Understanding AquifersBy Barbara Jaquish
A child’s globe shows the planet with clearly divided land and water. But spin it, and the blue, green and tan blur. While our experience of life on earth may seem to conform to the static globe, with water in the blue place and land in the green and tan places, reality is closer to the spinning blur.
The road enters the Savoy Experimental Watershed, 3,000 acres of forest, meadows and river with a typical karst landscape. Undergraduate Chris Hobza observes a water seep discharging a heavy flow of water.
Ground water is a source of sustenance for people, agriculture and industry worldwide, and as populations grow and economies develop, we increasingly draw on those stores of water beneath the surface. Our needs for water grow each year. But ground water is a finite resource, one that in some places is strained to the limit.
“Ground water” is more than just a term for water that resides below. The word can also imply a relationship between the water and the surfaces of soil, sand and rock that it touches. The water and the ground interact in complex and little-known ways. Discovering how ground and water interact in the dark places we cannot readily observe may be key to planning how to live in a sustainable manner with the finite amount of water available here on earth.
At the University of Arkansas, an eclectic cadre of researchers from all corners of campus – geosciences, civil engineering, biological sciences, soil sciences, chemical engineering, biological and agricultural engineering, and more – is doing something to understand the world of water below, the minerals and microbes it carries, how it renews itself and its relationship with surface water.
“The overriding concern, my greatest concern, about water is sustainability,” said Phillip D. Hays, associate research professor in geosciences, whose academic appointment is shared with the U.S. Geological Survey and the National Water Management Center of the U.S. Department of Agriculture.
“We assign a lot of value to water, but we don’t necessarily have an understanding of how to allot and most wisely use what we have,” Hays observed. “We need water to survive, and we can’t behave as though we can grow and expand exponentially, because we are limited by a finite resource.”
He notes that population growth and economic expansion in areas as divergent as China and Northwest Arkansas increasingly strain that finite resource. In the western United States, “water wars” have been a common problem, and we are beginning to see such signs of stress in formerly water-rich states like Arkansas, Rhode Island and Georgia.
Ralph K. Davis, associate professor in geosciences and director of the Arkansas Water Resources Center, has seen first-hand the effects of over-use of an aquifer. In the 1980s, he worked for several years in Kansas with a system commonly known as the Ogallala Aquifer, which has been recognized as troubled for decades. That aquifer extends from western Kansas through west Texas and has been extensively “mined,” leading to its significant decline. In an area that receives no more than 20 inches of rain each year, the aquifer is recharged – that is, replenished by drainage through the soil – at about half an inch a year. He calls the area from Dodge City to Garden City in Kansas “one great feedlot” where usage far exceeds the recharge rate.
“The Arkansas River no longer flows in the stretch from Garden City to east of Dodge City, a distance of over 100 miles, because the ground water is just too low. In much of the Great Plains, no new development is possible,” Davis said. “The Ogallala Aquifer is a poster child for non-sustainable use.”
He draws similarities between the situation with the Ogallala Aquifer and conditions in areas as far-ranging as Saudi Arabia and eastern Arkansas. In all of these areas, we are drawing geologic water that is 10,000 or more years old, water associated with the last glaciation. And Davis is blunt: “These aquifers can’t be replaced.”
Over-use of aquifers can cause irreversible damage. With over-pumping, the water table drops, and eventually there’s a collapse of the pore space – the space between the components of soil that had been occupied by the water. The effects of over-pumping can be seen in places like Mexico City or Houston, where the sediment has compacted to the point that people have to walk down steps to get to the front doors of buildings. Once this damage occurs, the aquifer can no longer recharge as effectively – even should more water be introduced.
Tracing Water Through Karst
Geosciences professor Van Brahana is proud of the development of the Savoy Experimental Watershed, “a premier water quality research collaborative,” originally involving the university’s department of animal science, the U.S. Geological Service and other UA departments. Located about 15 miles west of the main campus, Savoy is, in Hays’ estimation, “an exceptional field station and a unique research setting.”
Brahana predicts Savoy will be particularly beneficial as a long-term site to help researchers understand the multiple questions related to ground water in a karst environment, including how water enters the system, where it goes and how it is processed. The karst terrain is produced by the dissolution of rocks, mainly limestone and dolomite. On the surface, the landscape is characterized by dips and depressions known as sinkholes, produced by the collapse of the supporting rock after it has been eaten away by water. Water seeps, where water oozes out of a hillside, are also common. Beneath the ground are cracks and fissures – and sometimes caves – produced as the rock dissolves.
Savoy’s 3,000 acres of forest, meadows and river offer researchers an outdoor laboratory with a typical karst landscape underlain with porous limestone and involving a mix of interflow zones and focused-flow conduits. In the interflow zone, water moves laterally, and it slows down. In contrast, water in the focused-flow conduits moves quickly, at rates of miles per day, rather than feet per year.
Savoy has been important to Sue Ziegler, associate professor in biological sciences, who is interested in learning more about a poorly understood area: how nutrients are processed in the ground water system, particularly nitrate.
Nitrate is a common, naturally occurring form of nitrogen that can be introduced into the soil by the application of fertilizers and animal wastes and move from there into surface and ground waters. While more research is needed into the health effects of nitrate on humans, it is known to reduce oxygen transport in the blood and to affect thyroid function, with particular concern for infants and pregnant women. According to the World Health Organization, nitrate-related toxicity problems are only beginning. Their 1998 report on nitrate in drinking water found, “Because of the delay in the response of ground water to changes in soil, some endangered aquifers have not yet shown the increase expected from the increased use of nitrogen fertilizer or manure.” Many factors affect whether nitrate in the aquifer is converted to nitrogen gas, such as the height of the water table, the amount of rainwater and the presence of other organic material.
For Ziegler’s work, “the infrastructure at Savoy is fantastic.” In addition to the naturally occurring seeps, Savoy features a trench cut into the ground to enable researchers to observe and collect from the water flow through the interflow zone and the soil above. Researchers also have sunk a series of lysimeters, small, tubular wells, to pull water from the surrounding soil zone for analysis.
One goal for Ziegler is to determine how much of the reduction in nitrate levels at a series of sample points is due to biological processing and how much is due to simple dilution. Understanding how important processing of nitrate is and where it takes place is critical to managing land-use practices on the surface to reduce ground water nitrate contamination. Additionally, Ziegler notes, it is important to understand what controls the actions of the micro-organisms, known as denitrifiers, which use nitrate to gain energy, like humans use oxygen. These denitrifiers are ultimately responsible for removing nitrate from the aquatic environment.
“These are the guys that everyone’s really interested in when we think about dealing with nitrate contamination,” Ziegler said. “It’s the main way nitrate can be lost from an ecosystem. It’s the natural way. The interflow zone is likely to be the place where denitrification is occurring in karst ground waters, and we want to understand what surface processes do to that zone. We want to go from just a single plot to using multiple plots within the Savoy watershed. Then we can test different agricultural practices to see how they impact the surface and how that impacts the flow through the interflow zone, and ultimately, how this all impacts denitrification and nitrate processing within these vulnerable watersheds.”
Other UA researchers seek to understand the processes of water moving through karst in relation to common bacteria. Davis, Brahana and Greg Thoma, the Bates Teaching Professor in chemical engineering, have worked with other researchers and graduate students to learn more about the life of bacteria, in particular Escherichia coli (E. coli), in a karst aquifer. They had noticed that when a heavy rain hit at Savoy, bacteria levels would shoot up in the water that was flushed out in springs. This was odd on two counts: Normally, with more water, common sense would suggest that any bacteria present would be diluted. Also, since the optimal environment for E. coli is the warm gut of a mammal, it seemed that the bacteria should die out in the 57 to 58-degree temperatures of the aquifer.
One of Thoma’s doctoral students, Tiong Ee Ting, collected E. coli and tagged it with a metal trace element that allowed them to quantify when the bacteria went into storage and when it was flushed out. What they discovered was that E. coli goes into a kind of stasis in colder temperatures, remaining viable for extended periods in the aquifer until flushed out.
This finding is important for two reasons. First, karst aquifers lay under as much as 20 percent of the United States, supplying water to rural residents, and much of this water is untreated. Also, the researchers used E. colibecause it is easy to detect and easy to grow in the lab. It is what they call an indicator organism. That is, when E. coli is present, there is also the potential for other serious viruses and bacteria to be part of the mix.
This Savoy field is dotted with water sampling devices such as lysimeters, wells and surface weirs. Back in the lab, Sue Ziegler and graduate student Byron Winston test water samples for nitrate contamination.
Arkansas and the World
“By understanding what we can learn at Savoy, we can extrapolate to put the best science and social considerations into account when making decisions on development questions,” Brahana said. “The excitement about Savoy is seeing the complexity.”
Hays uses his experiences in El Dorado, Ark., as an example when he speaks nationally and internationally about effective strategies to improve sustainability. El Dorado was the first region in Arkansas to be designated a critical area under a federal program that makes resources available to improve such sites. After a century of pumping, the Sparta Aquifer had dropped by 200 feet. With assistance from state, federal and university experts, the local conservation district was able to make changes that allowed the aquifer to see some recovery for the first time in 100 years.
Brahana has also worked with a graduate student on a project aimed at reducing the demands on the aquifer by using water from flooded coal mines in Greenwood, Ark., to supplement city water. Although the project is still in the beginning stages, initial studies have indicated that up to 500 million gallons of usable water may be available.
For the people of Bangladesh, India and Cambodia, it is vital today to unravel ground water processes that produce arsenic-tainted wells. As countries around the world put more demands on the finite resource that is ground water, it will become increasingly important to us all to understand how our aquifers work.
Phillip D. Hays is associate research professor in the J. William Fulbright College of Arts and Sciences geosciences department and shares his appointment with the U.S. Geological Survey and the National Water Management Center of the U.S. Department of Agriculture.
Ralph K. Davis is associate professor in geosciences in the J. William Fulbright College of Arts and Sciences and director of the Arkansas Water Resources Center.
Susan Ziegler is associate professor of biological sciences in the J. William Fulbright College of Arts and Sciences.