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Aerial maps and a geologist's erudition: in the mind of Steve Mabee, clues to the location of water.
CLAD IN THE CHILDHOOD UNIFORM FOR PUDDLE-JUMPING yellow plastic overalls and knee-high rubber boots Steve Mabee '92G goes 400 feet underground to see what Earth's cracks and crannies can tell him about scientific prospecting for water.
In a rare opportunity created by the construction of a tunnel that will carry water from Wachusett reservoir to the greater Boston area, Mabee (pronounced "maybe"), an assistant professor of hydrogeology at UMass, has the chance to view seventeen miles of the Earth's crust from the inside. As the boring machines munch forward through the miles of granite, quartz and other rocks, Mabee and graduate students Patrick Curry and Kit Williams follow along behind, making a record of the fractures, faults, and rock-types uncovered in the sixteen-foot-diameter tube.
A band of "gouge," where the scraping of two rock faces has reduced the stone to clay, represents a fault line. A belt of basalt rock, black against the gray wall of the tunnel, demonstrates where one rock type invaded another. A crack circumscribing the great belly of the tunnel marks a fracture where the stone split under tremendous pressure. Even more exciting than the rock features themselves, to these hydrogeologists, is evidence of water. Saffron ferrous streaks and inky manganese veins on the sides of the tunnel indicate a water source nearby. (The occasional subterranean rapids suggests, less subtly, the same thing.)
The Metrowest water supply tunnel, which will eventually bisect the towns of Marlborough, Southborough, Framing-ham, Wayland, and Weston, was commissioned by the Massachusetts Water Resources Authority to supplement an existing aqueduct. But with each foot it progresses, the tunnel also unveils geological features that have previously been matters of speculation. Among these are the distribution, orientation, and size of the fractures and faults created when the earth was shifting and creaking into the mass we now call Massachusetts.
Mabee's interest in these fractures and faults is not merely academic. The features of the tunnel get this scientist excited because they're clues to the location of water. Faults and fractures in the earth's crust serve as conduits underground for water that has infiltrated from the surface. By studying where these fissures intersect, hydrogeologists can predict the location of subterranean water.
The predictions are important because in many areas these hidden sources of water are the only fresh water for miles around. Fifty percent of the rural New England population depends on water drawn from bedrock, Mabee says. During the nine years he worked as a consultant before entering the Ph.D. program in geology at UMass, Mabee learned that there is nothing more basic than that dependency. If you're building a house that needs a well, you need to know where to drill. "Otherwise," he says, "it's just poke and hope." (Which is, incidentally, his scientific assessment of the folk technique of dousing rods.)
IT'S THE POTENTIAL of a technique known as "lineament analysis" that sends Mabee and his graduate students "down the hole" as tunnel workers (or "sand-hogs") like to call their subterranean destination. The technique dates from the advent of accurate topographic maps and satellite imagery, and was a major research interest of Mabee's mentor at UMass, now-emeritus professor of geology Donald Wise, whom the younger man calls "the lineament guru." Strewn across Mabee's desk in the south wing of Morrill Science Center, amid crayon drawings signed by his daughters Caroline and Elizabeth, are aerial photographs of the state on which he's mapped out major fractures and fault zones discernible from faint topographic depressions on the landscape. Mabee speculates that these depressions form because fractured rock weathers more easily than solid rock, and can cause the ground to sag in places where it is weak. "If you draw a bunch of these lines and find a spot where many lines intersect," Mabee says, "in many instances you'll find a lot of water!"
The trouble is, he adds, in many cases you won't. Lineament analysis has been used with considerable success by experienced hydrogeologists, such as Wise protegés Ken Hardcastle '89G and Mark Wingsted '93, '97G of Emery and Garrett, a New Hampshire consulting firm dedicated entirely to finding subsurface water. But the use of the technique is limited by lack of information on how variables such as proximity to a lake or different types of rock affect the accuracy of predictions. That's why older, more costly, physical methods are still the norm for mapping subsurface water. "Seismic refraction," for instance, involves hitting the earth with a sledgehammer or setting off explosives, and analyzing the resulting sound waves as they pass through the subsurface. "Electrical resistivity" works the same way, but involves electrical instead of percussive charges.
To test predictions made with the elegant technique of lineament analysis, and to gather information that will add to its usefulness, Mabee visits the Wachusett tunnel as often as he can. Predictions, or no, the tunnel also holds a primal appeal for him. "I don't get to spend as much time as I would like in the tunnel," he says. "My graduate students get to have all the fun." His idea of fun involves sloshing around in boots full of water and scurrying up the side of the tunnel every time the "lockie," or locomotive comes through carrying workers. Never mind the lack of natural light. "There are no telephones, no computers, no nothing, just work," he smiles.
Mabee and his students arrive equipped to work in very precise ways. You may think, he says, "that geologists are just people who hit rocks," but in fact they're people who make and record meticulous observations. The composition of a rock, the angle at which one rock abuts another, the specific characteristics of a particular rock face all contribute to how that rock will convey water. Among the tools in the multiple pockets of Mabee's orange-mesh vest are a Freiberger compass, a contour gauge, and a vial of dilute hydrochloric acid; each of these objects helps him characterize the details of a rock face. The compass allows him to measure the "strike and dip," or orientation, of a rock plane. The gauge, a two-sided comb with freely moving teeth, can be pressed up against the surface of a rock to take an impression of its profile. As for the vial, two apparently similar minerals can be distinguished when the acid is dropped sparingly on their surface: Fizzes mean the mineral is calcite, no fizzes mean the mineral is probably something else, perhaps quartz.
THE EXCITING QUESTION IS, of course, are there water sources in places where the lineament analysis has predicted them? Perhaps suppositions of water sources near the Sudbury river in Framingham will prove most accurate, while those in regions of the state devoid of lakes and rivers will not. Or perhaps limestone will prove more likely to harbor water than granite. Until now, because of the limited number of examples where lineament analysis did and did not work, it's been impossible to consider all the factors that contribute to its accuracy. The construction of this tunnel through a variety of rock types and topographic areas was thus serendipitous beyond a hydrogeologist's happiest dreams.
With the benefit of these observations, says Mabee, "I'll be able to say, `When we do these lineaments, it's a matter of luck.' "Or, `It doesn't work.' Or " and here his blue eyes sparkle "`You know what, these lineaments that we see on this imagery: 90 percent of the time they delineate high-yield fracture zones in the subsurface.'"
In childhood terms, puddles worth jumping over.