Volatile organic compounds (VOCs) are relatively light hydrocarbons (molecules made up of carbon and hydrogen) that readily evaporate. They can be toxic to humans and they sometimes persist in drinking water. The BTEX
compounds are of particular concern in Marcellus Shale drilling. BTEX stands for benzene, toluene, ethylbenzene, and xylene, all VOCs that co-occur with the methane in the Marcellus Shale, and are frequently found in the formation water that
comes to the surface after hydraulic fracturing.