The wastewater created by hydraulic fracturing, or "fracking," used to recover oil and gas from underground shale formations may present more environmental risks than previously believed, researchers say.

Cornell University scientists say chemicals in fracking wastewater can cause a release of microscopic particles from soils that can bind some pollutants to heavy metals.

That binding makes the pollutants an even greater risk to humans and to wildlife above-ground should there be an unexpected spill, the researchers say in their report of their study in the journal Environmental Science and Technology.

Accidents are not uncommon, they say; some studies have shown between 10 percent and 40 percent of chemically enhanced water solutions injected deep into rock strata in fracking operations eventually makes it way to the surface.

Along with separating large quantities of natural gas from surrounding shale, the "flowback fluid" breaks down soil particles that then bind with pollutants, eventually leaching out to contaminate both groundwater and soil, the researchers warn.

The particles are known as colloids, smaller than can be seen by the naked eye, which can cling to soil and sand because they possess an electric charge.

In an experiment run by graduate student Wenjing Sang and other researchers, sand mixed with synthetic polystyrene colloids was placed in glass columns to mimic various kinds of soils.

When deionized water was flushed through the columns, 5 percent of the colloids in the columns were released, but when they were flushed with fracking flowback fluid as much as 36 percent of the colloids were released, they said.

The researchers said they believe the chemical makeup of the fracking flowback fluid was reducing the power of the electrical forces that keep the colloids bound in the soil and sand, causing them to be ejected from the material.

The discovery and understanding of the science behind it could help highlight risks associated with fracking and help lead to mitigation strategies, study co-author Cathelijne Soof said.

"Sustainable development of any resource requires facts about its potential impacts, so legislators can make informed decisions about whether and where it can and cannot be allowed, and to develop guidelines in case it goes wrong," she said. "In the case of spills, you want to know what happens when the fluid moves through the soil."

The research, conducted by members of Cornell's Soil and Water group, was funded in part by the National Science Foundation, the U.S. Department of Agriculture and the National Natural Science Foundation of China.