USE OF GEOCHEMICAL TRACERS TO IMPROVE UNDERSTANDING OF GROUNDWATER DYNAMICS IN THE WILLISTON BASIN, MONTANA AND NORTH DAKOTA

In 2013, 30 randomly distributed domestic wells screened in the Upper Fort Union Formation in the Montana and North Dakota parts of the Williston Basin were sampled for a broad suite of inorganic and organic chemical constituents to characterize current water-quality conditions in the aquifer. This work is part of the U.S. Geological Survey’s Williston and Powder River Basins Groundwater Availability Study. Ten of those wells were also sampled for multiple isotopic and groundwater-age tracers to improve the understanding of groundwater dynamics in the aquifer. Most of the age-dated samples had apparent ages of several thousand years on the basis of carbon-14 and helium-4 data, indicating low recharge rates and slow groundwater velocities in the aquifer. Helium-3/helium-4 ratios show that the oldest samples contained helium from geologic units below the aquifer, indicating that the aquifer was open with respect to a deep gas flux. Methane was detected in 29 of 30 samples (mostly at concentrations less than 0.01 milligram per liter), but it appears to be biogenic in origin on the basis of hydrocarbon compositional and isotopic data. Thermogenic methane was not detected in any of the samples. Slow groundwater velocities in the shallow aquifer imply that the presence of hydrocarbons in the shallow aquifer from sources below the aquifer, if present, could go undetected if monitoring does not occur in close proximity to the source. Results from this study indicate that better understanding of groundwater dynamics is important not only with respect to water availability but also for interpreting water-quality data and improving the effectiveness of monitoring programs.