Wastewater generated by hydraulic fracturing could release tiny particles in soils that often strongly bind heavy metals and pollutants, exacerbating environmental risks during accidental spills, according to Cornell University researchers.
The properties that make fracking fluid effective at extracting natural gas from shale also make associated pollutants such as heavy metals, leach out, according to the Cornell researchers. “
The study’s findings can be used by people looking to prevent or cleanup fracking fluid spills,” co-author on the paper, Cornell postdoctoral associate Cathelijne Stoof said.
The tiny particles they studied are colloids which are larger than the size of a molecule but smaller than what can be seen with the naked eye and which cling to sand and soil due to their electric charge.
Previous research has shown 10 to 40 percent of the water and chemical solution mixture injected at high pressure into deep rock strata, surges back to the surface during well development. Scientists at the College of Agriculture and Life Sciences studying the environmental impacts of this ‘flowback fluid’ found that the same properties that make it so effective at extracting natural gas from shale can also displace tiny particles that are naturally bound to soil, causing associated pollutants such as heavy metals to leach out. They described the mechanisms of this release and transport in a paper published in the American Chemical Society journal Environmental Science & Technology.
Scientists are reporting that when spilled or deliberately applied to land, waste fluids from fracking are likely picking up tiny particles in the soil that attract heavy metals and other chemicals with possible health implications for people and animals.
The study attempts to understand the prevalence of colloids in groundwater from soils exposed to flowback fluid via accidental hydrofracking spills. They filled tubes with soil mixed with synthetic colloids that shined red under a bright light microscope. In one tube, the researchers poured deionized water. In the other tube, they poured flowback fluid from a drilling site at the Marcellus Shale.
In the experiment, glass columns were filled with sand and synthetic polystyrene colloids. They then flushed the column with different fluids, deionized water as a control and flowback fluid was collected from a Marcellus Shale drilling site at different rates of flow and measured the amount of colloids that were mobilized.
On a bright field microscope, the polystyrene colloids were visible as red spheres between light-grey sand grains, which made their movement easy to track. The researchers also collected and analyzed the water flowing out of the column to quantify the colloid concentration leaching out.
They found that fewer than five percent of colloids were released when they flushed the columns with deionized water. That figure jumped to 32 to 36 percent when flushed with flowback fluid. Increasing the flow rate of the flowback fluid mobilized an additional 36 percent of colloids.
They believe this is because the chemical composition of the flowback fluid reduced the strength of the forces that allow colloids to remain bound to the sand, causing the colloids to actually be repelled from the sand.
Fewer than 5% of the colloids were leached out of the soil with the deionized water when it was released. But 32-36% of the colloids were released with the flowback fluid. “This is a first step into discovering the effects of flowback fluid on colloid transport in soils,” said postdoctoral associate Cathelijne Stoof, a co-author on the paper.
The authors hope to conduct further experiments using naturally occurring colloids in more complex field soil systems, as well as different formulations of flowback fluid collected from other drilling sites.
Stoof said awareness of the phenomenon and an understanding of the mechanisms behind it can help identify risks and inform mitigation strategies.