Spring water gushing from a cave mouth.
Dorothy Vesper takes her geology class on a field trip to a new road cut. She points out a hole in the rockface about eight inches across. "Is it a big hole or a little hole?" she asks. "Is it big enough that a little critter can get into it? Can you get your head into it? How much water can it carry?"
Then she asks, "What's the size of a water line in State College?"
She smiles at her students' guesses. She tells them to look again at that hole in the rock. "It's about that," she says. "Eight inches diameter. You don't need a gigantic opening to move a lot of water." In fact, all you need is a crack a centimeter wide—less than half an inch—to call it a "conduit."
"Those little holes can be the ones that do all the work in a water transport system," she explains now, back in her office in Deike Building. "At one centimeter you can start carrying sediments through, little tiny clay particles, just enough to make your water cloudy."
Vesper, a doctoral student in geosciences at Penn State, is an expert on how water—and its contaminants—moves through rock. Particularly through karst, the water-carved limestone formation responsible for central Pennsylvania's landscape of springs and sinkholes, and for its water. And karst is not only important in Pennsylvania: Twenty percent of the world obtains its water via karst systems.
"Do you know Rock Springs?" she asks. "The University's agricultural research area?" Vesper pulls out a piece of notebook paper and scribbles a diagram in black felt pen. The spring that gives the area its name lies at the bottom of Tussey Mountain. She marks the top of the mountain sandstone, the slopes shale, the base of the mountain limestone, and the valley bottom dolomite. "At Rock Springs, the water runs down the mountain here—and here—and here—through the water gaps," she explains, tracing three arrows down through the shale layer. "Then it hits limestone and makes a sinkhole. The farmers couldn't farm around the sinkholes, so you have these fingers of trees coming down into the fields." Where the limestone gives way to dolomite, the water's flow is blocked. The underground streams change course and link up, flowing through the limestone along the interface until the water surfaces at Rock Spring. "The spring discharges all that water, and it travels down the valley as a stream."
There's no way to predict, in a karst landscape, exactly where a spring will be—"why it's there and not 100 feet away. There's no way to say." But there will, eventually, be one. "Almost all ground water comes back up to the surface eventually in karst systems," Vesper explains. "Things don't generally stay underground indefinitely."
And by "things," she means the water and all that it carries: pollutants, contaminants, sediment, and debris. By testing the water at a spring, you can get a pretty good idea of its quality all along the underground water course—but only if you understand the geometry of the rock cavities it's flowing through.
"Different springs are telling different stories," she says. "You have to know the geometry of each spring to know when to test."
James Collins
Dorothy Vesper is an expert on how water moves through rock. To know when to test spring water for contaminants, she says, you have to know the geometry of the water system: the size and shape and conductivity of the conduits.
How does water move through rock?
For her Ph.D. dissertation, Vesper has been studying metal contamination in a series of springs in Kentucky and Tennessee, not too far from the most famous of all karst systems, Mammoth Cave, with its more than 300 miles of mapped passageways.
Her work builds upon what she calls a "landmark paper" by Evan Shuster, who, like Vesper, received his master's degree under Will White, a professor of geosciences at Penn State and a world expert on karst and caves.
Shuster compared the water output and the calcium concentration in springs over a year's time. "When you dissolve limestone," Vesper explains, "you get calcium and carbonate. The carbonate reacts with water to produce bicarbonate. Testing bicarbonate is how you tell the alkalinity of your water." Monitoring the calcium concentration, as Shuster did, tells you how the water is moving through the rock. "After it rains, the calcium concentration in the spring can dip down. That tells you the fresh rainwater didn't have time to dissolve much rock," that the rain just rushed right through the rocky channels.
Water flow is another indicator of a spring's hidden geometry. If the water level peaks after a storm at the same time as the calcium level is lowest, "you know it's not a full pipe." The cracks and conduits in the rock have space to carry extra water, and there are no underground chambers pooling up into lakes with tiny drainage tubes. "The fresh water is rapidly transmitted to the spring."
But the opposite can also happen: the water peak and calcium low can occur at different times, hours or even days apart. "Then it's the pipe-full problem, like with your garden hose," Vesper explains. Imagine the hose is stretched out on the lawn on a sunny day. "If you turn on the hose, and the water coming through it is warm, you know it's not fresh water coming yet." The cold fresh water first has to push out the warm water that's been sitting there. Like the hose, the conduits leading to a spring can be too full to handle more water: there's a lag time before you can measure the effects of the rain.
"Those are the two extremes of springs," Vesper concludes. "But the same system can go in and out—it can change over time. Sometimes it's completely underwater and sometimes it's not."
So when do you check the water, if you want to know if run-off from a storm has contaminated a spring? "For water-quality monitoring, if the change is offset by eight hours or a week, that makes quite a big difference on when you need to collect samples."
Will White
"Caves, like roadcuts, are just made for hydrogeology," says Dorothy Vesper. They let scientists see how water flows through rocks.
The springs Vesper is monitoring for her dissertation are on the site of Fort Campbell Army Base, the largest army airfield in the United States. "It's beautiful farming area, rolling hills," she says. "But Fort Campbell has the same range of potential contaminants as any small town with an airport: jet fuels, engine oils, degreasers."
After finishing her master's degree at Penn State in 1988, Vesper worked for an environmental consulting firm on such problems as how to keep abandoned wells from harming the dune system in California, one of the Pacific coast's most fragile ecosystems, or how to track petroleum compounds and chlorinated solvents in the freshwater wells of St. Thomas in the Virgin Islands. She had been working under contract to the Army at Fort Campbell, monitoring the water quality in their springs, when she decided to return to Penn State for a Ph.D.
"Some of these springs are pretty remote and getting equipment to them is difficult," she says, noting that it took five people six weeks one year to keep tabs on them. "We often fill over 1000 bottles of water a year. But the car batteries are the worst," she grimaces. "They're so heavy." Once in place, the batteries power a pump that automatically begins sampling the water in the springs soon after a rain sets in. The Army wants to know the maximum difference in levels of various contaminants, to make sure they fall within safe water-quality standards. Says Vesper, "I want to look at the fine detail." So after filing her reports with the Army, she ships the water samples to Penn State for further analysis.
She's seen, for instance, how the concentration of aluminum and trace metals increases in a spring after a hard rain.
You would think that the additional water running through the system would dilute the contamination. But just the opposite happens. "If it's something that on a dry day doesn't get into the water at all, then the dilution effect is way overwhelmed by a sort of flushing behavior." The concentration increases, she explains, because of the storm water's turbidity, its ability to scour up and carry along dirt.
"We're moving all these suspended solids, all these little tiny clay particles, and the metal is adhered to the particles. The amount of contaminant you move is proportional to the surface area of the particles, so it's the little particles that cause the problems. If I filter the water, almost all the aluminum goes away. It drops from 2,700 parts per billion to 12 parts per billion.
"The same is true with iron, lead, and arsenic. And bacteria—like giardia or fecal coliform—they're going to come through in a pulse exactly like this. But not calcium. Calcium is in the water phase and is not associated with the particles that come through during a storm."
Knowing if a contaminant is free in the water or bound to a particle tells you how it will affect animals or humans. "How bioavailable things are depends on how they're moving," Vesper explains. "Generally, we consider something to be bioavailable if it's in solution. If it's attached to a clay particle, it's less available, and if it's incorporated into a material, it's generally not available at all. The next stage of my dissertation will be to look at the sediments in springs. Some accumulate a lot of sediments, some don't. The longterm risks and the longterm impacts of any of these contaminants are in the sediments, in the effects on the critters that live there, the microorganisms, the insects, the crayfish."
Oddly, Vesper has found in her study of springs that organic pollutants in underground water can be harder to track than inorganic ones like metals. "The organic contaminants are not coming out in the springs. They're just not going anywhere: They're trapped in cavities.
"At Fort Campbell there was one place that had ten feet of oil on top of the water—the Army sampled into it by drilling wells—and that oil was not going anywhere. The water coming out of the springs showed no sign of it. The associated springs had less oil contamination than drinking water standards."
Will White
Spring water might gush from a cave mouth; but underground, the same stream might detour into deep, still pools like these.
Perhaps the reason for this, too, lies in the geometry of the water system. It's hard to say, Vesper admits, what a watercourse really looks like as it travels through the rock. "It's not as if what I'm doing doesn't require a big leap of faith," she admits. "Do I really know what's happening down there under the ground?"
That's why roadcuts, with their occasional cross-sections of water systems, are "just made for hydrogeology," Vesper jokes.
"And why it's great to have examples like Mammoth Cave, where you can go down and walk around in passages carved by water. You can see how the water has been flowing."
In Mammoth Cave you can walk through a phreatic tube, the kind of passageway made when the conduit resembles a garden hose, the water filling it totally. "A phreatic tube is elliptical in shape. The rock was dissolving in all directions at the same time. They can be beautiful."
Where the water level in the cave was lower, not fully filling the space in the rock, it carved a vadose passage. "It eroded like a canyon. It only eroded down," Vesper explains.
And there are combinations of the two. "There's a wonderful passage in Mammoth Cave called a keyhole passage," Vesper says. "It was phreatic originally, then the water level lowered and it became vadose. It's called Fat Man's Misery. Because if you're fat, it's really difficult to get through. The bottom is a really narrow canyon and it meanders around"—she leaps up from her desk to mime walking through it, sashaying with mincing steps as if the cave walls were pinning her legs close. She laughs. "Where the phreatic part and the vadose part meet," she wiggles her hands, "that level is so smooth. Everybody puts their hands there and worms their way through. So you could say the passage was made not just by water and rock, but also by human hands."
Dorothy Vesper is a doctoral candidate in geosciences, College of Earth and Mineral Sciences, 437 Deike Bldg., University Park, PA 16802; 814-865-3321; vesper.d@geosc.psu.edu. Her adviser is Will White, Ph.D., professor of geochemistry, 210 Materials Research Lab; 865-1152; wbw2@psu.edu. Her research is funded by the U.S. Army Research Office, the Cave Research Foundation, the Clay Minerals Society, the Geological Society of America, Sigma Xi (the Scientific Research Society), and the Penn State Geosciences Krynine Fund.