The impact of groundwater age and flow patterns on water quality in the Milk River Aquifer, Canada

The Milk River Aquifer (MRA) is a regional transboundary aquifer covering over 26,000 km² across northern Montana (USA) and southern Alberta (Canada). Extensive groundwater extraction since 1960s has led to a decline in groundwater levels, thereby emphasizing the need for informed water management strategies. The objective of this study was to improve the understanding of spatial variations in major ion concentrations with respect to groundwater age and flow paths, and to  identify key geochemical processes that influence groundwater quality within the aquifer. A comprehensive digital database was developed using hydrogeological and geochemical data from 1,429 water samples collected from 549 wells. Additionally, 20 new  groundwater samples and associated gases were collected during a 2022 field campaign, and these samples were analyzed for concentrations of major and minor ions, gas composition, stable isotope ratios (²H/¹H and ¹⁸O/¹⁶O of water, ¹³C/¹²C of DIC and ³⁴S/³²S of sulfate, ¹³C/¹²C and ²H/¹H of methane), and radioactive isotopes (⁸¹Kr, ³⁶Cl and ¹⁴C_DIC).

Utilizing a newly updated groundwater numerical flow model (FEFLOW software) in combination with recent ¹⁴C and ⁸¹Kr-based groundwater age dates, distinct patterns in chloride (Cl) concentrations dependent on groundwater age and flow path were identified. Groundwater less than 34,000 years old exhibited Cl concentrations < 25 mg/L near the recharge zone, while groundwater exceeding 200,000 years in age had Cl concentrations > 100mg/L at distances of 125 km from the recharge zone. Increasing δ²H and δ¹⁸O values in older groundwater with elevated Cl concentrations indicate possible mixing of fresh recharge water with formation water from northern regions of the aquifer (Taber and Bow Island formations) or associated aquitards (Pakowki and Colorado formations). Ongoing analysis explores variations in other major ions with a specific interest in redox-sensitive species as a function of flow distance and groundwater age. Preliminary results reveal that elevated sulfate concentrations (> 1200 mg/L) in recharging groundwater are due to pyrite oxidation, but at groundwater flow distances between 50 and 75 km bacterial sulphate reduction becomes dominant resulting in sulfate concentrations < 1 mg/L. At flow distances >80 km, redox conditions become favourable for methanogenesis resulting in occurrence of biogenic methane in groundwater. A particle tracking algorithm within the updated numerical flow model was employed to compare residence times with groundwater ages determined from ⁸¹Kr measurements. The tracer ages (¹⁴C and ⁸¹Kr) were confirmed using a numerical particle tracking model based on an existing numerical steady-state groundwater flow model (FEFLOW). The outcomes of this study that utilizes innovative groundwater age dating tools (⁸¹Kr) are new insights into how geochemical processes evolve with respect to flow distance and groundwater age thereby modifying spatial variability of key water quality parameters within the Milk River Aquifer.