Oil and gas have contaminated groundwater in 17 percent of the 2,078 spills and slow releases that companies reported to state regulators over the past five years, state data show.
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.
Starting today, Colorado Oil and Gas Conservation Commission regulators struggling to maintain a consistent set of state rules governing the industry will begin grappling with the groundwater issue.
The COGCC is weighing proposed changes to state rules that would require companies to conduct before-and-after testing of groundwater around wells to provide baseline data that could be used to hold companies accountable for pollution.
"There is an impact," COGCC environmental manager Jim Milne said, reviewing the groundwater data. "We don't know if it is unreasonable or not."
While spills reaching groundwater are concentrated at the surface, the baseline testing proposals that state commissioners are weighing focus on harm from well bores to deep aquifers.
The proposals do not specify the depths from which samples would be taken. Any existing wells - shallow agricultural wells or deep domestic wells - could be used to draw before-and-after water samples from areas around oil and gas production wells.
One proposal developed by Shell and the Environmental Defense Fund would require testing of three samples of water within 12 months before drilling using wells within a half-mile of a vertical oil or gas well.
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