A new study may have a solution for both acid mine drainage and natural radioactivity in hydraulic fracturing – fracking – wastewater that can be found in 'flowback fluid.' In hydraulic fracturing, water is injected at high pressure down wells to crack open shale deposits buried deep underground and extract the natural gas trapped within the rock. Some of the water can flow back up through the well, along with natural brines and the natural gas.
Trace amounts of potentially harmful natural contaminants can be found in Marcellus Shale wastewater, for example, but trace amounts can eventually cause radioactivity to accumulate in stream sediments nearby. This "flowback fluid" from fracking typically contains high levels of salts, naturally occurring radioactive materials such as radium, and metals such as barium and strontium. Acid mine drainage also flows out of abandoned coal mines in the Appalachian Basin. It can be toxic to animals, plants and humans, and affect the quality of waterways in Pennsylvania and West Virginia.
A Duke University-led study finds that much of this naturally occurring radioactivity in fracking wastewater might be removed by blending it with the wastewater from acid mine drainage. That would also reduce the need for local freshwater resources, by giving drillers another source of usable recycled water for the hydraulic fracturing process. Some fracking sites currently use no freshwater at all, it's all recycled, but the finding means an acid mine drainage problem could also be solved.
By blending them in the right proportions they were able to bind some of the fracking chemicals into solids that can be removed.
"This could be an effective way to treat Marcellus Shale hydraulic fracturing wastewater, while providing a beneficial use for acid mine drainage that currently is contaminating waterways in much of the northeastern United States,"
said Avner Vengosh, professor of geochemistry and water quality at Duke's Nicholas School of the Environment.
"It's a win-win for the industry and the environment."
Fracking could be the perfect way to repurpose acid mine drainage but to test the hypothesis, Vengosh and his team blended different mixtures of Marcellus Shale fracking wastewater and acid mine drainage, all of which were collected from sites in western Pennsylvania and provided to the scientists by the industry.
After 48 hours, the scientists examined the chemical and radiological contents of 26 different mixtures. Geochemical modeling was used to simulate the chemical and physical reactions that had occurred after the blending; the results of the modeling were then verified using x-ray diffraction and by measuring the radioactivity of the newly formed solids.
"Our analysis suggested that several ions, including sulfate, iron, barium and strontium, as well as between 60 and 100 percent of the radium, had precipitated within the first 10 hours into newly formed solids composed mainly of strontium barite," Vengosh said. These radioactive solids could be removed from the mixtures and safely disposed of at licensed hazardous-waste facilities, he said. The overall salinity of the blended fluids was also reduced, making the treated water suitable for re-use at fracking sites.
"The next step is to test this in the field. While our laboratory tests show that is it technically possible to generate recycled, treated water suitable for hydraulic fracturing, field-scale tests are still necessary to confirm its feasibility under operational conditions," Vengosh said.