Unconventional natural gas development uses horizontal drilling in conjunction with hydraulic fracturing to gain access to natural gas deposits which may be tightly held in shale deposits and unavailable to conventional vertical drilling operations.

The intensive work required to extract this source of energy results in higher than usual numbers of vehicles involved, potential release of emissions from those vehicles in congested zones surrounding the drill site, and release of other contaminants from materials drawn back out of the borehole after fracturing of the shale.

Typical contaminants would be diesel exhaust particulate and gases, volatile organic compounds and other hydrocarbons both from diesels and the drilli...