GROUNDWATER FLOW MODEL INVERSION TO ASSESS WATER AVAILABILITY IN THE FOX HILLS-HELL CREEK AQUIFER

In much of western North Dakota the Fox Hills-lower Hell Creek (FH-HC) aquifer is the only source capable of producing large quantities of fresh groundwater. Historically, it has provided water for municipal, domestic, stock and industrial users in western North Dakota. More recently, the Southwest Pipeline has provided Missouri River water for many municipalities. However, the FH-HC aquifer remains an important water source for domestic, stock and industrial users. In valleys along the Yellowstone, Little Missouri and Knife Rivers the potentiometric surface of the FH-HC aquifer is above the land surface, creating flowing head wells. The pressure head is currently declining at an average rate of approximately one foot per year in western North Dakota. Western North Dakota is experiencing an increase in water demand for use by the oil industry. The purpose of this project is to gain a better understanding of the hydrogeology of the FH-HC aquifer through the development of a groundwater flow model using MODFLOW-2005. The model will provide a foundation for the development of a long-term management policy.

The active model area is approximately 25,000 square miles. Vertically the aquifer is represented as one confined layer. The bottom of the aquifer was delineated as the top of the Pierre formation and the top of the aquifer includes the lower portion of the Hell Creek formation. The steady-state simulation was developed and calibrated by using hydrologic data that represented average conditions for 1972. The transient simulation was developed and calibrated using hydraulic head observations collected from 1973 to 2009. Parameter values for hydraulic conductivity, recharge, specific storage, discharge from flowing wells, and conductance were estimated during model calibration using UCODE_2005. In general, the model simulates the observed water levels and can be used as a prediction tool for additional groundwater management scenarios. To assess future impacts on water levels the current level of use was assumed to continue from 2009 to 2039. At the current level of use, a large portion of the aquifer will continue to decline indicating that in parts of the basin additional large-scale groundwater withdrawals may not be sustainable.