Produced water from hydraulic fracturing varies widely from well to well, according to a new study.

Researchers from the U.S. Geological Survey (USGS) examined 13 hydraulically fractured shale gas wells in north-central Pennsylvania, and found significant variations in the microbiology and organic chemistry of the produced waters.

While the inorganic (noncarbon-based) chemistry of produced waters from the shale gas wells in the Marcellus region was fairly consistent from well to well, there were large differences in the organic geochemistry (carbon-based, including petroleum products) and microbiology (e.g. bacteria) of the produced waters. Volatile organic compounds (VOCs) were detected in only four of the samples.

"Some wells appeared to be hotspots for microbial activity, but this was not predicted by well location, depth or salinity," explained Denise Akob, a USGS microbiologist and lead author of the study. "The presence of microbes seemed to be associated with concentrations of specific organic compounds — for example, benzene or acetate — and the length of time that the well was in production."

Reporting on the findings, USGS noted that it was not surprising to see a connection between the presence of organic compounds and the detection of microbes. "Many organic compounds used as hydraulic fracturing fluid additives are biodegradable and thus could have supported microbial activity at depth during shale gas production," the agency said.

Going forward, the notable differences in VOCs in produced waters could play a role in how these waters are managed. High levels of VOCs such as benzene may be a health concern around the well or holding pond. Meanwhile, in wells without VOCs, treatment and disposal strategies could focus on issues related to other hazardous compounds.

USGS also pointed out that microbial activity detected in the samples could turn out to be an advantage by contributing to the degradation of organic compounds present in the produced waters.

The findings of the study have been published in Applied Geochemistry's special issue on shale gas geochemistry.