plains aquifer and yield science-based implications for assessing and managing the quality of this water resource. Understanding groundwater quality conditions and the natural and human factors that control water quality in this aquifer is important because of the implications to human health, the sustainability of rural agricultural economies, and the substantial costs associated with land and water management, conservation and regulation.

USGS scientists analyzed water for more than 180 chemical compounds and physical properties in about 300 private

domestic wells, 70 public-supply wells, 50 irrigation wells and 160 shallow monitoring wells sampled between 1999 and 2004. The study also assessed the transport of water and contaminants from land surface to the water table and deeper zones used for supply, to predict changes in concentrations over time.

Major finding from the USGS study are summarized below.

The importance of groundwater

Groundwater provides much of the nation’s public and private (domestic) water supply, supports agricultural and industrial economies and contributes flow to rivers, lakes and wetlands. About 40 percent of the nation’s public water supply is from groundwater in regional multi-state aquifers such as the High Plains aquifer, and more than 40 million people, includ­ing most of the rural population, obtain drinking water from private wells.

The High Plains aquifer underlies about 174,000 square miles in parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas and Wyoming. Nebraska hosts the largest segment and square mileage of the water source. In 2000, the High Plains aquifer had an estimated 2,980 million acre-feet of drainable water in storage, making it one of the largest aquifers in the world. The High Plains aquifer is a system of six hydraulically connected hydrogeologic, or water bearing, units. The most extensive of these units is the Ogallala Formation, which makes up about three-fourths of the total High Plains aquifer—hence the local name Ogallala aquifer.

The depth to water below land surface ranges from 0 to approximately 500 feet, averages about 100 feet and is generally greatest in Kansas, Oklahoma, New Mexico and Texas. The saturated thickness of the High Plains aquifer ranges from less than 1 to more than 1,000 feet and averages about 200 feet. The saturated thickness varies geographically and is greatest in Nebraska.

Evaporation rates exceed precipitation rates across much of the High Plains, so little water is available to recharge (replenish) the aquifer. Recharge to the High Plains aquifer occurs by infiltration of irrigation water, diffuse infiltration from precipitation, focused infiltration of storm and irrigation water runoff through streambeds and other topographic depressions and upward movement of water from underlying aquifers. Discharge from the High Plains aquifer is primarily to irrigation well pumping, streams and underlying aquifers, groundwater flow across the eastern boundary of the aquifer and evapotranspiration (evaporation and transpiration from plants).

Use of groundwater from the High Plains aquifer as a source of irrigation water has transformed the High Plains into one of largest and most productive agricultural regions. The High Plains aquifer is the most intensively used aquifer in the United States for irrigation, public supply and self-supplied industry, producing almost two times more water than any other U.S. aquifer. Groundwater withdrawals from the High Plains aquifer in the year 2000 accounted for about 20 percent of total groundwater withdrawn in the United States. Most (97 percent) of the water withdrawn from the High Plains aquifer is used for irrigation. Although withdrawals for drinking water account for a relatively small percentage of the total groundwater use, they provide drinking water for about 82 percent of the 2.3 million people who live within the aquifer boundary.

Part two of this article, to be published in the June issue, will cover the major findings and implications from the USGS study of groundwater quality in the High Plains.

 

This article includes modified excerpts from the U.S. Geological Survey report titled “Water Quality in the High Plains Aquifer,” USGS Circular 1337, available online at http://pubs.usgs.gov/circ/1337/.

 

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