In North Dakota’s Bakken region, the fracking boom has generated nearly 10,000 wells for unconventional oil and gas production—and along with them, almost 4,000 reported wastewater spills resulting from the activity. A new study shows that these spills have left surface waters in the area carrying radium, selenium, thallium, lead, and other toxic chemicals that can persist for years at unsafe levels (Environ. Sci. Technol. 2016, DOI: 10.1021/acs.est.5b06349). Soils and sediments at spill sites also harbored long-lasting radium contamination, the study found.
In hydraulic fracturing, operators inject fluid into shale formations to release natural gas and oil. During production, the well brings up a brine that carries the fingerprint of the rock formation below, including naturally occurring toxic or radioactive elements like selenium and radium. This wastewater, called produced water, may be reused, injected underground for disposal, or processed—though not always successfully—in water treatment plants. But as fracking has increased in the Bakken region, so has the incidence of wastewater spills, often resulting from leaks in pipelines that transport the brine to injection wells.
To trace the impact of these spills on the environment, Avner Vengosh of Duke University and his colleagues analyzed four samples of produced water from shale gas wells in North Dakota, and chemical data on produced water from the U.S. Geological Survey. They also took water, sediment, and soil samples at sites of reported brine spills—including the two largest spills in the state’s history—which had occurred months to years earlier. In the largest of these, the Blacktail Creek spill of 2015, an underground pipeline leak introduced almost 11 million L of brine near the creek, which flows into a tributary of the Missouri River.
The team used several geochemical tracers, including strontium isotopes, to detect wastewater residue at the spill sites. The ratio of 87Sr to 86Sr in fracking wastewater carries a distinctive signature of the rock formation where it was produced. By measuring strontium isotopes in the produced water and in water samples taken from spill sites, the researchers could identify brine residue from a spill. Other tracers present in both types of samples confirmed the link.
In the water samples from spill sites, the team found that high concentrations of salts, trace metals, and other toxic contaminants persisted from the spills. Selenium, thallium, and radium exceeded maximum contaminant levels for drinking water in some samples. Additionally, ammonium and selenium concentrations were above recommended levels for aquatic life. In soil and sediment samples downstream from the Blacktail Creek spill site, radium concentrations were up to 100 times as great as in samples upstream.
Brian W. Stewart, a geochemist at the University of Pittsburgh who studies fracking wastewater, says this is the first time to his knowledge that systematic sampling has been done downstream of known brine spills to detect the impact of wastewater from fracking. “I was surprised that it persists that long,” he says; in two cases, the group found elevated levels of contaminants from a spill four years later.
In a separate study, Vengosh and colleagues showed that a radioactive tracer technique could accurately date recent spills, including the one at Blacktail Creek. The method relies on measuring the ratio of 228Th to 228Ra in the soil and sediment. 228Ra, which is soluble in water, comes to the surface with spills and is absorbed by soil, and then decays into insoluble 228Th with a half-life of about six years. The technique can date spills up to 10 years old (Environ. Sci. Technol. Lett. 2016, DOI: 10.1021/acs.estlett.6b00118). It could finger recent pollution from fracking wastewater in regions with a history of conventional oil and gas production, such as the Marcellus Shale region, potentially resolving issues in which producers claim that contamination resulted from earlier production, Vengosh says.