UNIVERSITY PARK, Pa. — Chemical reactions deep below ground affect water quality, but methods for “seeing” them are time-consuming, expensive and limited in scope. A Penn State-led research team found that seismic waves can help to identify these reactions under an entire watershed and protect groundwater resources.
“About one third of the U.S. population gets their drinking water from groundwater, so we need to protect this valuable resource,” said Susan Brantley, distinguished professor of geosciences and director of the Earth and Environmental Systems Institute (EESI) at Penn State. “At this point, however, we don’t know where the water is or how it moves in the subsurface because we don’t know what is down there. In this study we used human-generated seismic waves — similar to the waves from earthquakes — to look under the surface.”
Traditional geochemical tests involve drilling a borehole 3 to 4 inches in diameter deep into the ground, collecting the soil and rock samples, and grinding and analyzing the chemical makeup of the samples in a laboratory.
The process is expensive and laborious, and it only reveals the geochemical information for that specific point in a watershed rather than the entire watershed, said Xin Gu, a postdoctoral scholar in EESI.
“In this study, we had the advantage of having previously drilled boreholes, so we knew at which depths geochemical changes happen,” Gu said. “We also had the materials from the boreholes, so we knew the mineral abundance and element composition. Here we tried to expand our knowledge by doing geophysics, which is relatively more efficient.”
The researchers logged — lowered instruments that can send and receive signals, or even take high-resolution images, down a borehole — a 115-foot deep borehole drilled into the valley floor at the NSF-funded Susquehanna Shale Hills Critical Zone Observatory, a forested research site in Penn State’s Stone Valley Forest that sits atop the Rose Hill shale formation.