HYDROSTRATIGRAPHY OF THE TERTIARY FORT UNION GROUP IN THE MISSOURI COTEAU NEAR MINOT, N.D

The city of Minot, on the banks of the Souris River, has experienced rapid growth and increasing water use related to development of the Williston Basin Oil Shale, and water levels are dropping in alluvial aquifers throughout the region. Water levels have dropped as much as 60-feet in wells in Souris River outwash (Sundre Aquifer) during the past 40 years. Ground-water recharge to the Minot and Sundry alluvial aquifers that are heavily used by Minot and the surrounding area occurs as: 1) seepage from the Souris River bed; 2) distributed infiltration during precipitation and snowmelt events; and 3) lateral flow of Missouri Coteau groundwater recharge through the Fort Union Group towards the 200-ft thick outwash fill underlying the Souris River. The relative importance of these three mechanisms of recharge is poorly understood.

Water levels were canvased in wells to the southwest of Minot in 2012, and ground-water levels were compiled for the Missouri Coteau and Lowland Drift Prairie near Minot primarily using data from the North Dakota State Water Commission. Glacial till forms a confining layer over the Fort Union Group and fine grained clay layers within the Fort Union Group confine it further. The Missouri Coteau upland area has steep downward vertical hydraulic gradients (up to 0.77) suggesting it is a region of ground water recharge. The water table in the Lowland Drift Prairie slopes northeast towards the Souris River with a regional (horizontal) gradient of 0.008, and upwards gradients beneath the Souris River are around 0.14. Hydraulic conductivity of the Fort Union Group aquifers is calculated from Driller’s Logs using Lohman’s Specific Capacity and the well screen length. The geometric mean of horizontal hydraulic conductivity is 11 ft/d for alluvium, is 7ft/d for till; and is 2 ft/d for Fort Union Group in this area. A detailed analysis of hydrostratigraphy has been produced for the Fort Union Group showing good correspondence between location of: sand and coal layers with higher hydraulic conductivity, and silt and clay layers with lower hydraulic conductivity.