Residents of Pavillion, Wyoming, have been concerned over drinking water contamination for some time, believing that it originates from E&P activities by EnCana Oil and Gas USA. Recent testing done by U.S. EPA is inconclusive on origin, but recommends that are residents with private water wells find alternate sources of water for drinking and cooking. From the Star-Tribune: "The health concern is based on high sodium and sulfates that EPA officials believe are naturally occurring in the groundwater, and on the detection of petroleum compounds that officials believe shouldn't be in the groundwater."
For more on the issue, including a copy of the report, see here (U.S. EPA site).