U.S. GEOLOGICAL SURVEY GROUNDWATER-QUALITY ASSESSMENTS IN THE WILLISTON STRUCTURAL BASIN, MONTANA AND NORTH DAKOTA

Domestic oil and gas production and clean water are critical for public health, economic growth, and the national security of the United States. The Williston structural basin (WSB) has been a major oil and natural gas producing region since the 1950s. Oil and gas development from deep formations, such as the Bakken and Three Forks formations, has increased in recent years due to the improved accessibility of resources using hydraulic fracturing and horizontal drilling methods. Associated with the increased oil and gas development is increased public concern about the effects of energy production on surface-water and groundwater quality.

Quaternary (Glacial), lower Tertiary (upper and lower Fort Union) and Upper Cretaceous (Hell Creek/Fox Hills) aquifers in the WSB provide the majority of groundwater used for domestic, livestock, industrial, and public-supply purposes. A study initiated in 2013 characterized water-quality conditions in the shallowest bedrock aquifer (upper Fort Union) in Montana and North Dakota. Thirty wells completed in the upper Fort Union were selected within the WSB using a spatially-distributed randomized grid-based method. Four wells completed in underlying formations (lower Fort Union or Hell Creek/Fox Hills) were also selected for the study. Water samples from all 34 study wells were analyzed for field parameters, selected major ions, nutrients, organic compounds, uranium, and methane and ethane. Samples from ten of the study wells were analyzed for additional parameters to improve understanding of these waters.

In addition to the assessment of the heavily-utilized water resources in the WSB, a baseline chemical and isotopic assessment of produced water from the Bakken Formation was conducted in 2012. Water samples were collected from 8 production wells within the WSB in Montana and North Dakota, and analyzed for major ions, strontium, trace elements, oxygen/deuterium, and radium isotopes. The analytical results were used to identify water-quality constituents that characterize Bakken Formation water. Based on results from these water-resource assessments, additional studies are being proposed to further characterize the quality of production water, and surface- and groundwater resources within the WSB.