Nobody can say how many oil spills happened in U.S.
Of the states examined, North Dakota had the highest rate of spills while Colorado companies reported just 11 spills per 1,000 wells annually.
Some or all of that difference may be due to the huge differences in what the states ask oil companies to report.
North Dakota requires operators to report any spill of 42 gallons or more, while Colorado and New Mexico generally don't ask for anything smaller than 210 gallons.
Texas, the nation's top oil and gas producing state, wasn't even included in the study because detailed data was not easily accessible.
The authors did not examine data from Oklahoma because the state had not digitized all of its information.
The authors from several universities as well as the Nature Conservancy and the United States Geological Survey, looked at reports of 6,648 spills of oil, brine, chemicals and other fluids reported in Colorado, New Mexico, North Dakota and Pennsylvania between 2005 and 2014.
Some states require verbal reports within 24 hours but give companies days to submit paperwork with specific information. Until late last year, Pennsylvania did not ask for written reports at all and companies reported the volume of spills less than a third of the time.
The authors tried to look at all major oil and gas producing states, but chose four with the most complete publicly available data that also allowed them to distinguish between conventional and fracked wells.
Spills have presented a persistent problem for North Dakota, with thousands of releases of oil, chemicals and saltwater causing lasting damage in the state's oil patch.
Releases of brine—which occurs naturally deep underground and contains heavy metals, radioactive materials and extremely high levels of salts—are particularly damaging.
Such leaks have spoiled patches of farmland for years or even decades. ■