Geotimes — May 2004 — Western Aquifers Under Stress

Geotimes

Feature
Western Aquifers Under Stress

Although the rate of water consumption in the United States has not increased over the past five years, according to a recently released U.S. Geological Survey report, water problems are prevalent across the country. On both coasts, saltwater is intruding into coastal groundwater. And with widespread urbanization and population growth, water is becoming scarcer everywhere in the United States and particularly in the West, where irrigation and groundwater mining are essential to meet demand.

As communities become more dependent on groundwater, they are facing a number of scientific and policy challenges. Here, we highlight only two hotspots, both in the West: the Denver Basin and the High Plains aquifer.

The Dwindling Denver Basin
State of the High Plains Aquifer

The Dwindling Denver Basin

In 1970, 12,000 people lived in Douglas County, Colo., just south of Denver. Today, more than 200,000 people reside there, with the population expected to double in the next 25 years. Like other counties in the Denver area, Douglas relies on 10,000-year-old groundwater from bedrock aquifers located in the Denver Basin for its water supply. The rapidly growing population, however, is sucking the supply dry.

Aquifers that sustain suburban Denver, Colo., are dwindling at rates of 30 feet per year due to exponential population growth over the past few decades. Highlands Ranch, pictured here, is an example of the growth of subdivisions in Douglas County, south of Denver. Photo by Kirk Johnson.

In Castle Pines North, in Douglas County, water in wells is dropping an average of 34 feet per year. In nearby Parker, water is dropping 30 feet per year. And the story is similar in other municipalities. With well yields already considerably reduced on many properties, the citizens face a dire reality: “There is a finite amount of groundwater in this aquifer system,” says Robert Raynolds, a geologist at the Denver Museum of Nature & Science. At this rate, the south metropolitan area could run out of water — or at least affordable water — in the next few decades. “We will have to find alternate sources of water,” Raynolds says.

The 6,700-square-mile Denver Basin encompasses the entire Denver metropolitan area as well as rural farmland along the eastern front of the Rocky Mountains, says John Moore, a retired U.S. Geological Survey (USGS) hydrologist. Four thick, predominantly sandstone aquifers comprise the semi-arid basin and are, for the most part, hydraulically isolated from the rivers (surface water) that drain from the mountains, Moore says. The four aquifers, stacked atop one another, are also largely isolated from one another by confining layers. And water moves very slowly through each aquifer. These factors make recharge very difficult.

Together, the four aquifers contain an estimated 467 million acre-feet of water (one acre-foot equals about 326,000 gallons of water), of which 269 million acre-feet are recoverable. The Arapahoe aquifer, at 400 to 700 feet thick, is the most productive and most used aquifer in the region, extending throughout two-thirds of the Denver Basin aquifer system. In 1985, 12,000 wells withdrew water from the Arapahoe, Dawson and Denver aquifers in Douglas County. In 2001, that number reached 33,700, Moore says.

Ever since Denver residents drilled the first Arapahoe well in 1883, withdrawal has exceeded recharge. But until the population began to explode, having enough water was not a daily concern. While Denver itself does not withdraw water from the underlying aquifers today (the city uses surface water, which is recharged with snowmelt from the Rockies and rain), the outlying areas rely mainly on the deep aquifers, especially the Arapahoe. And with only 11 to 18 inches of annual precipitation, natural recharge of the aquifer is not possible at the current rate of extraction, Moore says.

Researchers still do not fully understand the aquifer system, Raynolds says. Private consultants, USGS, the state of Colorado and the Museum of Nature & Science have all undertaken studies to better understand the stratigraphy of the aquifers, to learn how long well levels can continue dropping at 30 feet per year and what will happen next. “Part of the problem is that we’re facing a lack of monitoring well data. And furthermore, we don’t know who is doing what research in the Denver Basin,” Moore says. Thus, he and Raynolds sponsored a workshop at the 2002 Geological Society of America (GSA) annual meeting in Denver, and will again host one at the GSA meeting in Denver this November to bring together various lines of research.

However, with water yield dropping from 500 gallons per minute to 100 gallons per minute in individual wells (leaving bathroom showers with barely a trickle), Moore says, time is of the essence for some metropolitan Denver residents. Thus, city water planners are already moving forward with a plan, based on previous studies and current information as it comes in from geologists.

“All of us recognize that the water shortage is a problem,” says David Little, who is in charge of planning and development for Denver Water, the provider for the city of Denver. “It’s important to see if and how we can help.”

As a first step, the municipalities have implemented conservation measures, says Pat Mulhern, a water consultant and director of the South Metro Water Supply Study Board. Some areas, he says, have based their conservation measures from lessons learned in California, such as limiting days and times of lawn-watering and heavily fining citizens who use significantly more water than average; they then use those funds for additional conservation measures (such as better irrigation systems).

The supply study board in a report released in January has also proposed a plan to import water from Denver in years when the city has a surplus of surface water (“wet years”). According to the report, a combination of conservation and water-sharing is the best and most cost-effective option.

The board estimates that their plan, which would involve building pipelines between Denver and the south metropolitan area to transport water and reservoirs in which to store it, will cost around $3 billion between now and 2050. However, they estimate that the alternative — drilling more and deeper wells — will cost closer to $4 billion by 2050. The plan suggests that these costs would largely be passed along to consumers. Average tap fees (the price levied on new service lines or wells) could rise as much as 100 to 500 percent with the water-sharing plan. However, drilling more and deeper wells could increase tap fees by as much as 700 percent.

Even now, water costs in Douglas County are rising exponentially. Tap fees on new homes have doubled over the past six years in some areas and keep rising as development continues. And as much of the undeveloped land in the county has already been sold and zoned, it is unlikely that development will slow anytime soon.

The geosciences community, Raynolds says, needs to get more involved to help the Denver metro area community at large. “We can draw from our experience working in oil and gas reservoirs to help the water community here,” he says, by figuring out just how much recoverable water is in the aquifers, best extraction methods and so on. “This is a geological issue,” Raynolds stresses.

Megan Sever

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State of the High Plains Aquifer

The High Plains aquifer spreads below 111 million acres of land, encompassing eight states, and waters the nation’s breadbasket. Since the 1940s, widespread groundwater pumping from the aquifer for irrigation has led to the annual production of millions of tons of corn, winter wheat, sorghum and other crops that feed cows and people around the world. But the water resource is rapidly dwindling, threatening the livelihood of the western states that rely on the High Plains aquifer for domestic, agricultural and ecosystem water.

This irrigation system in Dundy County, Neb., is run on a center pivot with tubes that drop down to the crop. More than 5 million acres of land were irrigated in Nebraska in the late 1990s, out of almost 14 million irrigated acres in the High Plains region. Photo by M.K. Landon, USGS.

The states — Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas and Wyoming — have been working on the groundwater depletion problem for half a century. Regulation of the aquifer, however, has been controversial. Last October, the U.S. House of Representatives heard from critics and supporters of a bill that would provide resources for joint research between the states and the federal government (Geotimes, December 2003).

“One of the frustrations has been that every state is different, because of state rights and so forth,” says Bill Hargrove, director of the Kansas Center for Sustainable Agriculture and Alternative Crops (CSAAC) at Kansas State University. “So far there hasn’t been much effort to work together,” Hargrove says.

To aid the states in assessing the High Plains aquifer, the U.S. Geological Survey recently released a report detailing depletion rates across the aquifer and general reviews of what each state is doing to protect the resource. Overall, the report says, the aquifer has lost 6 percent of its stored water since the 1940s, with higher losses in many large swaths and slight increases in a few small pockets of the aquifer. Some regions of the aquifer may soon contain no economically recoverable groundwater at all.

Across the aquifer, composed mostly of the Ogallala Formation, annual pumping from 1949 to 1974 increased from 4 to 19 million acre-feet. After 1974, the amount of water pumped annually out of the High Plains aquifer stabilized, according to the report. Nevertheless, over the past half century or so, the High Plains aquifer declined by about 200 million acre-feet (equivalent to approximately 65.2 trillion gallons — about as much as California’s annual rainfall).

“There’s been a tremendous amount of change in water-use technology on the High Plains from the time that development began,” says Bob Hirsch, associate director for water at USGS. The changes generally do not stem from regulations but are “basically economic,” he says. The cost of pumping is proportional to how high water users have to lift the groundwater. “In the 1970s, they faced high energy costs and economized by changing technology, and to some extent took some land out of irrigation.”

Other changes led to higher efficiency in applying water to crops, Hirsch says. High-pressure sprinklers, which had upward-facing heads that sprayed water up and out over fields, also resulted in water loss through evaporation. These gave way to downward-facing sprinklers at lower pressures, as well as to conservation measures. In some places, farmers water according to different crops’ needs and have introduced technology such as drip irrigation, says Tom Huntzinger, chief of water appropriations in the Kansas Division of Water Resources (DWR).

Such shifts are clear from the data collected by state, local and federal agencies (including USGS), compiled by USGS, to compare the “predevelopment” aquifer to its current state. Using records from some 20,000 wells, some of which extend back to 1920, the new USGS report summarizes groundwater storage from the beginning of high impact irrigation in the 1940s through 2000. The water-use data also come from flowmeter measurements and estimates of the amount of irrigation water required by certain crops.

The largest losses of groundwater took place in Texas and Kansas, where the aquifer is thinner and deeper below the surface, and where surface recharge from rain or from the return flow of irrigation water is less. In Texas, the aquifer stores 27 percent less water than it did 50 years ago; Kansas’ portion now stores 16 percent less.

Considering the extensive use of the aquifer, the overall decrease of only 6 percent of its volume is encouraging, says Jim Conkwright, manager of the High Plains Underground Water Conservation District No.1, in Lubbock, Texas. However, according to water district estimates, he says, the Lubbock region — where a large portion of the economy is based on agriculture — has used at least half of the aquifer’s local groundwater during the past 50 years.

The Lubbock-based water district is working to quantify recent decreases in irrigation water use. However, sustainability in this southernmost portion of the aquifer may be unattainable, Conkwright says. Some regional farmers are returning to dryland farming methods, he says, but even if crop irrigation were significantly curtailed, local municipal water use would still exceed recharge rates to the aquifer. The district is sponsoring research by local agricultural research stations and universities to develop crops that require less water, and it continues to improve efficiency for water delivery.

Despite similar activities in Kansas, Huntzinger says, “many areas in west-central Kansas are approaching depletion. Large-scale irrigation as in the past is no longer applicable.” Because the Kansas DWR regulates the state’s water use — and USGS uses the state’s detailed data on the High Plains aquifer — the report is not a surprise from a local point of view, Huntzinger says, but the multistate perspective is useful. “Obviously we’re going to consider what’s going on across our borders; we need to know where the water is flowing,” he says.

Some slight groundwater level increases took place mostly in Nebraska, says Virginia McGuire of the USGS in Lincoln, Neb., and the report’s lead author. But these increases were partly due to losses from leaking irrigation canals that recharged shallow portions of the aquifer, or from irrigation runoff that fed back into the aquifer near rivers and streams. Other possible explanations include less intensive irrigation in those areas and higher rates of precipitation over the past few years.

These regional differences have made regulation of the aquifer challenging, says Hargrove of the CSAAC. In Kansas, he says, some places are basically dry, others have a 20- to 50-year water supply, and still other areas could yield groundwater for a century or more at current use rates. “Coming up with policy that fits everybody has been the frustration,” he says.

Different levels of drawdown on the aquifer may also lead to varying effects on water quality, says Kevin Dennehy, USGS project manager of the High Plains Regional Ground Water Quality study, under the National Water Quality Assessment program. Declining water levels, he says, “have the potential to adversely affect water quality of the High Plains aquifer, and that’s something I don’t think is being adequately explored.”

Wells ideally draw water from the cleanest and most productive zones, which in the High Plains aquifer is usually the midpoint, Dennehy says. But decreasing an aquifer’s saturated thickness might affect the water likely to be mined. “Water that did not previously discharge from deeper units might do so,” he says, and that deeper groundwater might contain more dissolved chemicals or salts. And, he continues, “the upper part of the aquifer is vulnerable to the effects of human activities,” such as the application of pesticides or other chemicals that travel to the aquifer via recharge.

Dennehy also says that the need remains for an update to the previous in-depth assessment of the aquifer. The last full USGS report dates to 1978, and the legislation currently under consideration by Congress would contribute to extensive and detailed geologic and hydrologic characterization of the aquifer.

No matter what happens with that legislation, the most recent USGS report is helpful, says Huntzinger of Kansas DWR. “I hope that this will allow people outside the Ogallala to realize how important this aquifer is to this part of the country.”

Naomi Lubick

Links:
"Steady water use," Geotimes Web Extra, March 19, 2004
"Water is for Fightin’," Political Scene, Geotimes, December 2003
"Congress confronts a depleting aquifer," Geotimes Web Extra, March 20, 2003

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