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DENVER — As large scale oil spills off the coast of the U.S. continue to decrease, the sort of smaller-scale inland oil and gas spills that are far less likely to make headlines — and in some cases, unlikely to even get reported — are on the rise.

This increase is being reported by Environment & Energy Publishing (E&E), a nonpartisan media company in Washington, D.C. that covers environmental and energy policy and markets.

Despite having not received comprehensive data from North Dakota, one of the largest natural gas-producing states in the country, E&E reported the U.S. oil and gas industry averaged about 20 oil or gas spills a day in 2013, which is an increase of about 18 percent from 2012.

Why is some of the data from North Dakota missing? According to the Associated Press, nearly 300 oil spills and 750 “oil field incidents” that occurred in the state since January 2012 went unreported.

Why did those spills go unreported? Because North Dakota regulators are not required to report all oil spills under state law, the AP reported.

The same goes for Colorado, according to another report recently released by the Center for Western Priorities (CWP). Though the organization is also nonpartisan, its stated goal is to “ensure that Western land and resources are being used responsibly.”

As part of a comprehensive project that includes an interactive map of all oil and gas spills in both Colorado and New Mexico from 2000 to 2013, the CWP noted that the data shows far more spills in New Mexico (10,300) than Colorado (4,900) in large part because New Mexico has tougher regulations about the reporting of spills.

“For example, oil and gas companies in New Mexico are required to report the unauthorized release of natural gas in significant quantities,” the CWP wrote in a press release. “They are not required to do so in Colorado.”

Spills in Colorado tended to cluster in areas that have seen a large boost in drilling over the past two decades — specifically rural parts of Weld, Garfield, La Plata and Las Animas counties. However, more residential counties like Arapahoe, Adams, Boulder and Larimer were not exempt.

In 2013 specifically, there were 531 total spills of either oil, brine, drilling water or other types of chemicals in Colorado. While that’s a significant rise from the 398 spills reported in 2012, it’s only an average of 1.47 spills a day, which is far below the national average. And of the spills that have been reported thus far in 2014 (156), only nine have caused and ground or surface water contamination, according to CWP data.

No two oil and gas spills are alike, either. In 2011 in neighboring Arapahoe and Adams counties, there were spills ranging from 42 to 6,258 gallons. According to CWP policy director Greg Zimmerman, all spills above 5 barrels, which is about 210 gallons, must be reported. And due to legislation that was new as of 2014, a release of 1 barrel outside of containment must now also be reported in Colorado.

While some may believe the increase in Colorado oil and gas spills from 2012 to 2013 is simply an unavoidable byproduct of a boom in oil and gas drilling, Zimmerman isn’t so sure.

“Yes, drilling is high in Colorado, but it’s not at an all-time high,” he said. “In fact, there was more drilling in 2008 than there was in 2013.”

Considering Colorado had 140 fewer spills in 2008, Zimmerman said there appears to be no clear reason why the state saw more spills in 2013.

But one thing does appear to be clear: The number of oil and gas spills in Colorado has increased almost every year since 2000, and the pace set thus far in 2014 doesn’t indicate any significant reductions.

Whether the spills expose the environment to 42 gallons of toxic materials or 10,000, each has the potential to cause damage, Zimmerman said. And the fact that Colorado is averaging more than one of these spills each “isn’t cutting it.”

“These numbers shine a spotlight on the oil and gas industry’s significant spill problem,” Zimmerman said. “Part of responsible energy development is holding companies to the highest standards and minimizing the release of toxic materials onto land and into water.”

RELATED: Click for more on the CWP oil and gas spill data