A study conducted by Duke University scientists in North Carolina examined state-level data to characterize spills associated with unconventional oil and gas development at 31,481 wells hydraulically fractured in four states between 2005 and 2014.

The researchers identified 6,648 spills reported across Colorado, New Mexico, North Dakota, and Pennsylvania during the 10-year period. The analysis also showed that 2-16% of hydraulically fractured oil and gas wells spill hydrocarbons, chemical-laden water, hydraulic fracturing fluids, and other substances.

The highest spill rate, with 4,453 incidents, was posted for North Dakota, followed by Pennsylvania at 1,293, Colorado at 476, and New Mexico at 426. The number of spills reported is partly a reflection of the reporting requirements set by each state. For example, North Dakota required reporting smaller spills (42 gallons or more) than Colorado and New Mexico (210 gallons or more).

Half of the spills were related to storage and moving fluids via pipelines, although it was not always possible to determine the cause of the spill because some states explicitly required this data to be reported while others relied on narrative descriptions.

Across all states, the first three years of a well’s life, when drilling and hydraulic fracturing occurred and production volumes were highest, had the greatest risk of a spill. A significant portion of spills (from 26% in Colorado to 53% in North Dakota) occur at wells that experienced more than one spill. Researchers say this suggests that wells where spills have already occurred merit closer attention.