The damage is worse in Weld County, where 40 percent of spills reach groundwater, the data show.
Most of the spills are happening less than 30 feet underground ? not in the deep well bores that carry drilling fluids into rock.
State regulators say oil and gas crews typically are working on storage tanks or pipelines when they discover that petroleum material, which can contain cancer-causing benzene, has seeped into soil and reached groundwater. Companies respond with vacuum trucks or by excavating tainted soil.
Contamination of groundwater ? along with air emissions, truck traffic and changed landscapes ? has spurred public concerns about drilling along Colorado's Front Range. There are 49,236 active wells statewide, up 31 percent since 2008, with 17,844 in Weld County.