A QUALITATIVE ASSESSMENT OF RECHARGE TO BURIED VALLEY AQUIFERS IN NORTH DAKOTA USING STABLE ISOTOPE ANALYSES

Over 400 samples have been collected and analyzed for the stable isotopes of oxygen (¹⁸O) and Deuterium (D) from precipitation, ground, and surface water locations in and overlying glacial buried valley deposits in North Dakota. This relatively inexpensive isotope data can assist in identifying recharge of meteoric waters into buried valley aquifer systems. Two examples where stable isotope data resulted in a better qualitative assessment of recharge are the West Fargo aquifer and the Spiritwood aquifer.

The West Fargo Aquifer is a buried valley aquifer, which serves as a primary water supply to the city of Fargo, ND. The water is characterized by low total dissolved solids (TDS) concentrations, which could be construed to be recently recharged meteoric water. Sediments overlying the aquifer consist mainly of low-permeable lake clays and till that would limit recharge. Secondary porosity through fractures and/or fluvial inhomogeneities may be providing the necessary hydraulic pathway for recharge. However, stable isotope analysis revealed water in the aquifer was highly depleted in ¹⁸O and D (-16.2 to -22.4 ∂¹⁸O, and -122 to -169 ∂D) and was derived from a colder recharge environment, possibly during the Pleistocene.

The Spiritwood aquifer is a major buried valley aquifer, which provides water for irrigation, municipal and industry use in eastern North Dakota. The aquifer is divided into numerous discreet flow segments due to the existence of transverse barriers and groundwater divides. In one such segment, the Spiritwood aquifer is overlain by glaciofluvial clays, silts, till, and the surficial unconfined Warwick aquifer. In this segment, the Spritwood aquifer is characterized by low TDS water. Stable isotope analysis revealed the water has a similar isotopic concentration (-13.1 to -15.9 ∂¹⁸O, and -100 to -121 ∂D) to the overlying surficial Warwick aquifer – intermediate between winter and summer precipitation isotopic signatures. It was determined that water in the aquifer was relatively recently recharged through direct infiltration where direct hydraulic connection with the overlying Warwick aquifer occurs, or possibly through glaciofluvial clays and silts characterized by more permeable fluvial inhomogeneities.