More than 7,600 oil spills and leaks were recorded in 2013 in the top 15 states for onshore oil and gas activities, according to a new analysis by EnergyWire based on state data. This is a 13-percent increase on the previous year, even though overall drilling activity remained generally flat over that period.

The total number of incidents including spills, blowouts and leaks at drilling sites came in at 7,662, equaling more than 20 per day, EnergyWire said. The majority of these spills were relatively small and were contained quickly, before any serious damage was caused. However, the combined volume of oil, fracking fluid, fracking wastewater and other liquids that were released in them amounted to 26 million gallons. Some of the increase could be put down to new reporting requirements that were introduced in several states last year, but the number of spills went up even if those were not factored in.

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In terms of regional distribution, the Bakken Shale recorded some of the most dramatic increases in spills and leak rates. Specifically, Montana saw the sharpest rise in spills (48 percent), followed by North Dakota (42 percent). Anadarko Petroleum Corporation reported 321 incidents in 2013, landing at the top of the chart, leaving ConocoPhillips and Occidental Petroleum far behind with 256 and 240 spills, respectively, the data shows.