Benjamin Schwartz, a Ph.D. student in geosciences in the College of Science, who is from Doe Hill, Va., in Highland County, is using an innovative technique to characterize ground water movement in sinkholes. His goal is to recommend management strategies to reduce contamination of aquifers in regions that are rife with sinkholes. Hurricane Ivan's downpour in Southwest Virginia allowed him to measure changes in underground water over a short 4-day period.
Schwartz will present his findings at the 116th national meeting of the Geological Society of America in Denver on Nov. 7-10.
Sinkholes generally form over limestone and dolomite. That rock dissolves and the earth on the surface subsides. Water from the sinkhole either seeps into the subsurface or runs in through a fissure or cave opening and rapidly enters the aquifer.
"People up and down the Shenandoah Valley get their water from aquifers," Schwartz said. "Often, these aquifers are contaminated. Sources of contamination include runoff from paved surfaces or because a good portion of Virginia's agricultural land is on karst terrain. There is little filtration between surface water and karst aquifers."
Karst is a term used for a landscape where water movement is underground because of the voids in the bedrock.
Schwartz is using six sinkholes on Virginia Tech's Kentland Farm along the New River in Montgomery County to measure how water and contaminants move within a sinkhole, that is, thehydrology and chemical transport in a sinkhole. He is looking at depth to bedrock (soil thickness), slope within the sinkhole, drainage area, and land use - such as wood land, crop land, or pasture ("Cattle love to stand in sinkholes.") to determine if such sinkhole characteristics indicate what is happening underground.
He is using a geophysical technique called electrical resistivity. Current is passed through the soil using a car battery attached to two electrodes embedded in the soil. By using an array of 25 embedded electrodes, and by changing the locations of the current electrodes, voltages can then be measured at different electrodes in the array. Nearly 200 measurements are taken using a single array. "If you know the voltage and current, you can calculate the resistance," Schwartz said.
Different degrees of electrical resistance allows him to identify water, rock, soil, and voids to a depth of about 15 meters and create a model of the subsurface. Schwartz may also be able to determine the water's chemistry by the changes in electrical resistance measured in an aquifer. He explains that sitting or pooled water becomes saturated with minerals while fresh rainwater has a low dissolved mineral content. Fresh water conducts electricity poorly, compared to water loaded with minerals.
Depressions in the bedrock surface can also store contaminants. When rain and runoff pour water through caverns and fissures, the contaminants are flushed out of the depressions and into the aquifer.
Schwartz took advantage of Hurricane Ivan to measure a rapid change in water movement under a sinkhole. "I went out before the hurricane and ran two transects (measurements from lines of electrodes) as a control. I left the electrodes in place then made four measurements as the storm moved through and afterwards." He says he only actually got rained on once.
The resulting two-dimensional computer model showed the changes in water movement. "I saw that water was not sinking evenly or being taken up like a sponge, but that there are preferential flow paths."
Next, he will create a 3-D model by placing the electrodes in a grid, which will allow him to add the direction of water movement to his model. He combines the electrical resistivity measurements with a topographic map of the surface to create a 3-D model of the bedrock and land surfaces. But he won't have to wait for another hurricane. "If a rain event is predicted, I can take base line measurements then monitor the site. But another three to four inch rainfall would be nice."
Soon, to confirm his interpretations from this new use of electrical resistivity, Schwartz will drill a series of wells.
Schwartz will present the paper, "Hydrologic characterization of sinkholes in agricultural settings," at 2:15 p.m. Sunday, Nov. 7, in room 205 of the Colorado Convention Center. Co-authors are Madeline Schreiber, assistant professor of geosciences at Virginia Tech, and William Orndorff of the Virginia Department of Conservation and Recreation, Division of Natural Heritage, Karst Project.
Orndorff earned his master's degree in geosciences from Virginia Tech. Schwartz earned his bachelor's degree in geology from Radford University. He is just beginning his Ph.D. program.