Natural gas hydraulic fracturing - fracking - has been wonderful for CO2 emissions while keeping energy costs for poor people manageable but a few sites have been treating fracking wastewater and returning it to rivers. 

A new study finds that this is just as risky as dumping any municipal treated wastewater back into rivers. As runoff, it is safe but it shouldn't be done in volume. In the case of fracking wastewater, existing facilities are not equipped to thoroughly deal with halides so until they are ready, it's simply better to use fracking wastewater for fracking.

Writing in Environmental Science&Technology, William A. Mitch, Avner Vengosh and colleagues point out that the disposal of fracking wastewater can be radioactive and contain high levels of heavy metals and salts called halides (bromide, chloride and iodide). Most sites recycle the water but one approach has to treat it in municipal or commercial treatment plants and then release it into rivers and other surface waters.

Credit: Doug Duncan/U.S. Geological Survey.
Article: What's In Fracking Fluids? Are They Harmful?

The problem is these plants don't do a good job at removing halides. While there is no evidence yet, there is concern that halide-contaminated surface water subsequently treated for drinking purposes with conventional methods, such as chlorination or ozonation, could lead to the formation of toxic byproducts. 

In a laboratory experiment, the study authors diluted river-water samples of fracking wastewater discharged from operations in Pennsylvania and Arkansas, simulating real-world conditions when wastewater gets into the environment. They then used current drinking-water disinfection methods on the samples.

They found that even at concentrations as low as 0.01 percent up to 0.1 percent by volume of fracking wastewater, an array of toxic compounds formed. Based on their findings, the researchers recommend either that fracking wastewater should not be discharged at all into surface waters or that future water treatment include specific halide-removal techniques.