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Some natural gas extraction operations have sent the highly saline water left over from hydraulic fracturing, or fracking, to wastewater treatment plants for disposal. These plants then discharge their treated water into rivers that may feed drinking water plants downstream.

A new study finds a possible problem with this process: Even when made 10,000 times more dilute, fracking wastewater can increase levels of troubling compounds formed under conditions similar to those during drinking water disinfection (Environ. Sci. Technol. 2014, DOI: 10.102/es50281841). Halides in the wastewater lead to toxic disinfection by-products, some at levels that exceed allowed discharge limits for drinking water treatment plants, the researchers say.

Chlorine and chloramine used to disinfect water can react with organic matter to form compounds such as trihalomethanes and haloacetic acids. These disinfection by-products have been linked to cancer and nervous system problems, and some are regulated by the Environmental Protection Agency. In the presence of other halides such as bromide and iodide, the disinfectants can create by-products that are even more toxic than their chlorinated analogs. Wastewater generated during hydraulic fracturing can contain high concentrations of these halides, and they remain after the water goes through commercial or municipal wastewater treatment.

William A. Mitch of Stanford University and his colleagues wanted to know how wastewater from fracking might influence disinfection by-product formation at downstream drinking water treatment plants. The researchers obtained two samples of wastewater from hydraulic fracturing operations in the Marcellus Shale in Pennsylvania. To simulate the chemical composition of what a drinking water plant might take in, they diluted the wastewater with water from rivers downstream of the shale operations to get samples containing 0.01 to 0.1% wastewater by volume. Then the researchers treated the samples with chlorine, chloramine, or ozone, just like at a drinking water plant.

Using mass spectrometry, they measured levels of a range of disinfection by-products and compared them with levels in treated river water without any fracking waste. In chlorinated samples containing as little as 0.01% wastewater, concentrations of trihalomethanes and haloacetonitriles were higher than those in treated, unaltered river water. Samples containing 0.1% wastewater had 70 to 140% higher levels of trihalomethanes, and concentrations of some of those compounds exceeded EPA limits. In ozone-treated water samples, levels of bromate, a potential carcinogen, were also above regulatory limits. The researchers also noticed that the fracking wastewater led to increases in levels of brominated and iodinated disinfection by-products, which tend to be significantly more toxic than their chlorinated analogs.

In an actual drinking water plant, the organic precursors to these compounds may be removed prior to disinfection, so the regulated by-products might be found at lower levels, the researchers write in the paper. But in 2010, the Pittsburgh Water & Sewer Authority measured a significant increase in trihalomethanes in drinking water (J. Am. Water Works Assoc. 2013, DOI: 10.5942/jawwa.2013.105.0093). The agency traced the problem to elevated bromide levels in the source water, which might have come from industrial wastewater treatment plants handling fracking waste.

The Pennsylvania Department of Environmental Protection has started to discourage fracking operations from sending their wastewater to municipal treatment plants in the state. Some of that wastewater has been diverted to Ohio for underground injection instead, though this disposal method raises concerns about induced earthquakes.